An air conditioner's BTU (for British Thermal Unit) rating is a measure of its cooling power. An 8,000 BTU window unit, for instance, can typically cool a room of 300 to 350 square feet, according to the California Energy Commission. However, window AC units with the same BTU rating can use different amounts of power to achieve that cooling. A unit's Energy Efficiency Rating, or EER, gives consumers an idea of how efficient a unit it. Of units with the same BTUs, the higher thr EER, the greater its efficiency, and the less it costs to operate.
Check the air conditioner for the wattage, which is the amount of power the device uses per hour. This number may be printed on the side, on a tag attached to the electrical cord or in the manual. You may find amps instead of watts. If this is the case, multiply the amps by the voltage in your country, which is 120 in the U.S., to get the wattage. For example, if the unit is 7.0 amps, the wattage is calculated by multiplying 7.00 amps by 120 volts to equal 840 watts. Alternatively, if the unit has an EER, divide 8,000 by the EER to get the unit's wattage. For example, an 8,000 BTU unit with an EER of 10.8 uses 740 watts.
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Check your electrical bill to find your cost per kilowatt-hour, or kW, of electricity. You may need to add up the separate supply and deliver charges to determine your total cost per kilowatt-hour.
Divide the air conditioner's watts by 1,000 to convert it to kilowatts. For example, the 840 watt air conditioner would be 840 divided by 1,000, or 0.84 kW.
Multiply the kW by the cost per kilowatt-hour to get how much it will cost to run your air conditioner for one hour. For example, if your electricity costs 11.1 cents dollars per kilowatt-hour, you would multiply 0.84 kW by $0.111, which equals $0.09324 per hour, or 9.324 cents per hour.
Determine how many hours you will run your air conditioner each day, then multiply this by 30 to get approximately how many hours it will run each month. For example, if you plan to run the air conditioner for three hours each evening, multiply 3 by 30 to get approximately 90 hours a month. Multiply the hours per month by the dollars per hour from your earlier calculation to get the cost to run your air conditioner each month. For an air conditioner cost of $0.09324 per hour running at 90 hours a month, the total cost equals $8.39 per month.
Calculate the cost per month when comparing air conditioners with different efficiencies to purchase, to determine which will actually be cheaper to own over the course of a few years.