AC Wattage Calculator
This AC wattage calculator works out how much electrical power your air conditioner draws while the compressor is running, then turns that draw into daily electricity use and running cost. Enter the cooling capacity from the unit's label, in BTU per hour or kilowatts, along with its EER, the hours you run it on a typical day, and your electricity price per kilowatt hour. The calculator divides capacity by efficiency to get the input wattage, multiplies by your hours to get kilowatt hours per day, and prices that at your rate to show the cost per day and per 30 days. Air conditioners move heat rather than create cold, so a unit delivering 12,000 BTU per hour of cooling draws only a fraction of that energy as electricity, and how big a fraction depends on the EER. Many portable units sit around an EER of 8 to 10, while the high-wall heat pumps common in NZ homes generally do better in cooling mode. Use it to compare models before you buy, to estimate what a summer of cooling will add to your power bill, or to check whether a circuit can handle the load. Pair it with our BTU sizing tool to get the capacity right first.
EER is cooling output in BTU per hour divided by electrical input in watts. If your unit lists a COP instead, multiply the COP by 3.412 to get the EER. 1 kW of cooling is 3,412 BTU per hour.
The compressor cycles on and off once the room reaches temperature, so real consumption over a day is usually lower than the steady running draw suggests. Estimate only.
How it works
The running wattage is the cooling capacity divided by the EER. If the capacity is entered in kilowatts of cooling it is first converted to BTU per hour by multiplying by 3,412, because EER is defined as BTU per hour of cooling per watt of electrical input. Once the wattage is known, daily consumption is the wattage divided by 1,000 and multiplied by the running hours, giving kilowatt hours. The cost per day is that consumption multiplied by your electricity price, and the 30 day figure simply multiplies the daily cost by 30. The wattage shown is the steady draw while the compressor runs flat out; inverter units ramp down once the room is at temperature, and fixed-speed units cycle off entirely, so treat the daily figures as an upper estimate for a hot day rather than a year-round average.
Worked example
Take a 12,000 BTU per hour unit with an EER of 10. The running draw is 12,000 divided by 10, which is 1,200 watts. Running it 6 hours a day uses 1.2 kW multiplied by 6 hours, which is 7.2 kWh. At $0.32 per kilowatt hour that costs 7.2 multiplied by 0.32, which is about $2.30 a day, and $69.12 over 30 days. If the same unit had an EER of 8 instead, the draw would rise to 1,500 watts and the 30 day cost to about $86.40, which is why the efficiency rating deserves as much attention as the capacity.
Related calculators
- Air Conditioner BTU Calculator: size the capacity for your room.
- BTU to Tons Converter: cooling capacity units.
- Electricity Cost Calculator: cost of any electrical load.
- Appliance Running Cost Calculator: compare household appliances.
- Watts to Amps Calculator: check the circuit load.
Frequently asked questions
How many watts does a 12,000 BTU air conditioner use?
Divide the BTU per hour rating by the EER. A 12,000 BTU per hour unit with an EER of 10 draws about 1,200 watts while the compressor is running. The same unit with an EER of 8 would draw about 1,500 watts, so efficiency makes a real difference to the electrical load.
What is EER and how is it different from COP?
EER is the cooling output in BTU per hour divided by the electrical input in watts. COP is the same ratio expressed in consistent units, watts of cooling per watt of input. EER equals COP multiplied by 3.412, so a heat pump with a cooling COP of 3 has an EER of about 10.2.
How much does it cost to run an air conditioner?
Multiply the running wattage by the hours used, divide by 1,000 to get kilowatt hours, then multiply by your electricity price. A 1,200 watt unit running 6 hours a day at 32 cents per kilowatt hour costs about $2.30 a day, or about $69 over 30 days. Because the compressor cycles on and off, real consumption is usually a little lower than the running draw suggests.
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