How do you figure out how much your AC costs per degree?

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nanette1985

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Oct 12, 2005
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I got my AC repaired and wow, is it ever nice to have cool air inside on these 90 degree high humidity days (I know others here have it worse - my sympathies).

Anyway, I was wondering how to figure out how much this cool air is costing me. I don't really mind paying it but I'm curious. It's one of the nicest things I spend my money on.

Enjoying a cool iced beverage at the moment.

Hope you-all are comfy.
 

vi edit

Elite Member
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Oct 28, 1999
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It'd be hard to narrow down to a per degree basis. Getting the house cooled from 90-89 degrees is heck of a lot cheaper than getting it from 73-72 degrees if it's 90 degrees outside.
 

jagec

Lifer
Apr 30, 2004
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You can get a home energy monitor, or you can plug some numbers (size of house/humidity/outside temps vs. inside temps/SEER rating of AC/quality of insulation) into heat transfer equations to get a rough estimate.

A lot of web sources say that it's around 1-3% savings per degree above 72...but I don't know what numbers went into that calculation.

It's going to be 97 degrees with over 50% humidity in my city tomorrow. I agree that AC is one of the best things that civilization has invented.
 

vi edit

Elite Member
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A lot of web sources say that it's around 1-3% savings per degree above 72...but I don't know what numbers went into that calculation.

We had our AC installer tell us that that anything under 72 is considered "refridgeration" and it's much more difficult for a unit to keep up with that temp. I can attest to that in use. At 73-74 degrees it seems like it kicks in once an hour or so to keep the house cool throughout the day. Just bumping down another degree or two to 72 and it seems run 3x-4x an hour.
 

Spungo

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Jul 22, 2012
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I would guess that it's fairly linear. Take the temperature of the room with no AC and mark that as 0%. Set the temperature so low that the AC is running constantly and never reaches the set point, mark that temperature as 100%. Draw a straight line and use that to interpolate.

We had our AC installer tell us that that anything under 72 is considered "refridgeration" and it's much more difficult for a unit to keep up with that temp. I can attest to that in use. At 73-74 degrees it seems like it kicks in once an hour or so to keep the house cool throughout the day. Just bumping down another degree or two to 72 and it seems run 3x-4x an hour.
I have a portable 12000 BTU AC unit in my condo that I never run during the dya because it's in my bedroom. At night I set the AC to 65 degrees. It runs constantly throughout the night and never reaches that temperature.
 

gorcorps

aka Brandon
Jul 18, 2004
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We had our AC installer tell us that that anything under 72 is considered "refridgeration" and it's much more difficult for a unit to keep up with that temp. I can attest to that in use. At 73-74 degrees it seems like it kicks in once an hour or so to keep the house cool throughout the day. Just bumping down another degree or two to 72 and it seems run 3x-4x an hour.

Interesting. Also a shame because my preferred temperature right now is 71. I can definitely tell the difference at a setting of 73 and it gets a little uncomfortable in certain rooms. I know my house could be better sealed and probably could use new windows, but it just feels warmer than it should be. It definitely works hard to keep that 71 though so maybe I should start with 72 to see.
 

vi edit

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The problem I have is a two story house. I can get by with the AC set at 74 or so on the main floor. But then upstairs it's pushing close to 80 and that's just really uncomfortable for the kids. I have the blower running all the time since the AC is in the basement and it's circulating cool air from there. That seems to help a bit.
 

jagec

Lifer
Apr 30, 2004
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Interesting. Also a shame because my preferred temperature right now is 71. I can definitely tell the difference at a setting of 73 and it gets a little uncomfortable in certain rooms. I know my house could be better sealed and probably could use new windows, but it just feels warmer than it should be. It definitely works hard to keep that 71 though so maybe I should start with 72 to see.

Wow, you must have to pay through the nose for that. I keep the thermostat at 78 in the summer.
 

vi edit

Elite Member
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Wow, you must have to pay through the nose for that. I keep the thermostat at 78 in the summer.

I keep my 3200 sq/ft + full basement house in the muggy, hot midwest at 72-73 degrees. I've never had a bill higher than $220 degrees of which is a normal basline of ~$100 for basic electricity use.

Yay insulation and tight building standards!
 

Dr. Zaus

Lifer
Oct 16, 2008
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We had our AC installer tell us that that anything under 72 is considered "refridgeration" and it's much more difficult for a unit to keep up with that temp. I can attest to that in use. At 73-74 degrees it seems like it kicks in once an hour or so to keep the house cool throughout the day. Just bumping down another degree or two to 72 and it seems run 3x-4x an hour.

This is like saying that going from 45 to 60 costs twice the gas of going from 30 to 45. Maybe, depends on a whole host of other things.

I'm sure that if its 75 and you have great insulation going to 71 is a lot lest costly than if its 80 and you go to 76 but have terrible insulation.

If you have the money for electricity I suggest you simply set it to comfortable and be happy.
 

vi edit

Elite Member
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This is like saying that going from 45 to 60 costs twice the gas of going from 30 to 45. Maybe, depends on a whole host of other things.

I shouldn't have worded it as "going" rather sustaining. And in terms of cars, yes it usually is more fuel demanding to sustain 80 MPH than it is 65 due to wind resistance and general higher RPM's of the engine.
 

jagec

Lifer
Apr 30, 2004
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I shouldn't have worded it as "going" rather sustaining. And in terms of cars, yes it usually is more fuel demanding to sustain 80 MPH than it is 65 due to wind resistance and general higher RPM's of the engine.

Wind resistance scales as the square of speed. Engine performance is more complex and also depends on gearing, but I doubt that it is linear either. Once you're in top gear going 50+, you can expect a nonlinear dropoff in mileage as speed increases.

Heat transfer rate, however, depends directly on temperature differential times a bunch of things (area, heat transfer coefficient, thickness) which are constant for a given house. So, how much heat is ENTERING the house increases linearly as the outside temperature goes up (assuming constant inside temp).

However, the coefficient of performance of your AC which is removing the heat is NOT linear, and decreases as the temperature differential between the hot and cold sides increases. Which means that as the watts of heat entering your house increase, you have to spend more watts of electricity per watt of heat to pump them back out.

Which is why ground-source heat pumps are so great. The heat entering your house is still determined by air temperature (linear), but the COP of your heat pump will be reasonably constant, since ground temperatures won't increase nearly as quickly.

Of course, electricity is pretty cheap in the US, and ground-source heat pumps are super expensive to install if you don't DIY.
 

Chaotic42

Lifer
Jun 15, 2001
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Wow, you must have to pay through the nose for that. I keep the thermostat at 78 in the summer.

I keep my house at 67 (I live a few miles from the Gulf of Mexico) in the summer and my bill was something like $110 last month. :p
 

Mixolydian

Lifer
Nov 7, 2011
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It would be difficult to calculate, but could be done.

Some things you'd need to know:

-How much electricity (watts) the system uses
-How much your electric co. charges per KwH
-How long the system must run to lower the temperature one degree
-How often it must run to maintain that temperature

etc....
 

Imp

Lifer
Feb 8, 2000
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Bastards... I keep my A/C at 81 F. Lowest we've gone is 77 -- cheap bastard.


OP needs to calculate cooling degree days and energy flow through building profile. It takes me a few days to gather all the information to do that and I've done it at least twice... Good luck.
 
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