# How Efficient Is A/C AT Removing HeatAugust 1, 2011 10:47 AM   Subscribe

Say I run a 1000W oven in an air-conditioned home for 1 hour (using in the process 1kWh). How much power does a typical home A/C system require to remove the heat generated, all else being equal?

posted by lohmannn to Science & Nature (14 answers total) 2 users marked this as a favorite

Well, one generally doesn't run an oven for an hour with the windows closed, and one generally doesn't run an AC for an hour with the windows open. Which one will you be doing?
posted by griphus at 10:52 AM on August 1, 2011

What is the ambient temperature outside the house? How energy efficient is the house? Both factors will strongly influence how rapidly heat dissipates from the house.
posted by biffa at 11:06 AM on August 1, 2011

Best answer: Let's assume the heat is perfectly trapped within the house, so that only the AC can remove the heat.

1 kWh is 3413 BTU. The amount of electricity it will take to produce that amount of cooling depends on the Energy Efficiency Ratio of your AC. Very roughly, you can divide the BTUs by the EER to get the number of watt-hours required to produce that much cooling. A very high efficiency AC (for example) has an EER of about 17.5, which would require .2 kWh. A low efficiency AC would require about twice that.
posted by jedicus at 11:14 AM on August 1, 2011 [1 favorite]

Response by poster: Ahh, EER is the term I was searching for (more specifically, COP). That got me on the right track. Thanks, jedicus.
posted by lohmannn at 11:19 AM on August 1, 2011

The oven (most likely) has an efficiency rating as well, although it's generally pretty low.
posted by muddgirl at 11:26 AM on August 1, 2011

muddgirl writes "The oven (most likely) has an efficiency rating as well, although it's generally pretty low"

It does: 100%. Electric resistive heating is always 100% efficient.
posted by Mitheral at 11:57 AM on August 1, 2011

Best answer: It will use 0.33 kWh, roughly. Air conditioners have coefficients of performance (COP) of around 3 (equivalent to an EER of 10.2), meaning that for each unit of energy input it can move 3 units of heat (from inside to outside).
posted by fzx101 at 11:58 AM on August 1, 2011

Electric resistive heating is always 100% efficient.

That's assuming there's nothing being cooked inside the oven.
posted by muddgirl at 12:21 PM on August 1, 2011

That's assuming there's nothing being cooked inside the oven.

Unless the OP takes whatever was being cooked and throws it outside, the heat that entered the thing being cooked will eventually warm the house (again, assuming a perfectly insulated house).
posted by jedicus at 1:36 PM on August 1, 2011

muddgirl: When you cook food, you don't wind up with more chemical energy than you started with (usually you have less or the same). So all of that energy is heat*, and you still have 1kWh of heat in your house. Some if it is in the oven, some of it is in the food, some of it is in the air, some of it is in your belly, but it all is extra heat that you added to your house.

* I guess technically some of it goes into phase change stuff, like converting water in your pizza or whatever into water vapor, but that energy will readily turn back into heat when you cool the water vapor down and it turns back into liquid water.
posted by aubilenon at 1:40 PM on August 1, 2011

It will use 0.33 kWh, roughly. Air conditioners have coefficients of performance (COP) of around 3 (equivalent to an EER of 10.2), meaning that for each unit of energy input it can move 3 units of heat (from inside to outside).

Are you sure you've got that right? sounds to me like you just invented perpetual motion.
posted by Chocolate Pickle at 1:55 PM on August 1, 2011

It's not perpetual motion, because turning the heat energy difference created by the aircon back into electricity can only be done at very low efficiency, and some heat will always escape somewhere.

But by paying very careful attention to details, it may be possible to use a heat pump as the heart of a not-too-inefficient battery.

posted by flabdablet at 5:45 PM on August 1, 2011

Say I run a 1000W oven in an air-conditioned home for 1 hour (using in the process 1kWh).

But a 1000W oven doesn't use a constant 1000W. Ovens are thermostatted and insulated. It will only use as much power as necessary to bring the interior cavity to a fixed temperature and hold it at that temperature (for an hour).

The temperature difference between the oven and the rest of the house, as well as the nature of the insulation in the walls and door of the oven, will determine the amount heat that flows from the oven into the house. The power rating of the oven isn't particularly relevant.
posted by mr_roboto at 8:31 PM on August 1, 2011

Nevertheless, by running a 1kW oven with the door open it is indeed possible to add 1kWh of heat to the house, which was the premise specified in the question.
posted by flabdablet at 8:36 PM on August 1, 2011

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