Green / Eco-Friendly

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

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  • Sep 20th, 2018 1:28 am
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Mar 21, 2012
79 posts
44 upvotes
MARKHAM

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.
20 replies
Deal Addict
May 23, 2009
1912 posts
570 upvotes
Mississauga
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.
Image

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.
Image
Deal Fanatic
Feb 9, 2006
9079 posts
2977 upvotes
Brampton
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.
Deal Addict
Dec 5, 2009
4823 posts
2648 upvotes
Ecobee just released a feature that does exactly what you are describing (I think). It’s called peak relief. When it’s turned on, it will automatically adjust the temp of your home based on peak and off peak times. The difference is subtle though and I’ve found it works well. For example if your day time target temp is set at 25, it will cool to 24.5 just before peak times kick in, and then set the target temp to 25.5 during peak.

Not sure of the savings at years end. But I just turn the feature on and don’t think about it. And I don’t even notice it’s on.
Deal Fanatic
Feb 9, 2006
9079 posts
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Brampton
fdl wrote:
Sep 10th, 2018 9:39 am
Ecobee just released a feature that does exactly what you are describing (I think). It’s called peak relief. When it’s turned on, it will automatically adjust the temp of your home based on peak and off peak times. The difference is subtle though and I’ve found it works well. For example if your day time target temp is set at 25, it will cool to 24.5 just before peak times kick in, and then set the target temp to 25.5 during peak.

Not sure of the savings at years end. But I just turn the feature on and don’t think about it. And I don’t even notice it’s on.
I Wish there was a thermostat that released a "relative temp setting" (optional of course). Eg. When it's 35C out set the AC to 27C when it's 25C out set to 23C because I Find after a long day outside my house I don't need to come back to a "preset temp" I just need it to be slightly cooler and less humid than outside to feel comfortable.
Deal Fanatic
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Nov 28, 2016
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SK
Is off peak hours an Ontario thing. Our power costs the same all day and night long

And run the AC when its cooler at night? Thats when you let nature cool your house with open windows.

You are doing this 100% backwards

+35 and not have it kick in, loosen the freaking purse strings, why even own a/c

Good luck
Deal Fanatic
Feb 9, 2006
9079 posts
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Brampton
WikkiWikki wrote:
Sep 10th, 2018 2:20 pm
Is off peak hours an Ontario thing. Our power costs the same all day and night long

And run the AC when its cooler at night? Thats when you let nature cool your house with open windows.

You are doing this 100% backwards

+35 and not have it kick in, loosen the freaking purse strings, why even own a/c

Good luck
Yes it is. Welcome to our ass backwards province. Before the crooked Government mortgaged our future and further raised the cost of hydro you were talking about doubling the cost of hydro during "On Peak Hours" 1.5x for the mid compared to "off Peak". Our Off peak hour costs are equal to many other provinces regular all day price.
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Nov 28, 2016
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tebore wrote:
Sep 10th, 2018 3:07 pm
Yes it is. Welcome to our ass backwards province. Before the crooked Government mortgaged our future and further raised the cost of hydro you were talking about doubling the cost of hydro during "On Peak Hours" 1.5x for the mid compared to "off Peak". Our Off peak hour costs are equal to many other provinces regular all day price.
So then why do so many on here say Toronto and Ontario are the best places to live in Canada?
Deal Addict
Dec 5, 2009
4823 posts
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WikkiWikki wrote:
Sep 10th, 2018 2:20 pm
Is off peak hours an Ontario thing. Our power costs the same all day and night long

And run the AC when its cooler at night? Thats when you let nature cool your house with open windows.

You are doing this 100% backwards

+35 and not have it kick in, loosen the freaking purse strings, why even own a/c

Good luck
Not sure if you are replying to the ecobee peak relief thing , but no it does not run the AC at night. Peak hours start at 11am and end at 5pm. So the set point would drop 0.5 C lower just before 11AM, and then 0.5 higher just after 11AM. I haven’t noticed a difference comfort wise and if it’s saving me a few bucks , hey why not.
Deal Addict
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Oct 9, 2010
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Windsor
WikkiWikki wrote:
Sep 10th, 2018 3:35 pm
So then why do so many on here say Toronto and Ontario are the best places to live in Canada?
Because they don't base their entire decision on the cost of electricity. Also, I don't think there are so many on here that say that.
One who is offended by truth, has no place among those who seek wisdom.
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Oct 9, 2010
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Windsor
flightsimmer wrote:
May 30th, 2018 11:35 am
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.
Largely depends on how well insulated your home is, and the technology in your A/C, but you will very likely save some cash, though probably less than you think. In most homes, you'll reasonably be able to avoid peak hours, but not mid-peak; probably "smarter" to run your off-peak and mid-peak at the same temp ... would suck to come home from work to a house that is 28°C at 5pm (which it probably will be).

I recently tested this in my home, and the savings were not enough to justify the comfortability to me, largely because my A/C takes too long to bring down temps. In my 1970s home (ranch, zero tree coverage), I keep the temp at 22.5°C normally. If I turn it off at 7am, on a hot day, my house will be at least 27°C by 5pm, which is uncomfortably warm for me (working from home). Turn on A/C, it will take it until at least midnight to cool the house back to 22.5°C. The savings per day were in the area of $0.25/day, or $7-ish/month, which was not enough for me to justify it.

Now, if I close blinds, it's a dramatically different story; my house will likely stay cool enough to avoid peak (I haven't tested it; I always forget to close them). I would guess I'd save closer to $10-12/month, but it's a guess.
One who is offended by truth, has no place among those who seek wisdom.
Deal Addict
Feb 4, 2015
3391 posts
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Canada, Eh!!
From Ecobee website, no android peak relief option, yet?

To use Peak Relief, you’ll need to be on a Time of Use rate with your current energy provider and have an electric heating or cooling system. You’ll also need to have an iOS device and either an ecobee3, ecobee3 lite or ecobee4 Smart Thermostat in your home.

https://www.ecobee.com/introducing-peak-relief/
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Jan 17, 2002
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Toronto
Thank goodness we have a ductless mini-split, only have to cool a few rooms at night.
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May 16, 2011
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Clarington
tebore wrote:
Sep 10th, 2018 3:07 pm
Yes it is. Welcome to our ass backwards province. Before the crooked Government mortgaged our future and further raised the cost of hydro you were talking about doubling the cost of hydro during "On Peak Hours" 1.5x for the mid compared to "off Peak". Our Off peak hour costs are equal to many other provinces regular all day price.
The person you were talking to lives in Sask. Their cost is 14.228¢/kWh at all times. Ontario's peak is (when including deliver fee costs into it) is ~16.5¢/kWh. It's not that outlandish.

An average customer in Toronto will get charged 16.32¢/kWh on average while a customer in Regina will get charged 15.94¢/kWh.

Time of use is horrifically stupid - not disagreeing there. Just more so saying the cost isn't that out to lunch. I particularly find it funny that the time they charge us more for our power is when we have more of a surplus of it. Ontario has a fairly large amount of solar arrays which produce best right at peak hours lol.

Our hydro is actually quite cheap when compared to a lot of the rest of the world (kinda the exact opposite of our cell phone companies).

Average prices for USA cities:

NYC - 29.67¢/kWh
San Fran - 31.05¢/kWh
Miami - 13.39¢/kWh
Detroit - 21.22¢/kWh
Boston - 28.45¢/kWh
Houston - 12.34¢/kWh
Seattle - 15.05¢/kWh

The average European price is 30.48¢/kWh (20.02 euro cents per kWh). Australia costs 31.83¢/kWh (34.41 AUD cents per kilowatt-hour).
Deal Fanatic
Feb 9, 2006
9079 posts
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Brampton
Bawler wrote:
Sep 17th, 2018 10:51 pm
The person you were talking to lives in Sask. Their cost is 14.228¢/kWh at all times. Ontario's peak is (when including deliver fee costs into it) is ~16.5¢/kWh. It's not that outlandish.

An average customer in Toronto will get charged 16.32¢/kWh on average while a customer in Regina will get charged 15.94¢/kWh.

Time of use is horrifically stupid - not disagreeing there. Just more so saying the cost isn't that out to lunch. I particularly find it funny that the time they charge us more for our power is when we have more of a surplus of it. Ontario has a fairly large amount of solar arrays which produce best right at peak hours lol.

Our hydro is actually quite cheap when compared to a lot of the rest of the world (kinda the exact opposite of our cell phone companies).

Average prices for USA cities:

NYC - 29.67¢/kWh
San Fran - 31.05¢/kWh
Miami - 13.39¢/kWh
Detroit - 21.22¢/kWh
Boston - 28.45¢/kWh
Houston - 12.34¢/kWh
Seattle - 15.05¢/kWh

The average European price is 30.48¢/kWh (20.02 euro cents per kWh). Australia costs 31.83¢/kWh (34.41 AUD cents per kilowatt-hour).
Ok but what's the rate for our neighbor Quebec ?

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