How many watts an air conditioner uses, and how to work it out from the label

Watts, kilowatts and kilowatt-hours in plain words

The U.S. Energy Information Administration (EIA) says electricity is measured in units of power called watts. It says one kilowatt (kW) is equal to 1,000 watts.

The EIA separates power from energy. Watts describe the draw at a single moment, while watthours describe how much electricity is used over a stretch of time. Utilities generally measure electricity in kilowatt-hours (kWh), and the EIA says one kWh is one kilowatt for one hour.

So "how many watts" and "how much electricity" are two different questions. For a full year of use on a heat pump, see how much electricity a heat pump uses.

Method 1: read the label

Hawaiian Electric, an electric utility, says an appliance's wattage is usually stamped on a metal plate, called the nameplate, or in the plastic covering on the back or bottom of the appliance.

The International Association of Electrical Inspectors (IAEI) says HVAC nameplates can supply a vast amount of information about the equipment's electrical needs. HVAC means heating, ventilation and air conditioning.

Method 2: divide capacity by the efficiency rating

Cooling capacity is the amount of heat the unit can remove each hour. Penn State's energy course text says a ton of cooling is 12,000 BTU per hour. Our arithmetic: a 3-ton system is rated at 36,000 BTU per hour.

The Energy Efficiency Ratio (EER) is the rating for room air conditioners. Penn State says it lists how many BTU per hour the unit removes for each watt of power it draws. The higher the EER, the more efficient the unit.

That gives a simple formula: watts equal BTU per hour divided by EER. Penn State's example is a 5,000 BTU per hour room unit with an EER of 8, which works out to 625 watts.

Our arithmetic on a second example: a 10,000 BTU per hour unit with an EER of 10 draws about 1,000 watts. Penn State says room air conditioners generally range from 5,500 to 14,000 BTU per hour.

ENERGY STAR defines the Combined Energy Efficiency Ratio (CEER) as the energy efficiency of a room air conditioner measured under the U.S. Department of Energy (DOE) test procedure at 10 CFR 430, Subpart B, Appendix F, or a DOE-approved test procedure waiver under 10 CFR Part 430.27, expressed in BTU per watt-hour.

For central systems, see our guide to SEER and SEER2 ratings.

Method 3: multiply amps by volts

Hawaiian Electric says that if the wattage is not listed but the amps and volts are, you can multiply them. Amps times volts equals watts.

Never open the electrical panel or the outdoor unit's cover to measure current yourself. Readings inside live equipment are work for a licensed contractor.

Why the draw changes

An air conditioner does not draw the same watts all day. Hawaiian Electric says air conditioners are controlled by thermostats that cycle on and off, using energy only when the motor is on.

ENERGY STAR says a conventional room unit's compressor works like an on/off switch. It runs at maximum capacity when the thermostat calls for cooling, then shuts off. Variable speed compressors, it says, can vary the motor's speed to control the temperature continuously.

From watts to kilowatt-hours, and what Florida changes

Hawaiian Electric gives the steps. Divide the wattage by 1,000 to get kilowatts. Then multiply the kilowatts by the hours the appliance runs at full load to get kWh.

Our arithmetic: a 1,000 watt window unit is 1 kW. If its compressor runs a total of 8 hours at full load in a day, that is 8 kWh. Hawaiian Electric notes you have to estimate the time a cycling appliance actually spends at full load.

Florida pushes those hours up. Tampa Electric, the Tampa Bay utility, says air conditioning is the biggest cause of higher bills during hot weather. It says that in Florida's heat and high humidity, systems run longer and more often, sometimes nearly nonstop for months.

Tampa Electric adds that in homes with poor insulation or undersized units, it may not cycle off or reach the set temperature. A 2024 Tampa Electric release advises setting the thermostat at 78 degrees or higher.

Size matters as well. ENERGY STAR says an oversized air conditioner wastes energy and does not provide better cooling. For matching a window unit to a room, see our window AC size chart.