Saving $$ on AC

July 25, 2009 6:38 PM

From an energy-/cost-saving POV, is it better to ramp down your AC to a lower temp when you're not in the house or using the space, or turn it off completely and then crank up when you're back? What about, say, if you're away for a week or more. I know this is not like heat where pipes can freeze if you turn off. I should know this by now but I don't. Thanks for any advice.
posted by terrier319 to home & garden (11 comments total)

In my experience, turning it off completely gives you far better savings than just turning it down. (My energy expenditure nearly doubles in the months I use my one window unit, and I only use it while I'm sleeping, and then only on nights warm enough to require it.) A window unit seems to use nearly as much energy when it's on low as it does when it's on high.
posted by ocherdraco at 6:57 PM on July 25

Absolutely. Turn it off when you're not using it.

Look at it this way: the energy loss is a function of the temperature difference between inside and outside. The cooler it is inside, the more energy is being expended to keep it that way.

Yes, it's true that it chews a huge amount of power if the house is warm and you turn it on, but less than it would have chewed if you'd left it on the entire time you were away.
posted by Chocolate Pickle at 7:00 PM on July 25

If you leave it at the same setting all day, it'll cycle on x times throughout the day to cool the house by 1 degree. If you turn it off all day, it'll cycle on 1 time when you get home to cool the house by y degrees. As long as that end-of-day run is shorter than the combined amount of all the individual cycles it would have used throughout the day, you're saving money. There is absolutely a threshold where turning the AC off will save you more money than leaving it on, but whether that threshold is minutes, hours, or days depends on your local climate, the insulating capability of your home, etc. You can probably figure it out based on how often your AC cycles on on a typical day, and how long each of those cycles lasts. Keep a log and add it up, then compare to one single end-of-day cycle and see which is shorter.
posted by Nothlit at 7:04 PM on July 25 [1 favorite]

We found that on a hot day, if we waited until afternoon to turn on the AC, it was never able to catch up. Eventually, it would be cooler outside than inside, so we just gave up at that point. In the meanwhile it was uncomfortably warm. So, we keep our house around 80 during the day and then cool it off some more when we get home. However, we have a high efficiency, whole house AC unit so yours may be very different. Since our climate tends to cool in the evening, if we are going to be gone overnight or longer then I don't start the AC until the morning we will be coming home.
posted by metahawk at 7:23 PM on July 25

Frankly, I doubt there is any situation where you'd save energy by leaving it on. Put it this way: heat is always leaking into your house, and the rate it leaks is proportional to the temperature difference (if it's the same temp. inside and out, no heat would leak in). Obviously, if you keep the house cool more total heat will leak in than if you allow it to warm up. This heat has to be pumped out, and at a lower efficiency to boot (because of the greater temperature difference).

Now, comfort, of course, is another issue.
posted by alexei at 7:26 PM on July 25

I've always been told it's better to turn the AC off when you're out. You can do a few other things to need less cooling in the first place, too:

Closing your curtains will also help a lot- I have north and east facing windows that never seem to be sunny, but my apartment can be very oveny. Recently I pulled an all-nighter and realized why- the rising sun was beating through my windows very early in the morning and adding several degrees of heat to the room, even though by the time I usually wake up the sun's arc has passed those windows, so I never realized they were the culprit. I've started closing the curtains at night so as soon as the sun's up, it's blocked out- and it's made a noticeable difference.

After you use the stove, put pots with some cool water on the hot elements when you're done to absorb the heat, or else it radiates back into the room. If you heat up the sink or tub by washing dishes or yorself in hot water, splash it down with cool water after. These heat sources seem minor, but I have a small place and I really notice the difference.

Also, switch to fluorescent bulbs and there'll be less radiant heat there, plus the cooler colour temperature of those bulbs will make the place "seem" cooler than the warm yellow light of incandescent or halogen bulbs.
posted by pseudostrabismus at 7:38 PM on July 25 [1 favorite]

There's not enough information in your question to give an unqualified answer, one way or another. Here's why:

Some structures intentionally include large thermal masses, to mitigate heat cycling. If your home has such features, they tend to average heat cycling from night to day, and you are better off keeping your A/C in a normal range around the clock, to let them do their job.

If you live in a temperate climate, like the southeast U.S., your home may be equipped with a heat pump, which is optimized for cooling, even to the point of your home being zoned for heating and cooling. A heat pump in the mountains of North Carolina, pushing a thermal load of a single, well insulated house against a 10°F temperature differential to the outside on a windy summer day, is a pretty efficient device. Shutting it down, absorbing an afternoon's heat gain to the structure, and then starting it again in the evening, to dissipate that stored heat at a faster rate than it was gained, will not be as efficient as pumping it out at low duty cycle while it is coming in. If you were willing to take 5 or 6 hours to dump that heat load, and take advantage of direct radiative cooling while doing it, your efficiency could be better than keeping the A/C on throughout the day, but you'd be warmer for much of the evening than you'd like.

If you took that same structure to Miami, where there is a much greater solar load, and higher average daily temps, and less average wind, all those factors working against you could change the situation. For one thing, the same heat pump won't be as efficient on its cooling cycle in Miami, on average, because of the greater average outdoor temperature. Second, the greater solar load is going to put perhaps, 50% more heat into the structure, for a given day, and there will be more annual days when this occurs, than in North Carolina. Finally, the atmosphere provides a much better blanket for retaining heat at night in sea level Miami, than it does in mountainous North Carolina, so that nighttime radiation cooling is less effective in Miami. On the whole, it is going to cost you more to cool that same structure in Miami. But, it might behoove you to try to save some energy on the air-conditioning in the daytime in Miami, because no matter how much you ran it, you might not keep up with the cooling load entirely, any way, with a North Carolina sized heat pump. In that case, shifting your cooling load to the night is a better strategy, and you'd be using the building's thermal mass to cool it below your desired average daytime temperature at night, in order to absorb excess heat during the day, and still keep a reasonable average temperature through much of the day, even with a North Carolina sized heat pump.

If you looked at the differences in the ways houses are constructed in North Carolina and Miami, including their HVAC systems, you'd see how much local conditions factor into housing design and operating costs. In North Carolina, few people pay for airflow, and everybody buys insulation. In Miami, everybody wants airflow features, and insulation has only become a cost effective feature in new construction in the last 10 years, as electric rates have continued to rise.

All of which is to say, location, HVAC system type, energy rates, and many other factors play into a factual answer to your question.

There is, of course, a point where not using energy at all, over a period when a structure is unoccupied, pays off, compared to averaging out the heat cycling that structure undergoes. For most homes, in most U.S. locations, a period as long as a week is plenty of time to build up savings in energy costs, if you are not there, and nothing will be harmed by turning off HVAC. But for many homes, in temperate climates, it wouldn't pay to cut off AC, if you were only going to be gone for the day, and then ran the AC wide open for several hours in the evening, trying to cool down the structure from absorbed daytime heat gain.
posted by paulsc at 9:48 PM on July 25 [4 favorites]

Not an efficiency answer, but a note of warning from experience: check into how hot your home actually gets without the air conditioning on during whatever time you plan to be away. If you have fish tanks and you cut the A/C for a week you may boil your fish, and your refrigerator may spend the whole time trying to waste heat into air that will take no more.
posted by EnsignLunchmeat at 11:55 PM on July 25

Paulsc is absolutely right. It all depends on the cooling capacity of your AC unit(s), whether you need to just cool your house down or also remove a lot of humidity from the air, and your house's insulation and location.

For example, where I live we have both heat and humidity but we usually only need air conditioning for about a month. Our two window A/C units are not really big enough to cool our whole house, but by running them almost all the time they get the job done. I can't turn them off because it can take a full day for the house to cool down again and when I get home from work I want to be comfortable and be able to do things without sweating.

We also help the A/C out by keeping our curtains and blinds shut and using a window fan to suck the hot air out of a second floor room that gets a lot of afternoon sun.

If you are going to be gone for a week, or even a few days, then you should probably turn yours off.
posted by 14580 at 5:23 AM on July 26

If you live someplace humid, it may be necessary to run a dehumidifier. It sucks to come from a trip to find mildew. It's happened to me twice; both times, I left windows open a little. When you're gone, the air doesn't move much. Still, moist air can allow a fair amount of mildew.
posted by theora55 at 9:09 AM on July 26

to offer another point not exactly answering your question: i leave my AC on all day, and actually turn it colder when i leave. here in AZ, it gets to be 115 easy in the afternoon, not counting direct sunlight heat. if i leave my AC set at 78, by the time I need the AC to be on, there is no way that it will catch up and my apartment will be 85. If i set it to 74 when i leave for work, yes, we're paying a lot (a LOT) for electricity, but when i get home from work, i dont have to turn around and leave to spend 5 hours at the mall or the bookstore.

see my previous thread if you want to wallow in my misery.

last month was $235 for a 950 sqft apartment. my parents spent slightly more than that for over 3000 sqft.

lets hear it for poorly insulated 3rd floor west facing apartments
posted by phritosan at 1:52 PM on July 28