# Does the temperature you set your house thermostat at make any difference in the electric bill?

Asked by Dutchess_III (42274) July 24th, 2017

My son recently got a new AC installed in his house. As is his nature, he started researching How It All Works.

He was telling me that it doesn’t really matter what you have the thermostat set at. It’s summer, so if you have it set at 78, where I keep mine, it just keeps the house at 78. It simply kicks on when the thermostat reads 77 and blows until it hits 78 again.

Setting it at 76 doesn’t make a difference, because it just does the same thing. It just kicks on at 75, and kicks off at 76. It doesn’t blow colder air or anything. I’m thinking it’s how often it kicks on is the difference. But, then again, what he said kind of makes sense.
Does it take longer to get the house from 75 to 76 than it does to get it from 77 to 78?

I was shaking my head at him to tell him I thought he was full of crap, but, on the other hand, the logic was sound. He’s done that to me for as long as I can remember. He’ll just give me crap under the guise of a really good argument.

So, how does the temperature regulator work?

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I’ve wondered this. I think it does matter a little, because the sun coming through the windows, and beating down on the roof, an the hot air that seeps in through cracks in the house are part of the equation.

I know in the winter if I turn my heat way down it makes a significant difference in my bill.

Air from a setting of 77 or 78 might not make a huge difference, but probably a setting of 72 vs. 78 is noticeable.

JLeslie (59526)

Trying to figure out why.

Dutchess_III (42274)

It matters, if you set the thermostat at 75 and it’s 95 degrees out it will take longer a run time to maintain 75 than it would if you set it at 80. Those kw hours cost \$

Setting the Stat at 69 for summer will make the A/C run way longer and more often then setting it to 78. Longer more often means more money for utility company \$ \$ \$ \$ \$ !

Why does it take a longer run time @ARE_you_kidding_me? I don’t disagree, but we are still talking about a degree of difference to bring it back to what you have it set at.

@Tropical_Willie Just work with me…you set it at 69, and it kicks back on when it hits 70. If it’s set to 78, it kicks back on at 79. Does it take longer to get from 70 back to 69 than it does to get from 79 to 78?

Dutchess_III (42274)

Ideally if your house is at temperature and stays in that range in theory it should not cost more but “ideal” is only “in theory” Your house is not ideal, you open doors, heat finds its way in through gaps and cracks. It simply takes more “work” (\$) to maintain the larger temperature differential.

Yes, it makes a difference.

Newton’s law of cooling means that the temperature differential between your house and the outside air affects how quickly your house heats up. As the differential increases (either because it’s hotter outside or because it’s cooler inside – e.g. if you have your AC set very low) the rate of heat exchange between your house and the outside air increases. So, the cooler you set your AC, the more frequently it will have to turn on in order to maintain that temperature.

Mariah (25863)

Thank you @Mariah! I shall go wack my son with that, with your permission.

Dutchess_III (42274)

@Dutchess_III I have installed a “setback” thermostat that let the set temperature to rise to 76 during day for A/C and drop down to night time temp at night; it does the reverse for winter. Year after installation I saved over 8% on electrical costs. You have a loss of cool air from house during day & night, the A/C has to make up for loss of cool air. The higher the outside temp the larger the loss.

Thanks you guys. I knew “how often it turned on,” had to be the answer. I just couldn’t put my finger on why.

Dutchess_III (42274)

With the thermostat set at lower and lower temps, the compressor unit and blower fan will run more often the lower it’s set and the hotter it is outside. Like @Tropical_Willie said.

kritiper (19316)

Heat loss (or cool loss) is a function of the difference between the inside of the house and the outside. If you set your house cooler in summer there is a bigger difference between the inside and outside so the loss is greater. Your cooling system will run longer and/more often.

This difference is measured in CDD cooling degree days. There is data for most cities and regions. Enjoy!.

LuckyGuy (38286)

A lot of things make the difference. How insulated is your home? How often do you open your windows? How do you have your vents circulating the air in your home? Do you use an ceiling fans? Do you run a lot of machines during the day that heat up your home. Do you leave on the lights. Are they cool lights or do they run hot. Do you let the sunshine in when the sun is beaming down on your house, or do you have sun block curtains?

My point it would be hard to say. When I first moved in my home, I kept cooling down and putting up the temperature in the house. Then I tried leaving it alone. None of them improved my bill. What improved it was getting new windows, putting seals around the doors where I could feel a draft or heat enter, and get sun block curtains. Changed all my lights to cool lights and I cook meals that take a long time in the evening when the temp has gone down and early morning. My lunches are either cold or microwaved.

I played with my vents till it adequately cooled the rooms I wanted only and use nature do it’s work. Heat rises and cool air stays down. So I close the vents in my always cool basement, and on the first floor I closed two vents so it would blow most of the cool air where we spent most of the day. Then for the upstairs all the vents to my room is open. and the fan above pushes the air down the stairs. I also have fans that use little energy. All together, this keeps my home at a comfortable 70 to 72 degrees at all time in the summer and my bill is 50 dollars a month cheaper then when I first moved in.

Another thing also determines your bill. Do you maintain your A/C unit. Get it cleaned once a year and change the filters regularly? A poorly maintained unit will cost you more in the long run. I use those 3 month filters and I change them every 45 days to 60 days. I never keep it 90 days. The cleaner it is the better airflow and less energy your unit uses.

Almost forgot. A broken thermostat can also cost you quite a bit of money in energy.. Had one break on me once in the winter and it ran on emergency heat for a few days. I had a really drafty house then so I was just glad the house felt warm. I didn’t realize it was on emergency heat till my kids noticed it. My bill for that month was a killer.

Pandora (28975)