DIY Home Improvement, Remodeling & Repair Forum > HVAC > How to calculate KWH usage of A/C?

07-09-2009, 01:03 PM
onecrazyfoo4u
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How to calculate KWH usage of A/C?

Just wondering how you can calculate the amount of killowatt hours my A/C condensor outside uses (That uses way more than the attic blower fan correct)? What numbers do I need to calculate it? I can provide a picture if needed. Thanks!

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07-09-2009, 04:04 PM
kok328
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You'll need to know how many amps the compressor pulls and how many amps the condensor fan pulls. You'll also want to factor in the in the blower in the furnace cabinet that circulates the cool air in the home too. These numbers won't be hard to get if you have some documents to tell you, otherwise, you have to pull some stuff apart to read the nameplates on each component.

Once you have this information, post back and I'll try to calculate your KWh usage.

Amperage of the compressor =
Amperage of the condensor fan motor =
Amperage of the evaporator fan motor (i.e.-blower motor) =

However, if you are simply trying to determine which use less power the attic fan or the A/C unit, the answer is the attic fan uses less.

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07-09-2009, 06:59 PM
onecrazyfoo4u
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Well I took some pictures of my system, hopefully that helps. First picture is the tag on the outside of my A/C unit. Second one is of the compressor motor label. And third is of the Fan motor label.

I'm guessing that the compressor Amps is 23.1 or 40, not sure though? And the fan motor amps are maybe 1.5? Not sure, that's why I through up the pictures. Does that sound about right? What is RLA and LRA? Thanks alot!

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07-12-2009, 06:55 PM
AU_Prospector
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I think you are missing one set of numbers which would be the blower which moves air through your duct system. This will be either in a utility room in your home, the attic, or like me the crawl.

A simple wattage calculation is amps times volts = watts. You can get a little more complicated than that if you factor resistance across the wire, but a quick down and dirty is simply amps x volts.

Heavy equipment in your home will run on 220 volts (from 208 and 240 depending on your power company, their consistiency, and the time of day). No kidding, power companys used to anyway, drop voltage during non peak hours like at night.

Your compressor looks to 'draw' 17.2 amps. 17.2 x 220 equals 3784 watts. If you run this for an hour, it will draw 3.784 kilowatt hours. Good thing the compressor cycles on and off and doesnt run constantly. Where I live 3.784 kwh costs about 40 cents.

Your compressor fan looks to draw about 1.6 amps or 352 watts. or about .35 kwh

Your minimum breaker requirement is different than the max breaker allowed. Yours probably is 30 or 40 amps.

Another example your 1500 watt hair dryer draws about 12.5 amps on your household 120 volt circuit. (1500 watts / 120 volts = 12.5 amps) If you ran it for 1 hour, it would draw 1.5 kilowatt hours and cost you about 15 cents or so. Electricity is cheap when you think about it, no?

BTW amps kills. It is a measure of force that can be converted to power. Think of a garden hose. The more water I FORCE through it, the more power I will feel at the nozzle.

You still need the amps and voltage of your evaporator/blower unit. I think you can do the math from here.

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07-12-2009, 08:19 PM
onecrazyfoo4u
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Thanks allot for the answer. Yeah, I'm missing the blower because it's up in the attic and I haven't had time to crawl up there and check it. It should probably be around the same as the condensor fan right, 1.6ish or so?

Thanks again for the exptination! For me it's around 50 cents an hour + whatever the blower puts out. Not too bad at all!

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