Setting the AC higher when away versus a constant temperature

I have a Nest thermostat at home. One of it’s features is that it detects whether you’re home or away based on your phone’s location, and sets your thermostat to “eco” temperatures when you’re not home. When the AC is on, this is generally a slightly higher temperature than you have it to while at home. This is supposed to help you save energy, and therefore money.

However, my utility company switched to time of use rates for electricity a few months ago. The details of those rates are here, but basically the most expensive time to use electricity is between 5 and 8 pm. I typically get home between 6 and 7, which means if I use Nest’s home/away feature it switches from eco to normal temperatures and cycles the AC on right in the middle of when the peak electric rate is in effect.

So I’m wondering if it might be cheaper to leave the AC at a constant temperature all day, so that it runs more earlier in the day when it’s cheaper, and less during the peak time. I’m thinking maybe even if it runs more it might be cheaper. Or maybe even program it to cool the house to a lower temperature than I want it before 5:00, to “pre cool” the house before the peak rates start so the AC won’t have to run between 5 and 8.

Some information that may or may not be relevant: The house is about 1150 sq ft. Built in 1973. I upgraded the attic insulation to R38 but the wall insulation is whatever was installed when the house was built. Most of the windows have been replaced with double pane, low-e glass. The only window that wasn’t is the big one in the living room, which is double pane but regular glass, but is shaded by a big tree in the front yard. The central AC unit dates from 2003.

Hard to say without empirical data. There are a lot of variables, but that’s a big difference between peak and mid-peak. I would certainly be tempted to crank it cold from 3:30 - 5:00 then roll it up a couple of degrees until 8:00. But that’s an opinion, not a fact.

I don’t have an absolute answer to this but I think this analogy is applicable.

Suppose when you get home at 6 pm you are going to bake a cake. Does it make sense to preheat the oven* at 3 pm when the electric rates are lower, keeping it a 375 deg until you get home at 6 o’clock or does it make sense to turn the oven on when you get home?

I think you will be using a lot more KW maintaining a cooler house temperature during your absence than any benefits you get from the discounted rate.

  • assuming you have an electric oven.

Not necessarily.
Say that the off-peak rate was zero. Would it then make sense to pre-cool the house?
Of course.
So, it all depends on the difference between on and off peak, and the amount of energy required to bring the house to a cooler temperature and keep it there for an hour or so,

FWIW, I used to do this back when our power company had a huge difference between on and off peak rates. It was something like 3¢/KWH off-peak and 13¢ on-peak.

Quite possibly the former. Ovens generally have very good insulation (as evidenced by them not heating the kitchen to 375 degrees, even when left on for days). It takes much, much more energy to heat the air in a closed oven than to hold it at a temperature. It wouldn’t take much of a discount to make up for the tiny amount of electricity used in the three “holding hours.”

A well insulated house is similar, and you can check this for yourself merely by keeping track of how often the A/C or heater kicks on when holding vs. changing temperatures.

You wouldn’t even have to keep it there for an hour. You just have to time it so when you get home at 5:00, the temperature is a couple of degrees cooler than the target. You then raise the thermostat so it will warm up to target (or just above) by 8:00 when you lower the thermostat again.

AGA makes a high-end oven that stays on all the time.

I’m used to seeing this question asked in terms of the energy used, which is a simple physics problem: You use more energy maintaining the temperature than you do by allowing it to approach equilibrium and then bringing it back to your comfort level.

But introducing the cost factor makes it more complicated: Now, it matters not only whether it decreases energy usage, but how much. If the difference in rates is great enough, it might even make sense to keep your AC colder when you’re away, to do most of your house cooling while the rates are cheap, and to then just let the house slowly creep warmer during the peak hours.

The main variable, I think, will be how well your house is insulated, and how much heat sources you have inside your house. Good insulation and minimal heat sources would favor always keeping the house at ideal comfort temperatures.

I don’t have a Nest but I do have an old(er)-fashioned programmable thermostat. I have it set to come on/lower temp for an hourish in the morning, starting before I get up & then about ½ hour before I get home in the evening until about ½ hour before I go to bed so that the house is comfy when I am typically there & awake.

Can you not have different set temps at different times with a Nest?

Yes, you can. It attempts to learn your schedule and figure what tempt it should set itself to at different times automatically, but you can override that and just manually set different temperatures and different times, too.

A lot of variables make it hard to give a good answer.
What is your occupied temperature, and what is your unoccupied temperature?
What is the outside temperatures?
How well is your house insulated?
How big is your house?
What is the sun load on your house?
Size of AC.
Age of AC.

OK I went back and re read. Ceiling R-38, Walls will be R-13 if I remember right, Low e Glass is around R-6. Double pane glass in early 70s R-1.

My swag
I am assuming you will be leaving around 8:00 AM. It will cost more to keep the house at a constant temp. Consuming more energy from 8:00 AM to 5:00 will not offset the savings for higher demand when you get home and turn down the AC.

The question is when to turn down the set point. For a properly sized AC unit I would say between 1 to 2 hours before you get home. But that is right at the start of the peak hours. So you might (and I say might) get a savings tp turn down the stat between 3 and 4 PM during near peak hours. Hopefully this will remove the heat gained throughout the day and the unit can cycle normally during peak time.

If you have the skills put a time recorder on the compressor to find out the various runtimes.

I think in all cases if you’re away I would just set it at 80 and leave it. AC works best maintaining a constant temperature.

I bet that’s assuming a constant load, right? That’s probably not applicable when the maximum load coincides with the “bring it back” period. This would be the case for a house with a large western exposure taking on a lot of late afternoon and evening sun when the temperature outside is still close to its maximum. If insulation is poor and/or the A/C is undersized then it may not be able to recover at all until after bedtime on particularly hot days. There’s just so many variables it’s hard to say, let alone throwing in peak electricity rates.