My girlfriend and I keep having this same debate. Every morning
we wake up and get ready to leave for work. I always want to open up the
windows and turn off the AC, knowing that when I get home it may be a little
warm and most likely I’ll close the windows and turn the AC back on. My
girlfriend insists that it will save us on our electric bill to leave the AC
on all day rather than making it work harder in the afternoon to cool the
place back down. So, which way will use the least electricity?
You are right, she is wrong.
If you can see your electric meter, try it your way today & her way tomorrow, checking the meter each day like a trip odometer (once in the morning & once just before you go to bed). Assuming both days have roughly the same weather conditions, this should result in you coming out the winner.
It’s usually (perhaps always?) more economical to turn off the AC when you’re not at home. But there’s a drawback: it will take some time for your house to “recover,” during which you might be uncomfortable. This, of course, is dependent on many factors. If you find that the recovery time is unbearable, might I suggest a compromise? Simply turn the temperature controller to a higher setting when you’re not at home, then adjust it to the desired temperature once you return. The duty cycle will be shorter (thus costing less money) when you’re not home, while the recovery time will be shorter.
What Crafter_Man said.
Also, you can easily buy & install a thermostat with weekday/weekend day/night settings. Set it so that during weekdays when you’re not home the temp is relatively high (80’s) and then turns itself down to comfort level before you get home, so its nice and cool when you walk in the door.
Also, if you do leave the AC on during the day, consider closing up the house really well – close every shutter & curtain so the house is a cave. Keeping out the UV works wonders; when I do this at home, the AC doesn’t even kick on until 5pm when the attic finally heats up enough to affect the rest of the house. I’ve seen it 100F+ outside and 71F inside when everything is shut tight.
Of course, a lot depends on your local climate and your house, so YMMV.
A big factor for the “recovery time” factor, I think, is the humidity level. If you live in Miami, the A/C is going to spend about a half hour just bringing the humidity down before it can really start to cool down. If you live in Albequerque, the A/C may be able to start cooling right away.
Leaving it on during the day at a higher temperature will keep the humidity level under control.
(Note: I don’t think the A/C is actively sensing the humidity. Rather it is because high humidity air has a higher thermal mass, and takes more energy to change the temperature. You wouldn’t really want high humidity air to cool fast anyway, since you would get “dew” all over everything.)
Leaving it on at the normal “at home” setpoint all day is a waste of energy. A setback thermostat would be a happy medium for you since it covers elements of both your arguments. What you do is set the thermostat to change the cooling setpoint from say 75 to 85 from 8 am to 4:30 pm, and then drop it back to 75 at 4:30 pm so the house is comfortable when you get home. The work an A/C does is based on how much energy it has to remove from the house. Anything inside consuming energy (such as lights, refrigerators, TVs, etc.) dumps heat into the space. Heat outiside also comes in at a rate proportional to the difference in temperatures (outside minus inside). If it is 95 outside and 75 inside then you gain twice as much heat from outside as you would if it was 85 inside. A setback thermostat saves energy by reducing your external heat gain.
If you live in a place with high humidity then this strategy would save a bit more energy because of the huge amounts of energy the A/C’s consume removing moisture from the air. This has nothing to do with sensors, and is strickly based on cooling the air. As air cools water vapor converts to liquid water just as liquid water evaporates when hit with warm dry air. Evaporating water takes energy, just as condensing water releases energy. Water vapor hitting a cold A/C coil condenses to liquid water and makes the coil work harder to cool the air. Once the air has lots of water removed the A/C doesn’t have to work nearly as hard. It is nice to freshen the air in the house now and then, but as an everyday strategy to reduce energy use it doesn’t work well.
Although I actually work for an air conditioning company, I am not an expert. HOWEVER, in the instructions for our heat pumps, it says that you will save energy by keeping the thermistat at a consistent level. I’m not sure that this is true for all cooling systems, but for heat pumps, that is our recommendation.
When it’s very hot it’s better to leave the windows closed. It will stay a bit cooler so that when you turn the AC on it won’t have to work as hard to bring the temperature down.