Start with electrical input, not cooling capacity
An air conditioner’s running cost depends on the electricity it draws and your utility rate. Cooling capacity in BTU/h or tons is not electrical power in watts. A three-ton label therefore cannot be inserted directly into a running-cost equation.
EIA defines a kilowatthour as one kilowatt used for one hour. The arithmetic is straightforward once you have a credible input-power estimate:
Energy (kWh) = electrical input (kW) × operating hours.
Cost = energy (kWh) × electricity price ($/kWh).
If your power figure is in watts, divide it by 1,000 first. The AC running-cost calculator performs these steps and displays hourly, daily, monthly, and energy totals.
Three illustrative hourly costs
These are example power draws and rates, not measured averages for specific equipment sizes.
| Assumed electrical input | At $0.12/kWh | At $0.18/kWh | At $0.30/kWh |
|---|---|---|---|
| 1 kW | $0.12/hour | $0.18/hour | $0.30/hour |
| 2.5 kW | $0.30/hour | $0.45/hour | $0.75/hour |
| 4 kW | $0.48/hour | $0.72/hour | $1.20/hour |
For the middle example, 2.5 kW × 8 hours/day × 30 days = 600 kWh. At $0.18/kWh, the monthly estimate is $108. Four operating hours daily produces $54; twelve produces $162. The same equipment can therefore have very different bills in different weather or households.
Count operating time correctly
Eight hours with the thermostat enabled is not necessarily eight compressor operating hours. A fixed-output unit may cycle, while a variable-output system may operate longer at lower input. If you use average power over the entire observation period, do not multiply by a duty-cycle adjustment again. That would count cycling twice.
Write down which method you are using:
- Active input power multiplied by active operating hours.
- Average input power across a known period multiplied by all hours in that period.
- Directly measured kWh over the period, multiplied by the applicable rate.
The third method avoids an assumed average power draw, but accurate equipment-level metering may require professionally installed monitoring. Do not open an electrical panel or clamp conductors to obtain a homeowner estimate. Use appropriate existing monitoring, documentation, or a qualified electrician.
Find the relevant electricity price
For a simple estimate, add the utility’s variable energy and delivery charges per kWh, including applicable variable riders. Keep fixed account fees separate. With time-of-use pricing, calculate each rate period independently and add the results. A monthly bill divided by kWh can be useful as a rough blended price, but fixed fees may make it unsuitable for estimating the cost of one additional hour.
For example, 100 kWh at $0.15 and 50 kWh at $0.30 cost $30 in total. Using the lower rate for all 150 kWh would understate the result by $7.50. These are arithmetic examples rather than claims about any utility’s current tariff.
Why SEER2 cannot give an exact hourly bill
SEER2 is a seasonal cooling-efficiency rating, not a constant wattage label. DOE’s central-AC purchasing guidance uses efficiency and climate-specific assumptions for comparisons. Actual input varies with conditions, output, and installation.
A simple capacity-divided-by-efficiency calculation may support a rough seasonal comparison under stated assumptions, but it should not be presented as a measured hourly draw. Read SEER2 explained for the correct use of relative-efficiency estimates.
Turn the estimate into a useful decision
Record low, middle, and high runtime scenarios, then compare them with actual billed kWh as the season progresses. Note weather and thermostat changes. Include indoor blower electricity if your input estimate covers only the outdoor unit. Keep non-AC loads separate where possible.
If the estimate differs sharply from observed use, check assumptions before buying new equipment. A changed rate, additional household load, or hotter billing period may explain the gap. The bill increase after replacement guide provides a structured investigation. The goal is an estimate you can revise with evidence, not a precise-looking number built on an unknown wattage.