Updated October 23, 2026 · All wattages and rates below are illustrative
A useful AC estimate starts with energy use, not the temperature on the thermostat. You need the unit’s average operating power, how long it draws that power, and the electricity price that applies during those hours.
The basic calculation
Daily energy cost = daily kWh × rate in dollars per kWh
If you have measured kWh for the system over a representative period, multiply that directly by the applicable rate instead. This avoids guessing a constant wattage.
Compare three chosen scenarios
These are arithmetic examples, not expected consumption for a category of AC. Central, window, portable, and variable-speed systems can have very different inputs.
| Average operating power | Hours/day | kWh/month | Energy cost |
|---|---|---|---|
| 1,000 W | 4 | 120 | $19.20 |
| 1,000 W | 8 | 240 | $38.40 |
| 3,000 W | 8 | 720 | $115.20 |
To reproduce the second row in the appliance calculator, enter 1,000 watts, 8 operating hours, 30 days, and 16 cents. Use your measured or model-specific information for a household estimate.
Thermostat-on time is not compressor runtime
A system can be enabled all day while its compressor cycles. On some equipment the fan can run separately. Variable-speed systems can run longer at a lower power level, so comparing hours alone may mislead.
Choose a consistent method: either average watts over the entire observation period and all hours in that period, or operating watts and equivalent operating hours. Do not combine an already averaged low wattage with reduced runtime a second time.
Find a better power estimate
- Review the manufacturer’s documentation for the exact model.
- Use suitable energy-monitoring data when available.
- Include the relevant indoor and outdoor equipment if estimating a complete system.
- Keep measurements from comparable weather and household conditions.
A maximum nameplate rating is not necessarily an average. Do not attach an ordinary plug-in monitor to a hardwired or high-current AC system. Use appropriately rated equipment and qualified help where required.
Use the result to compare a change
At the chosen 3,000-watt example, reducing equivalent operation by one hour daily changes 30-day energy use by 90 kWh. At 16¢, the calculated difference is $14.40. Whether a change actually produces that result depends on weather, comfort needs, controls, and equipment behavior.
What the estimate leaves out
The calculator does not model humidity control, cycling behavior, heat gain, tariff tiers, or time-varying prices. An annual projection assumes the same daily pattern for 365 days, which is often unsuitable for seasonal cooling. Use a realistic number of cooling days for a seasonal calculation.
Before buying a replacement
Compare purchase and installation costs with a realistic energy difference, rather than assuming a new unit pays for itself. The Department of Energy’s guide discusses sizing and efficiency as parts of a system decision. Ask for current model-specific information; do not use an older guide’s standards as current purchasing requirements.
Source: DOE Consumer Guide to Home Heating and Cooling (2021 background guide), checked October 23, 2026. Example calculations are RegionalRates arithmetic. Calculation assumptions.