Pick a 2 hours period where the temperature isn't likely to fluctuate much. At the beginning, read your meter and then turn on the air conditioner (low enough to keep it running). At then end of the first hour, read the meter again and turn off the air conditioner. At the end of the 2nd hour, read the meter one more time.
Subtract the first reading from the second. Subtract the second reading from the third. Subtract the first result from the second result. Now you know roughly how much current the A/C draws in an hour. Your bill probably tells you what you pay per kwh and that times the result of the last calculation * the number of hours per day your unit runs is your answer.
Can you really find out how much it uses? Yes but it would require you put a meter on it to figure out exactly how much it uses. You can just do the math for it but the numbers you come up with will be wrong.....
#1 Doing the math will not figure in the down time of the unit. The AC unit is always on and the fan is always running. However the parts that draw the most power cycle on and off. They will turn off when the correct temperature is reached or to give itself a break.
#2 You need a wattage or amperage number to do the math. The number printed on the unit is not a real world number. The unit my say 12amps but that is normally surge amps and once the unit is settled it will consume less.... say around 8 amps.
#3 Getting the correct numbers are based around knowing what voltage you are using. A normal US house will see voltage in the 110-125 range.... you need to know what your is to find the exact number.
Doing the math is simple, but the results will be higher than they really are. You could however come up with a rough number.
Really the easy way would be to compare a bill from last year when you didnt have an air conditioner.
If you have a 12 amp AC unit it uses roughly the same as 12 100 watt light bulbs. About the same as a Microwave oven when its running. About half of what a hair dryer uses. 15-20 Times what a TV uses.............
Your AC probably does not tell you what its wattage is but it will tell you its voltage and amperage. As long as you know 2 of the 3 you can always figure out the third.
lets use a 60 watt light bulb on a 110 volt circuit(120 for this exercise)
to find amperage:
divide wattage by voltage - 60 watts divided by 120 volts = .5 amps
to find voltage:
divide wattage by amperage - 60 watts divided by .5 amps =120 volts
to find wattage
multiply voltage by amperage - 120 volts x .5 amps = 60 watts
By understanding this you will notice it will cost you half as much to operate your AC if it was 220 volt instead of 110
Once you have figured out your wattage you will need to figure out your usage or how many hours a month you will use it, this is called watt hours.
For this exercise we will still use a 60 watt light bulb, turned on for 12 hours a day 28 days a month.
60 x 12 x 28 = 20160 watt hours
Your electric company charges you by the kilowatt 1kwh = 1000 watt hours.
20160 watt hours divided by 1000 = 20.16 kwh
lest use a figure of 10 cents per kwh
.10 x 20.16 kwh = 2.016
To run a 60 watt light bulb on a 120v circuit, 12 hours a day, 28 days a month at 10 cents per kilowatt hour would cost $2.02 per month.
This does not negate what was mentioned above, motors and compressors use higher amperage to start then settle. so for the purpose of an air conditioning unit your figure cannot be exact but very close. In the instance of an incandescent lamp you can be exact.
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