The most important AC cost input is not how many hours the thermostat is switched on. It is how long the compressor actually draws power, together with the system's average electrical demand.
Estimate power draw
Use measured watts when possible. A product label, technical sheet, smart thermostat, whole-home monitor, or clamp meter may provide a usable number. Rated maximum power can overstate average use for variable-speed equipment.
For central systems, include the outdoor unit and indoor blower if your measurement does not already cover both.
Estimate compressor runtime
An AC may be available for eight hours but cycle on for only part of that time. Runtime changes with outdoor temperature, humidity, insulation, air leakage, thermostat setting, equipment sizing, and maintenance.
Use a representative hot-day runtime and multiply by the number of similar days. For seasonal estimates, calculate mild and extreme periods separately.
Apply the electricity formula
Divide watts by 1,000, multiply by compressor runtime, and multiply by the number of days. Apply your electricity rate to the resulting kWh.
The Department of Energy notes that actual cost depends on both utility rates and use. Fixed charges on a bill do not normally change because the AC ran another hour, so keep them separate when evaluating incremental operating cost.
What this estimate cannot diagnose
A cost calculator cannot determine whether equipment is correctly sized, charged, ducted, or safe. Large unexplained changes in consumption may justify professional inspection.
Do not infer savings from a replacement system using wattage alone. Climate, capacity, part-load behavior, installation quality, and usage changes all matter.
Use measured or realistic average power and compressor-on time; thermostat-on time alone usually overstates consumption.
Frequently asked questions
Does an AC use full power whenever the thermostat is on?
No. The compressor and fan cycle or modulate. Cost estimates should use representative active runtime and average system power rather than the total time cooling is available.
Can an AC cost estimate predict my exact bill?
It can estimate incremental energy cost, but fixed fees, tiered rates, weather, humidity, equipment condition, and household use prevent it from reproducing an exact bill.
Sources and further reading
Sources support the methods and practical guidance above. Product specifications and local requirements should take priority for a specific project.
U.S. Department of Energy: Energy Saver Guide Energy-cost guidance and efficiency considerations for household systems. ↗