Air-conditioner electricity use depends on electrical input, efficiency, size, outdoor temperature, and actual runtime. The nameplate or specification gives a power reference, but monthly cost is best estimated from kilowatts, operating hours, and your local electricity rate.
Find electrical input, not just cooling capacity
The label may list watts or amps along with cooling capacity. For an energy estimate you need electrical input; British thermal units per hour (Btu/h) describe cooling capacity, not direct electricity consumption.
Convert power and runtime into kilowatt-hours (kWh)
Divide watts by 1,000 to get kilowatts and multiply by operating hours. A 1.5 kW unit that actually runs for six hours uses about 9 kilowatt-hours (kWh) that day; compressor cycling may reduce effective runtime.
Thermostat setting and weather change the result
A large indoor-outdoor temperature difference, air leaks, or loaded filters make the system work harder. Comparing similar days and meter readings gives a more realistic estimate than assuming maximum power draw every hour.
Efficiency matters when comparing units
Two units with the same cooling capacity can use different amounts of electricity. Use the efficiency metric relevant to your market and compare properly sized models; oversizing or undersizing can also hurt comfort and energy use.
Sources
- U.S. Department of Energy — Energy Saver — Residential energy efficiency, heating and cooling, and equipment use.