SEER rating is the efficiency metric
Central air conditioners are rated by SEER (Seasonal Energy Efficiency Ratio). A 14 SEER unit uses about 30 percent more energy than a 20 SEER unit for the same cooling output. The higher the SEER, the less electricity per ton-hour of cooling.
Nameplate wattage is not always listed. Estimate running watts by dividing the cooling capacity in BTU/h by the SEER rating, then multiply by 1,000. A 36,000 BTU/h (3-ton) unit at 16 SEER draws roughly 2,250 watts while the compressor is active.
Runtime drives the bill more than capacity
An oversized unit short-cycles, wasting energy on startup surges. An undersized unit runs continuously on hot days. Proper sizing (Manual J calculation) and a programmable thermostat that raises the setpoint when away are the biggest levers for cost control.
- Set the thermostat to 78°F when home and higher when away.
- Seal duct leaks; 20 to 30 percent of conditioned air can escape in unsealed ducts.
- Clean or replace the filter monthly during cooling season.
Monthly cost example
At an illustrative 18 cents per kWh, a 3-ton 16 SEER system running 8 compressor hours per day uses about 18 kWh daily, or 540 kWh in a 30-day month. That is roughly $97 per month in energy charges. A 20 SEER unit for the same runtime would use about 432 kWh, or $78 — a $19 monthly savings.
Compare with window units for partial cooling
If you only need to cool one or two occupied rooms, window units or a ductless mini-split can be cheaper to run than central air. See the window unit and ceiling fan guides for those comparisons.
Rate source and limits
The default rate is the EIA U.S. residential average for April 2026, retrieved July 16, 2026. It is an average revenue per kilowatt-hour, not a quote for your utility plan. Fixed fees, taxes, tiers, and time-of-use prices can change the bill.
Open the EIA source