What is the most energy efficient way to use our programmable thermostat?

Question

We seldom use our air conditioner until the outside temp hits 100 or more. When we do use it, we have our thermostat set at 85 when we're home. Is it better to have the AC off or set at, say, 95 while we're gone, and then have to cool the house down when we return? Or should we just leave the thermostat at 85 and maintain a constant temperature?

Answer 1

Depends on how long you are going to be gone. If you are going to be gone for a couple of hours, like a short shopping trip or the movie, then leave it on. It will take more energy to cool the house than to maintain it once it is cooled.

If you are going to be gone for a day or so, then turn it off and save the energy. It will take less to cool it than leave it on for extended periods of time.

Also, a ceiling fan on the slow setting used to move the air will save even more energy. By circulating the cool air, the thermostat won't turn the A/C on as often.  The A/C uses many times more energy than a ceiling fan on the slow setting.

Answer 2

This may get long, so bear with me for a moment.  

When you set up your thermostat from 85F to 100F, the first thing that happens is that the house slowly heats up to the new setpoint.   The AC doesn't run as the temperature floats up, but your really aren't saving any energy yet, because every single degree that you "lose" will need to be "gained" again when you get the temperature back down again.  

But once your house gets up to the new setpoint, this is when you start saving energy - because now the air conditioner will genuinely have to work less to keep the house at 100F than it did at 85F.    

When the thermostat is turned back down, your AC will run to bring the temperature back down.  This is where you "spend" the energy that you "saved" when the temperature floated back up.

So the question is really how long it takes your house to reach the new setpoint, and how long are you planning on setting the temperature up?  

As long as the temperature is turned up long enough for the house to reach AND MAINTAIN that higher temperature, then you're saving energy (and money).     But if the house never actually reaches that higher setpoint, then all you're doing is "saving" energy when the temperature floats up and "spending" it again to cool the house back down.

I hope this helps.  Sorry its so long.