Rocking the AC

When it comes up in casual conversation that I do not generally heat or cool my house, people either move to another seat or look at me with some mixture of admiration and disbelief. When non-Californians then find out that I live in San Diego, they might huff or spew, which often involves some embarrassing projectile escaping their mouth. But the locals are more consistently impressed—more so by my forsaking heat than AC (San Diego has very mild summers by U.S. standards). This summer, I turned on the AC for the first time since we bought the house three years ago. All in the name of science! I was blown away. Here is what I learned.

A Happy Convergence

I occasionally embark on a campaign to track temperatures throughout my house in order to understand how it behaves thermally in response to fluctuations in sunlight and external temperature. I keep telling myself that I’m establishing a baseline against which to compare the results of a hypothetical insulation upgrade/replacement. So far, I keep getting baseline data.

Most telling are campaigns when the external temperature is extreme in some way. The forecast showed a hot week was coming up, so I deployed the Thermochrons for an 8.5 day mission, set up to sample temperatures to 0.06°C precision every three minutes (and roughly synchronized with each other).

As it happened, the first day into the campaign was unusually hot and muggy for San Diego (high 30 °C, or 86 °F, 62% humidity). And we had invited 2500 W over for a surprise party (non-physicists might instead say 25 people). Okay, so Thermochron campaign, hot and muggy, loads of people coming over…if there was ever a right time to fire up our AC, this was it!

We Have Liftoff!

I set the thermostat to 75°F (24°C) flipped the switch at 4PM, and immediately rushed to TED to see what he thought of our unfamiliar visitor. I had never seen anything like it. TED was screaming that we had over five kilowatts running! Previously, the highest rate of electricity consumption I had ever seen in our household was in the neighborhood of 3 kW when both the microwave and toaster oven were in simultaneous operation.

I rushed outside to watch the synthetic dial “spin” on my electricity meter. Usually, I have to wait a full minute to see one block change, but now blocks were coming and going faster than once per second. Night and day difference!

100 W at a time, our guests arrived and were appreciative of the cool—though those who knew me well admitted to being surprised. The air conditioner ran constantly for 4.5 hours, and I began to wonder if the system had enough oomph to overcome the party load. But it finally started cycling, spending about 45% of the time in the on-state over the next five hours.

I clocked 32.6 kWh that day. This may not sound like a lot. In fact, it’s pretty close to the U.S. average daily household electricity consumption. But for me, it was out of this world. For perspective, over the previous 600 days, our typical daily expenditure (median) was 1.64 kWh (average 1.82, with 0.79 standard deviation). The minimum was 0.85, and the max soared to 6.3. 32.6 kWh beat our previous maximum by a factor of five, and our typical usage by a factor of 20! Welcome to America, my friend.

Daily energy use surrounding the air conditioning test. The test stands out like a huge, sore thumb.  The vertical scale is kilowatt-hours.

Are We There Yet?

As much as I learned from this experience, the day of the “test” could hardly be called typical. Lots of people, lots of in-and-out traffic, a late start turning on the AC, etc. So I decided to dedicate the next day to a more stable test. Setting the thermostat at 76°F (24.4°C), the compressor kicked into action just before 10AM, executing 24 cycles over the next 11 hours.

The funny thing is, it felt unnaturally cold in the house, even set at 76 °F. My bare feet, dangling over the couch arm as I prepared a blog post, were on the verge of feeling cold.

Okay, a few anecdotes from “normal” America. I grew up in the South. It’s hot and humid in the summer, the humidity keeping it relatively warm even overnight. AC is ubiquitous, and I get this. But the thermostats are usually turned down so far that if I wear sandals indoors for any length of time, my feet get downright cold. No one in the entire state should have cold feet in the summer! I’d call it overcompensation. It really strikes me as absurd—especially in light of the stiff energy cost, and complaints about the swollen electricity bills.

In a second instance, my wife and I were visiting relatives in eastern Washington where it gets pretty hot in the summer and pretty cold in winter. We noticed the winter-time thermostat turned to 76°. Pretty toasty! When queried, we were told that so-and-so’s joints have difficulty if it’s any cooler than that in the house. Okay, bummer. A return visit in the summer found the thermostat at 72°. Yet so-and-so was moving around just fine, thank you very much. Maybe there’s some complicated medical thing going on here, but it sure seems to me that they likes their comfort, and that it’s largely psychological: they want to feel that home is a luxurious retreat from the nasty outdoors.  Wearing shorts inside in winter and a sweater inside in summer somehow feels right as rain.

So here we are during our day-long “test,” suffering the unnatural cold of 76° in the summer. My wife asked me several times if we could just turn it off. “Aren’t you done yet?” I was a bit surprised by this. I’m usually hardier in terms of adapting to too-warm or too-cold conditions. What might have been considered a special treat had turned into a grueling march.

Adding to the trial, on the same day the New York Times Sunday Review had a fascinating article on air conditioning and its energy/environmental cost—to which I was now contributing. It also felt very strange to read about reluctance among many to bump up their thermostats to 75° or higher, while I was itching to ditch 76° as unpleasantly, unnaturally cold. Noting the outsized energy demand imposed by air conditioning, the article shared the scary statistic that if Mumbai, India adopted universal air conditioning for its inhabitants at American standards, the associated energy demand from this city alone would come to a quarter of the cooling demand for the entire U.S. That’s scary, folks.

Evaluating the Damage

Okay. Experiment Complete. What’s the damage? The air conditioning ran for about 400 minutes that day (nearly seven hours out of eleven). In this time, the air conditioning racked up 31.5 kWh of energy use. Over a 24-hour period, this averages to about 1300 W of continuous power. Imagine my fright: two days in a row over 30 kWh: more electrical energy in two days than we normally expend in a month!

So what do the Thermochrons have to say? Below is a plot of the entire period for the bedroom, which was not as disturbed by the party (not that kind of party).

Temperature record for the bedroom (blue) and outside (black). A cooling trend is evident, as is the impact of the air conditioner abruptly pulling down the internal temperature starting late in the first day. The fourth day (234) is unusual, in that I disabled the attic fan for the day.

If we plot the second and third days on top of each other (roughly similar outside), we can examine the impact of the air conditioner.

24 hours of AC experiment, compared to the following, normal day.

Now we can see that the day on which the air conditioner was run (the “AC day”) was on the whole a bit warmer outside (black line) than the following day (gray line)—especially in the evening and night-time. Meanwhile, the cyan curve indicates what happened inside on the normal (no AC) day, which turns into something like the dark blue curve when the AC is turned on.

One possible clue as to why my wife and I were unsettled by the AC—feeling that we had created an unnatural environment for ourselves—may be seen in the jagged temperature profile resulting from the cooling cycles. We were being whip-sawed around all day. I think this unfamiliar jerking behavior grated on us, getting progressively more annoying.

Quantitatively, I can say that the AC day averaged 1.07 °C hotter than the following day, over a 24 hour period (or 1.3 °C over 12 hours when the AC was active). Meanwhile, the air inside averaged 1.88 °C cooler inside on the AC day than the following day (3.5 °C cooler during active AC time).

Assuming that a warmer day outside translates to a comparably warmer day inside, we can conclude that the air conditioner bought us an average of 3.0 °C of cooling (1.07 plus 1.88) compared to what would have happened without AC, when averaged over the day (4.8 °C for AC-on time).

We can get an energy cost per degree by taking the power expended and dividing by “comfort” delivered. For the 24-hour period, this is 31.5 kWh divided by 24 hours, divided by 3 °C, yielding about 440 W/°C. For the shorter 12-hour period, we get about 550 W/°C.

What does this mean? Is it good? Is it bad?

Comparison to Fans

A box fan might typically run at about 90 W and move something like 2000 cubic feet of air per minute (cfm), or about a cubic meter per second. Imagine enveloping the fan within a window to efficiently move air between inside and outside (pushing either direction). Roughly speaking, if the replacement air is 1 °C cooler than the interior air, then each second, the kilogram (cubic meter) of cooler air removes about 1000 J of thermal energy from the house (specific heat capacity of air is about 1000 J/kg/°C). All that for 90 J of input, each second: a leverage of 11×. Good bargain. Better than a heat pump.

Looking at the graph above, the overnight house temperature stayed about 4 °C warmer than outside through the night on the normal day (cyan vs. gray). If a fan operating overnight (10 hours at 90 W) could even take that down to a 3 ºC difference, then this 1 °C bonus may average to something like 0.5 °C over the full 24-hour day. At a 24-hour average fan power of 37 W, this comes to 75 W/°C. Again, this would be a substantial bargain, compared to our 440 W/°C calculated for the AC system.

How much ΔT might we expect a fan to deliver? In terms of volume flow, it looks easy. At 2000 cfm (~1 m³/s), a 2000 ft² house (185 m²) with eight foot ceilings could have a complete air exchange in 8 minutes—if air could be coaxed into such an orderly flow. Of course it can’t, and some cool air will escape out of the other open windows. But in a matter of an hour or so, it should be possible to manage complete air exchange.

Yet experience says it’s not that easy. That’s because all the items and walls in your house are storing thermal energy, and donate this heat to any cool air that happens by, warming it back up. Every kilogram of material in your house (including interior walls and some fraction of the exterior walls) takes another roughly 1000 J to cool down by 1 °C, gobbling up another second’s worth of air using the foregoing numbers. So in practice, a single box fan will have difficulty moving enough air through a large house to substantially take down its average temperature. But what effect it does have will be a total bargain. Especially, blowing cool air into an occupied room is a good idea. And a beefier air exchange system can get some serious cooling done at a much lower energy cost per delivered degree than AC. But this mode of cooling is not available until the night cools off, so a well-insulated house is key to help preserve the night-time coolness for the rest of the coming day.

Comparison to Heating

Some time ago, I evaluated how much energy it took to heat my home in a similar “special day” experiment—this time as a Christmas treat. Using the construct of degree-days, I computed a cost of 610 W/°C for heating. Compared to this number, 440 W/°C for cooling is only marginally better, while I expected the energy efficiency ratio (EER) of the heat pump to exceed the direct-energy heating value by a factor of three or so.

But I’m not comparing apples to apples. Revisiting the heating experiment data, we generate the following plot for the heating day and the (similar) day after.

Temperature record for heating experiment. Inside temperature is recorded in our hallway near the ceiling.

In this case, the day on which heating was applied (black curve) was warmer than the comparison day (gray curve) by 1.6 °C. As a result of heating, the interior temperature averaged 4.5 °C warmer than it was on the comparison day (and almost certainly an overestimate based on the sensor’s location near the ceiling). If we apply our “warmer out = warmer in” rule, we only really got 2.9 °C of warming impact over the expected passive behavior (4.5 minus 1.6).

Doing so cost 3.5 Therms from natural gas, or an astounding 102 kWh of energy. I compute 1465 W/°C, blowing the AC cost out of the water by a factor of 3.3. Hey! There’s the EER I was looking for!

So the same house is less energy-intensive to cool via heat pump technology than to heat by direct flame. But let’s not forget that my utility electricity is largely derived from fossil-fuels, driving heat engines at maybe 35% efficiency. So in terms of primary energy use, every degree of environmental conditioning costs about the same for me.

Winter/Summer Aside

An interesting feature emerges when comparing temperatures in our house in the summer vs. winter. In winter, the interior temperature tends to be about 5 °C warmer than the outside temperature, on average. In summer, this is more like 2–3 °C warmer inside. I attribute the difference to our use of an attic fan during the summer (usually PV-powered), opening the windows at night, closing curtains during the day, and a different solar loading profile (eaves keep light out of windows during mid-day in summer; more summer loading onto roof and less into windows and walls).

Back to Normal

Our experiment is over. On my way home from the bus stop in the evenings, I pass houses with their AC on. But around that time, the outside temperature is dropping, so that opening up the house makes for a pleasant temperature. Granted, I’m in San Diego, where things aren’t so tough. Yet people did used to live without AC (and still do). Houses were built for it, with many windows allowing cross-ventilation. Those practices have faded, as AC is a feature taken for granted in the design of new houses.

I was walking through Washington D.C. one afternoon in late summer, and saw some old government buildings whose windows were propped open. “Must be a little stuffy and warm inside,” I thought, compared to modern buildings. The people working within may abhor their old building with its inadequate cooling capability. But then I considered a future scenario in which energy scarcity may impose restrictions on how much is available for air conditioning. In such a case, having a building whose windows can open will suddenly seem like a huge bonus. A building built to be tolerable in the days before AC will fare better in a future deprived of that luxury, should such a time come. The directional flow of envy between modern and older buildings may reverse.

For my wife and myself, seeing our daily utility electricity consumption shoot up by a factor of 20 to keep our interior cool (though still warm by American standards) was completely off-putting. It’s not the money spent so much as it is the principle of the matter. It feels like an unnecessary luxury—like having a servant peeling grapes for us. Humans evolved to withstand hot summers and cold winters. We’re tough stock.

Now that I have personal experience with the amount of energy involved in running the AC, I’m both more frightened and more optimistic. The fright comes from the fact that more people in the world strive to live in air-conditioned comfort—and I don’t blame them. But it’s hard enough as it is to meet our energy needs. Add more demand, and the strain is greater. To the extent that air conditioning is a necessary component of advanced society, we may have a hard time paving our way to that future, globally.

The optimism comes from seeing how much low-hanging fruit there is to lop off, if people are willing to change their thermostats to moderate settings (like 25 °C, or 77 °F, or even higher). Then another part of me says I have too much faith in our fellow citizens to adopt austerity for the common good. Not sure when I’ve seen that happen, en masse.  I can’t do this alone, after all.

54 thoughts on “Rocking the AC

  1. I live in Brooklyn and only air condition the bedroom at night. I would probably leave the windows to the back open instead, but the back yard is the hottest and most humid place imaginable. I wonder how much of that is due to it being the exhaust area for other apartments’ air conditioners.

  2. Thanks again Tom.
    Love your blogs!
    We made it without AC till guests arrived last weekend. Our thermostat was set to a more reasonable 80. Still cooler than outside (In Temecula).
    We installed two exhaust fans two weeks back, one in the garage and the other in the attic. I have my home automation system plotting temperatures…..
    Our air con pulls 6kW, so the 600w for the two fans are night and day power wise and so far they are doing a great job just on their dumb thermostats. I will putting them on the automation system asap and will save even more power once we get some smarter rules programmed up (It takes an extra hour of running to pull down the last 1°C, just no point in running that much longer).
    I still cant stop giggling when we get our power bills. We signed up for the maximum savings allowing PG&E to remotely turn off our air-con…. Its never on! Summer its around 40 bucks. More than yours I know, but still pretty good on average, and we have a little more work to do (I have some salvaged solar panels to install, much like your system).

    Anyway, just wanted to say thanks for blogging. Love each and every one.



  3. “Then another part of me says I have too much faith in our fellow citizens to adopt austerity for the common good.”

    If it was clear that there was some significant common good, I think many/most people would do it. But what is the “common good” for which you are proposing “our fellow ciitzens to adopt austerity”?

    • The challenges of our time will likely revolve around resource limitations—energy in particular, climate change (also energy related), feeding the swollen population (consequence of surplus energy), etc. We’ll likely have to hit crisis mode before the challenge is convincing enough to spur behavioral changes.

      • Great post as always. The ability of “our fellow citizens to adopt austerity for the common good” could be your next question.

        You just showed (again) that behavior is just as important as technology. The problems we face are not so much about surplus population, but about surplus consumption. Most people still live below their means and I am not only talking about developing countries.

        If you can (loosely) connect energy usage with “money” social inequality is on the table. You made your calculations for the “national battery” for the energy usage of the total population. What would be the conclusions for the bottom 20% (looking at income), the bottom 80%, the top 1%.

  4. I live in Utah in an old house without air conditioning. Many older houses here have evaporative (“swamp”) coolers, but I don’t even have that. Generally I can cool the house quite well at night using fans and open windows. Shut the windows and lower the shades during the day, and the house stays comfortable until mid-afternoon, even as the temperature outside approaches 100F. By sunset, however, the inside temperature is often around 90F. That’s when the basement (where it’s never even 80F) comes in handy.

    Of course, all the new houses here have central air conditioning.

  5. It’s not uncommon for us to hear the ACs running in all the homes surrounding ours at 10PM at night (we live in Sacramento). Even during heatwaves with 100F+ during the day, the outside temp falls into the mid 70s by then. Increased heat pump efficiency for sure, but nothing compared to our whole-house fan! (250W vs 5KW). People can’t believe our power bill is never over $100.

  6. Tom,
    Keep up the great posts – each one is an absolute delight.

    You have a good point, that modern buildings lack even the ability to enjoy outdoor air. I have often lamented my 2-story engineering office in Costa Mesa lacks operable windows on beautiful days (which are common). Our positioning is perfect to take advantage of a natural breeze.

    California Energy Code and the newer “CalGreen” code do allow for natural ventilation, but they do not provide strong incentives for it. Owners simply see air conditioning as a given. There is an interesting finding from the EIA’s “Commercial Building Energy Consumption Survey”, which was on hiatus but is about to begin again. The finding: there is a sore lack of Tom Murphys in the world – i.e. the end-users of buildings commonly lack awareness on the tools available to them, for saving energy. The actual usage has been varying by about 30% from the expected energy based on our design (we use programs like EnergyPro and ComCheck to estimate total annual energy usage). The new round of surveys will delve deeper into this phenomenon.

  7. From the christmas graph, it seems like your winter is about equal to Swedish late/early summer which is a comfortable living temperature and no one is heating their homes.

    Only time people are heating there house in Sweden is in the midst of cold winter when it really is a necessity in order to survive and prevent the piping for cracking.

    • Another Swede boasting:
      I lived four years in Tamil Nadu, India, close to the city of Madurai, which average minimum temperature during summer is 25-26 degrees Celsius. That means that many nights were above 30 C late into the night and morning. It was difficult to sleep in that heat but I managed without AC but a ceiling fan. After a couple of years I also got an “air cooler” as well that evaporated water by fanning air through water soaked wood shavings, which I ran while reading a book in bed before sleeping. Back home in northern Sweden I have an average _maximum_ summer temperature of 11 C.

      We are adaptable!

      • We certainly are adaptable, but we also have our limits.

        Add ten degrees (Celsius) to each of your figures from India, and you’ve just described our summers here in the Valley of the Sun. Indeed, we’re only just now cooling off to what you experienced.

        Yesterday brought lots of rain. The overnight temperatures were lovely, all the way down to the lower 80s F / mid 20s C, but the humidity from the rain was too oppressive to open the windows.

        I dare say an environment such as ours would change your opinion as to the necessity of air conditioning….



  8. Boston this summer was hitting 95 degrees and 95% humidity, and high temperatures even at night. And I live in a thermally poor apartment. Borrowing an A/C unit made life livable. But it did do a number on my power bill, which went from a year around $20/month to $40 or more for just a few weeks of usage. And I’ve got one 5000 BTU unit for my bedroom. Though the window sealing isn’t good, I basically taped up a couple layers of Saran wrap.

    What I’d like would be a unit whose fan mode actually worked as a fan; mine just recirculates. As it is, when it does get cool, I have to contemplate switching with the fan (hard), putting a fan in my other window (loss of light and privacy), or just running the A/C anyway.

  9. We could save a lot of climate control energy if more people lived in places like San Diego, or the Pacific Coast in general. But that would mean much higher densities, and allowing building of new housing in large quantities, and accepting loss of local environment. You want to save the environment, vote for high-density zoning (or against zoning) and for public transit.

  10. Sorry for three posts in a row; last one I think. But your NYT Sunday Review article has interesting stuff on the need for A/C, as well:

    “every degree rise in temperature above 25 Celsius (77 degrees Fahrenheit) resulted in a 2 percent drop in productivity”

    “Worse still, perhaps, Mr. Tanabe calculated that the suffering was all for naught: When offices were kept above about 82 degrees, so many people were using inefficient fans at their desks that the total electricity consumption could be higher than if the building had been better cooled. “That’s just stupid,” he said. ”

    “Though people in Singapore tend to identify a range from about 68 to 75 degrees as “neutral” temperature — neither hot nor cold — studies found that work improved if the thermostat was lowered to about 72″

    People lived in hot humid climates, but there’s also a lot of stereotypes of lazy and indolent tropics dwellers — or US Southerners — which might have a basis in fact (but be about rational or unavoidable adaptation, not shiftlessness).

    Of course Spaniards coped by sleeping in the afternoon and having a busy night. But they have an arid climate that gets cooler at night, not a muggy one.

    • I did catch that, too, which is what prompted the sentence: “To the extent that air conditioning is a necessary component of advanced society, we may have a hard time paving our way to that future.” So this may make the connection to energy very explicit. If it’s not optional, then the threat of energy scarcity is that much more threatening to our current expectations about the path of civilization.

      • Well, your expectations. :) I’ve always expected 10 billion people at US levels of 10 kW/person as an endpoint. You think “San Diego levels of conservation”, I think massive solar/nuclear rollout. Maybe because I knew people weren’t going to give up A/C and heating.

  11. We had a crazy hot summer this year in Hungary, with heat waves peaking at 35-39C (95-102F) for several days. We manages to survive in our house without AC, just with proper ventilation. The temperature is usually at 24C (75F) in the house, for the end of the longest heat wave it went up to 26C (79F). Getting up at 5 and keeping windows open for 3 hours, cooking outside (which was a fun itself) were all the special measures. Of course, the triple glazed windows with low-e coating, the shadow of trees and bushes and especially the 1m roof overhang also helped a lot. The half meter thick solid brick walls also payed back a part of the energy spent 120 years ago to manufacture them.

    So, there is life without AC, but in many stupid planned buildings, people suffered inside temperatures over 40C (104F).

  12. We too seldom run our heatpump. We run it only when the weather is both warm (above 33C) and humid (+60%). The primary reason is to wring water from the air.

    In dry warmth up to about 38C, we keep fairly comfortable in light clothing with mild air movement and plenty of sun tea. Having been raised in SoCal, temperatures up to ~45C are hot but tolerable if one remains hydrated, protected from direct sun, and in a slight breeze.

    I contrast this with the ten June days I spent in Hawai’i where the temperature reliably remained between 23C and 27C, yet felt uncomfortably warm (and close). In Hilo, every two to four hours brought a condensation event where the air gelled into rain. It’s that damp.

    Box fans work great, but how much more heat are they moving out when the air is sopping? If they cannot dry the air out, moving the heat around may not make one any more comfortable.

    • Weird. I have trouble imagining 45 (113 F) as comfortable under any circumstances, while my memories of Hawaii are “wow, it’s 80 all the time, but it feels great!” Coastal breeze and all. And yeah, it rained every afternoon… while the sun shone. Rainbows every day.

      Fans: best use is when it’s cooler outside, so blowing cool air into a heated home. Works best in arid regions where it gets cooler at night; of course. Failing that, blowing dry same-temp air can at least help move heat away from your body. In humidity, as you note… probably still useful, but less so.

      One question I faced recently was “hmm, it’s cooler outside, but 90+% humidity, vs. warmer but drier inside. Should I open the window and run my fan or stick to A/C?” I opted for A/C.

  13. You talked about fans cooling the house by blowing in cooler air from outside. Fans also cool by causing your sweat to evaporate, so when the air is cooler than your skin, can’t small fan blowing on you can cool you more effectively than exchanging air in the entire house?

    • Absolutely. This can be an important effect, barring nearly saturated humidity. I actually never realized that perspiration was a cooling mechanism until I moved out of the South. I thought it was merely annoying. But even there, a fan blowing ambient air on you does feel better than air of the stagnant variety.

  14. I was rather surprised a while ago when my power company added another page on my bill congratulating me on the fact that my power consumption was 95% lower than households in the region. I was a little mystified at the time – sure, I’m thrifty with my power consumption, but that seemed a bit extreme. But now it all makes sense – I pretty much never used the AC, even living in tropical Australia.

    • I will add my congratulations. I got an advertisement in the mail for a solar installation, estimating based on my square-footage that my electricity bill was $156 per month (really $9). With their installation, they could knock that payment “down” to $40. Really? If I take your deal, my electricity bill will go up? They have no idea what they’re dealing with here!

      • I wouldn’t be too quick to mock them. Their service, if it’s as advertised, will do far more to further your stated aims than anything you’ve proposed.

        They’re talking about something that generates $156 – $40 = $116 of electricity a month, or $1,400 / year. At $0.10 / kWh, that’s 14 MWh / year. I’ll let you figure out what size array you need for that in San Diego, but that’d be roughly in the 10 KW range (give or take) here in Arizona.

        San Diego has a population of about 1.3 million. If only 5% of those people take that company up on that offer, that’s an entire gigawatt-hour of annual solar generation, and probably about a megawatt of total capacity. Put that in your coal-fired plant and smoke it!

        It’s rooftop solar that’s going to be our salvation, and it’s shamelessly commercial enterprises like the one seeking your business that’s going to be the means to get us there.



        • You’re right—I am indeed a fan of what they are trying to do, having solar on my own roof. I just found it amusing that their projection was so far off for my behaviors.

  15. Permit me to add a somewhat more radical datapoint.

    Here in the Valley of the Sun, airconditioning isn’t optional. Period.

    During this time of year, our average temperature is generally above the high temperatures elsewhere. Right now, it’s in the mid-90s, but a few weeks ago it was over a hundred. You’d have to go back to May for it to be in the 80s and 70s.

    Now, as I wrote, that’s the average temperature. Not the average daily high, but the 24-hour average. That hot streak a few weeks ago? Daily highs were in the upper teens and overnight lows were in the mid 90s. And, thanks to the heat island, it’s not unusual for it to still be 105 or more at ten o’clock at night. Some hotspots might not ever get below 100.

    So, not only is airconditioning not optional, you don’t even have the choice of turning it off overnight and opening the windows.

    I generally keep the thermostat in the lower 80s. Then again, I don’t wear a whole lot of clothing when I’m in the house; if I had to wear a business suit, I’d absolutely have it set ten degrees cooler. The office is the warmest room of the house (it’s a converted “Arizona room” with upgraded but less-than-ideal insulation and an undersized A/C duct), so I might lower the thermostat a couple degrees when I’m working.

    But, even with the A/C on 24/7, I’m still running a net surplus of electricity from the PV array, averaging several kWh / day. (For reference, this is with a 6 kW PV array with a separate closed-loop solar water heater, and I’m living by myself in a typical single-story two-bedroom suburban home with a couple rooms added on.)

    One factor that can’t be overemphasized is proper insulation. Never mind the efficiency aspect; it’s simply more comfortable to be in a well-insulated space. I have the thermostat set a couple degrees higher this summer than last, and I’m more comfortable. Before, you could feel the heat radiating off the ceiling; it was like having a whole-ceiling heat lamp on you. The fact that the air was cooler didn’t matter. Now, the ceiling doesn’t even feel warm to the touch. Not only is the ceiling no longer heating the air, it’s no longer radiating that heat onto me.

    Now, I know that not many people in the Western world live in places with heat as extreme as I experience here…but a lot of people, hundreds of millions, especially in the northeastern US, northern Europe, and elsewhere, experience cold that’s comparable to our heat.

    So, Tom, I appreciate that somebody in San Diego can be comfortable with little or no climate control. But we’ll go months at a time when our overnight lows are well above your “sweltering summer heat,” and Maine, Moscow, and Seoul can go months at a time when their daytime highs are lower than your “frigid winter cold snap.” You simply can’t extrapolate your own experiences to anybody living outside the Mediterranean climate zones.

    And, yes, airconditioning is a very modern invention, but heating isn’t. People didn’t settle northern Europe until after the invention of fire, and it couldn’t have happened without it. Either we’re going to have to continue to figure out a way to make our homes livable, or we’re going to have to move billions of people back into the temperate climates, or they’re going to die.

    The question isn’t one of how to do without climate control. It isn’t even one of how to do with less climate control, or how to make climate control more efficient; you’ve already done the math on those and figured out that we don’t have very many factors of anything to gain from either approach. Yes, all that will help and will inevitably be part of the picture — but, as you’ve also pointed out, so will all sorts of niche energy sources. Besides, projected population growth alone would likely outstrip all your suggested austerity and efficiency gains.

    The question is how to actually power civilization as we know it.

    You know the answer, and I’m living it: the Sun. The real question is how we’re going to come up with the initial capital investment we need to build that future — and that’s a sociopolitical problem, not a technological one.



    • There is a cooling technology which has been used in the Middle East for centuries (a place that gets really hot). Obviously, it uses ZERO electricity. They are called Air Ground Tubes. A trench about 100 feet long by 6 to 8 feet deep is dug, (it can zig-zag) and a ceramic (or plastic) pipe is buried with an outside air inlet and an outlet in the house. (The tubes are usually about 6″ in diameter.) Open a high window, or a skylight, (or something a little fancier called a sun chimney), and convection alone pulls 55F air through the house no matter how hot the outside temperature is. Unfortunately, this technology does not sell complicated mechanical devices and electrical service with dependent monthly bills, so it is little heard of in the U.S. European manufacturers of these tubes coat them with some kind of silvered material to mostly eliminate algae growth in the condensation that inevitably forms in the tubes. (Besides, show me an AC unit that doesn’t occasionally smell for the same reason.) One time investment in a ditch and a pipe – lifetime free cooling. (I don’t know how applicable this would be for really large buildings.)

      • That’s a great idea, but you’d have to go awfully deep to get to that 55° layer here. Remember? Heat island? Topsoil temperatures of 100°? And I I’m not sure our very dry soils here are thermally conductive enough for it to be ideally effective…you might get an initial cool shot, and then create a hot underground bubble that dissipates slower than you heat it.

        If the stars all line up, it might make sense for new construction, but retrofitting something like that into an existing home at best would be substantially more expensive than enough solar panels to run the A/C. After all, we’ve got no shortage of sun to power the A/C, and the panels work best when we need them the most.

        Incidentally, that last fact is a big part of the reason why the local power utility gave me thousands of dollars to put the PV array on my roof: I generate lots of the most expensive type of green peak power. Every watt my panels pump out is a watt that they don’t have to generate. And when I produce a surplus, that’s worth up to $0.75 / kWh and more to them (Palo Verde wholesale peak production was that on August 9th this year, though they’re averaging about $0.40 / kWh), but they only pay me a few pennies per kWh. In other words, they’re probably making 5% – 10% annually on their initial investment in my array — not a bad deal for the utility, eh?

        Sorry…totally off-topic…got carried away, there….



      • [We settlers] also brought huge masses of concrete and asphalt with us, resulting in that heat island effect I mentioned. It not only gets hotter during the day, it doesn’t cool off at night. Even the ultimate adobe building on my property lot would spend a couple months of the year with an interior temperature that never dipped below 100°, for the simple reason that the 24-hour average temperature is right around that point.

        Air conditioning isn’t just for comfort here. Without it, the infirm and elderly would drop like flies. They do, you know, when there’s a significant loss of power and they can’t get to a cooled space. That’s a big part of why life expectancy is double, if not triple, what it was here a few centuries ago.

        It really is that simple. Air conditioning in the Valley of the Sun adds at least fifteen years of life, if not more. My parents are in their mid 70s, but they’re probably the healthiest couple their age their GP sees. Even still, I wouldn’t place good odds on them surviving a summer here without air conditioning.

        The same applies to winter heating in cold climates, of course. How many people in their 80s, 70s, or even 60s could survive a New England winter without a source of heat? How many actually die when their home heating oil runs out during a nor’easter?



        • Unfortunately, this is very true. Look to the headlines after the next nor’easter, and you will find reports of homeless that refused to seek shelter, and froze where they were.

  16. The Federal Government would let employees go home in Washington D.C. when the temperature was over 90oF before they installed AC in buildings. The number of days over 90oF for that area is projected to go to around 75 days per year in the next 50-90 years. A lot of the South and Mid-west will see 150 days or more per year. Positive feed-back loop?

  17. Tom great post as are they all.

    I thought I would share what I have done to combat heat and humidity at our little slice of heaven.

    First we installed a complete ground based geo thermal system so it both heats and cools. We then added insulation to reach R55 in attic. Then we replaced all windows and finally put R7.5 over the existing brick (I hated that brick ever since we bought the place) and then put board and batten over it all. Note I keep the summer temp set at 24C.

    with all the added insulation the house does not reach 24C untill about 9PM and most times it will stay like that without triggering the AC. On exceptional days that breach 30C the AC does come on to hold that temp but does not cycle much at all maybe once or twice before holding.

    Then When we go to bed I will turn it down to 22C or 21.5C if it is really humid and we do not want to let in a lot of humid air. If it is cool enough and no humidity then the windows get opened instead. The first person up turns up the setpoint to 24C once again. it may sound extravagant but historical utility bills show we get this bit of luxury and still pay much less than we used to.

    What I would like to emphasize is how beneficial insulation really is. As a closing statement our bedroom has an eastern exposure. Prior to the upgrades every summer (month of Aug) I would wake up by 730AM on weekends even when trying to sleep in just because the room would heat up from the sun exposure. After the upgrades without setting an alarm It was scarry just how late I could manage to sleep.

    Keep up the great work


  18. Your comment about older buildings being designed without A/C in mind is so true. I once spent a summer in Baltimore, a city known for its hot, muggy summers. The large apartment building I was in did not have AC. However, being about 100 years old, it was designed so that air could circulate through each apartment and through the building as a whole. (One feature that I haven’t seen elsewhere is that all the apartments had a shuttered door, in addition to a main door. So if you were home, you could close and lock only the outer door, and get a nice breeze.) The temperature inside the apartment rarely became unbearable.

    At the same time, nowadays office buildings have strains on them that apartments do not, which you didn’t explicitly consider. Typically they have a higher density of both people and computers, which affects the need for AC. When I was in grad school in the midwest — more hot and muggy summers! — the AC in my building was turned off on the weekends, despite the number of grad students working on the weekends. It also had a habit of breaking down entirely during the hottest week of the summer. Between the lack of AC and the number of computers (this was before flat-screen panels were common, so we’re talking giant CRT monitors everywhere), it was utterly unbearable. I couldn’t type because my fingers were sweating too much. I am not someone who cranks up the AC — in fact, I have now lived in California for five years without AC — but it was truly an unworkable setting.

  19. Another data point –

    First, as a Texas resident, I’m jealous of your “unusually hot and muggy [day] (high 30 °C, or 86 °F, 62% humidity)”

    - Here, from mid June through mid September, we’re dealing with average highs close to 100 F and similar humidity (and considerably hotter last year especially).

    - We cool our house to 78 F during the day, and 72 at night (we like to sleep cool).

    - Our house is partially (30%) shaded by trees, and has a “mid toned” (tan) asphalt shingle roof. We don’t have specially reflective (“cool roof”) shingles or radiant barrier.

    - We have 6 inches of blown-in cellulose (paper) insulation over 8″ of fiberglass blown-in insulation. Sorry, I don’t have an exact R-factor. I would estimate > 30.

    - We have a single-story 2000 sq ft house with a generally rectangular footprint.

    - 14 SEER central AC and ceiling fans in every room

    Our cooling “activities” consume an average of 27kWh/day.

    So I suspect, given a home built with cooling efficiency in mind (which is arguably harder to justify in San Diego), you could have halved you’re AC power consumption.

  20. When I’m giving talks on topics like this, I like to tell a story about a friend of mine in Melbourne, Australia.

    In winter, when you visit the house you have to strip down to T-shirt and shorts because she keeps the house at 30C because she likes the warmth and it reminds her of when she lived in Ireland years before. If anybody has the temerity to turn down the thermostat to make it a bit more bearable she goes nuts and tells people to leave it alone!

    However, in summer, as the temperature appoaches 30C, she has the AC going flat out, set to 18C because, as she puts it,
    “It’s far too hot out there!”

    Behaviour is funny thing.

  21. It sounds like your home doesn’t have the best insulation or ducting. My home had poor insulation and ducting when we bought it. The airconditioning would run constantly like yours did. We updated the insulation and ducting and now our ac cycles and doesn’t run constantly, even if it is over 100f out. Turns out that we were blowing child air under our house through broken ducts and had virtually no insulation.

    I do have to say that a whole house fan is a real saver. We run it in the morning for 15 minutes and or house stays cool (<80f) till about 4pm on hot days (around 100f). The key is cooling down the home and attic since the attic can get very hot during the day.

    • You’re certainly right about whole house fans. In fact that is what we do for a living …
      However, running a fan for 15 minutes per day will not transfer much heat out of the house. You will most likely be able to change the air and drop its temperature very quickly, but the temperature of the mass of the house will change very little in that short time.

      Here is an example:
      2,000 sq. foot house with 8′ ceilings has 16,000 cubic feet of air. This air will weigh about 1200 lbs. Since air has a thermal capacity of .24 BTU/lb- deg.F., this means that cooling the air by 10 degrees F, will only remove 2,888 BTU (not much). A 3 ton air conditioner removes 36,000 BTU per hour.
      For this reason, we recommend running a whole house fan all night long, and that is why we build efficient and quiet whole house fans

      • Whole-house fans are even better with more sophisticated controls than normally offered. After many years with ours, my wife goaded me into building a smarter controller:

        Setting a target temperature and letting the software decide whether to run the fan on its high or low setting, and when to switch the speed down or turn the fan off, makes the overnight result much more predictable. During this next winter I’ll probably be reprogramming the device again to let it make smarter (or at least more complex) decisions.

  22. One option that you did not mention was fans for cooling people, in particular ceiling fans. According to the ASHRAE comfort model, you can get about 4 degrees of subjective cooling from 200-300 FPM air movement – and that’s within the Standard 55 comfort envelope (if you go outside of it, and start to sweat, evaporation increases this effect).

    Ceiling fans are a really good way to get uniform-ish low velocity air movement in a room. Traditionally ceiling fans are energy hogs relative to what they do, but the modern energy star ones are pretty good. And then there is Big Ass Fans.

    BAF historically makes industrial fans for destratification. Now they are moving into fans for comfort cooling, and into more commercial and institutional applications. They are also selling a fan to the residential market:

    The Haiku is a remarkably efficient device. IIRC, it’s about 8 times as efficient (in terms of CFM/watt, which is kind of a bad metric for ceiling fans, but whatever) as the next closest Energy Star fan. It totally blows away the non-Estar products.

    Anyway, thought I’d throw that out there. I have no direct interest in BAF, but I’ve worked with their products and I believe in the company and in their quality.

  23. I went through this summer, with strings of days over 100F, and did not use the AC. I was good friends with small fans and used them to move cooler air at night. My electric bill was about $40, with a fair amount of that energy lost to the wall warts associated with computer and TV. (I’m working on that…) As you say, no one can believe that anyone would do this volutarily. It is as if no one could have survived before AC was invented.

  24. I was looking at the cycling of your AC and how you didn’t like the temperature swings. Digital thermostats are horrible with this – it takes at least a 3degree swing to switch on and off. I just moved to an older house that still has the round Honeywell analog thermostat and it is AMAZING! I bet it keeps temp within half a degree by cycling much more frequently. Paying attention to how you really feel and adjusting an accurate manual thermostat is low-tech, but probably results in more comfort and equal energy savings than just relying on a programmable digital one.

  25. We are in the Washington DC area, and for us the primary purpose of AC is to remove humidity from the air. It so happens that this is a side-effect of chilling the air, but I can’t help but wonder that there *must* be other methods of removing humidity that are far more energy efficient. If there is, it isn’t something that is commonly used..

    A normal thermostat only measures temperature – not relative humidity. I have long thought that a better system would be for the thermostat to measure both and run the AC based upon a “comfort index” rather than just blindly trying to pull the temperature down. This together with a more efficient means of removal of humidity would make it possible to run the AC a whole lot less.

    In our house, my wife is always complaining that the house is “like a sauna”, but I am always freezing and wanting to turn it down or turn it off. And usually it pays to keep the wife happy, so we run the AC. I know it sounds crazy, but in July and August I end up needing a blanket to stay warm.

    • Dehumidifiers are a thing you can buy. Cost about as much as A/C, maybe less.

    • This week we’ve had temperatures around 100F and dew points in the mid to upper sixties here in southwest Arkansas. I keep my t-stat on 78F and use a small box fan. Around window and door areas the temps run closer to 80F. When I come in the house from the 100F temp outside, that 80F sure feels cool. We know that the greatest load on the grid is on the hottest days. I think it’s going to take a couple of overloads and blackouts to convince many of those that keep the t-stat on 72F that it’s in their best interest to kick it up to 78F or 80F on those hottest days. Less electricity is a lot better than no electricity. We seem to be on a collision course with something here in the South and it doesn’t look good. At least I have shade.
      I’m trying to cover my roof with vines on wires. My neighbors think I’m nuts.

      The condition Eric, in D.C., describes is the classic symptom of an oversized condensing unit. The unit can lower the temperature before it has run long enough to adequately dehumidify the air. Just GooGoo “oversized condenser” and you can probably read for a week.
      Thanks again Tom. That Mumbai statistic is OMG.

      • I doubt the unit is oversized – it is whatever the builder put in, and they always put in the minimum they can get away with. Our thermostat is usually set to 78 or above. The place I tend to sit is right near a vent, so when the programmable thermostat drops the set point, the thing will run for quite a while to cool the whole house down, and that’s when the blanket comes out.

  26. I live in central TX. Today it is 103. This is a regular thing for the months of June, July, August…and even September. I have not had air conditioning for the past five years by choice. My neighbors summer electric bills average $300-400 per month. They use air conditioning. My electric bill during this same time is between $35-45. I am not in misery. Living without air conditioning is definitely doable and is likely much better for you (in many ways).

    • I live in Central TX as well. My house is well insulated (spray foam), has a metal roof and 6kW of solar panels. My summer bill peaks at about $50 (net solar). About $30 of that is to charge my girlfriend’s car. We have our house at 76 degrees (which is freezing, I would prefer 79) and people are home at our house all day long, so we don’t get a period of the day where the air conditioning isn’t used. I think if we never used our A/C, our house would never get over 85 degrees inside (even when it approaches 110 outside).

  27. So glad I don’t live in the temperatures you guys endure!
    I’m in London, UK, where I find anything above 20C (68F) uncomfortable, and love best spring/autumn days about 10-14C (50-57F). I’m moving to the far north of Scotland to find coolth, investing in a ground source heat pump for hot water and space heating – apparently it can also be used for cooling, if the environment warms up that far…

  28. Already getting frosts here in the interior of Alaska. This morning it’s 26F / -3C, expecting a high of 46F today Two weeks ago the temps were 50 – 70F.

  29. Very nice write-up. I like the temperature testing all over the house. Any ideas square footage of your house and the size/age of the AC? Judging from your numbers you are in the 2.5-3.5 ton range. You can get your COP/EER from all the data you have (quite jealous), and see if an upgrade would better suit you (or the next owner). I think you should emphasize that this is typical nice west coast weather and on the east coast or other climates, one isn’t so lucky to get away with no AC in summer. But the main points are valid anywhere, if i may summarize:
    1. Wear clothes inside appropriate to the weather (go naked, save money/the environment! note: for couples this may result in more kids)
    2. Turn the t-stat up in summer/down in winter. You’ll get used to it.
    3. Ceiling fans help in summer for comfort because it helps your body remove heat (kudos Brent Eubanks), and you can better implement #2. It doesn’t necessarily help with bringing in outside air. Houses mainly rely on wind pressure for this, and not all designed well for it. In some apartments/houses large box fans can get good airflow through the house.
    4. AC is expensive, use it sparingly. Make sure you have a programmable thermostat.

  30. I just burst out laughing when I read “When it comes up in casual conversation that I do not generally heat or cool my house, people either move to another seat or look at me with some mixture of admiration and disbelief.” EXACTLY the same thing happens to me…..! And 30°C, or 86°F, 62% humidity weather is mild compared to the 36°C / 90% humidity we frequently get in subtropical Australian summers.

    The secret lies in minimising glazing surfaces, and lots of internal concrete to absorb both heat and humidity when the weather decides to go troppo. It also pays to cover all windows with pelmeted curtains, and close those curtains when extreme weather has been predicted.

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