Green / Eco-Friendly

Is it more economical to run the A/C only during off-peak hours?

  • Last Updated:
  • Jun 5th, 2018 1:39 pm
Mar 21, 2012
77 posts

Is it more economical to run the A/C only during off-peak hours?

So Ontario got its first heat wave couple of days ago and I managed to ride it out without turning on the A/C, the night temps. were low enough to cool down the house. I've been wondering though in preparation for the next heat wave, since the majority of Ontarians use Time-of-Use electricity is it cheaper to take advantage of off-peak hours?

I was thinking of setting the thermostat like this:
Off-peak 7PM-7AM: 22-24°C
Mid-peak 7AM-11AM: 28°C, A/C should be off anyways due to cooler mornings
On-peak 11AM-5PM: 35°C or off completely but windows closed, not home
Then wait after 7PM to bring the house back down to 22-24°C, letting the air conditioning running full blast to remove excess heat for a couple of hours. What matters to me is that the house is cool before I sleep.

There will be energy savings because of the cheaper rate and the fact it is easier to remove heat when it's cooler outside, but I'm wondering by how much, compared to before which was set to constant 24°C when home and 28°C when not home.

Also wondering if there is savings running the A/C full blast one hour before mid-peak starts, to something like 15°C, then have it off completely until 7PM.

Does anyone do this? Appreciate your thoughts.
2 replies
Deal Addict
May 23, 2009
1888 posts
Depends on who you ask. I think there was a thread discussing it last year.

Basically, thermodynamics heat transfer explains that as the wrong approach. The rate of heat transfer from the exterior to the interior increases when your house is cooler so you are better off cooling your house only when it is occupied. Just a rough non-factual example but it can be explained as during a heatwave your 25°C house is reheating at 0.75°C per hour; but your house at 22°C will be reheating quicker at 1°C per hour. So when you pre-cool before a hot day you end up cooling for more hours each 24 hrs day. But cooling for more hours is not really a bad thing with TOU pricing as I'll try to explain below.

With TOU pricing factored in, it might be economical cooling your house during mid-peak or off-peak. 1 on-peak hour pays for 2 off-peak hours(~1.5hrs mid-peak). So running my a/c for 2 hrs at night is equivalent to running it for 1 hr during the mid-day. With that said my house is occupied during the day and I follow the approach of not cooling my house during on-peak(11am-5pm). Having the blinds shut to control the temperature during the day is not an option for me. So it is a cost effective approach for me(cheaper bill with no on-peak usage) to overcool my house at night and have the A/C off but still have a comfortable house during the day. This is how I have cooled my house for the last 2 summers..both weekdays and Weekends. A/C is not working hard to cool in mid-day heatwave and I'm not paying the higher on-peak rates.

Here is what I do. Supercool to 21.5C nightly, A/C then stays off till around 4:30-5PM the following day. Indoor temperature usually rises to around 23.5C/24C by 4:30-5PM. A/C comes on around 5PM and repeats the process before bed time for the kids at 8:30PM.

Wife is happy. Kids do not complain that it is too hot and stand in front of the fridge with doors open. Its a win for us.

A/C run time.

This is my usage split with days that had no A/C use. May 21st and 26th were laundry days so might have had 5-6kWh added.
Deal Fanatic
Feb 9, 2006
8960 posts
Simple answer yes. Best to have your AC work with weather than against it. So have it start cooling down when the house and temps are dropping by themselves.
Long answer... it's actually more complicated than the question typically asked about when to heat your house. The reason? Because of how modern houses and ACs are designed.

In Southern Ontario (SOO) anyway most houses are designed to be energy efficient but they lean towards keeping the heat in during winter months (That's why code calls for HRVs vs ERVs, Windows don't call for Low E, etc..).

Modern AC systems (last 10 years) for the most part use TXV valves to help reduce power and modulate cooling. More advanced ones have additional staging for the fans and compressors. In addition ACs in SOO are spec'd to be what some would say "Undersized" (it's very lengthy the discussion). How it comes together, Temperature is only one part of the comfort equation another large factor is humidity. In SOO we're basically living in a "Swamp or pond" ecosystem very humid. So AC's are designed to run longer to dehumidify the house.
The TXV makes the AC run a bit "harder (use more power)" when there's a larger temp differential between interior and exterior, the closer the temp differential the less power.