Hong Life CalculatorsUnderstanding the air conditioner running cost calculator — concept and principle
This calculator estimates what an air conditioner costs to run per month and per year from its electrical power, the hours you use it, the electricity price and the number of days. It helps you compare units, judge the effect of using the air conditioner fewer hours, and plan a summer electricity budget.
How it is calculated
- kWh per month = power (W) ÷ 1,000 × hours per day × days per month.
- Monthly cost = kWh per month × price per kWh.
- Yearly cost = monthly cost × 12.
Power must be the electrical input in watts from the label, not the cooling capacity. A unit with 3.5 kW of cooling and an efficiency (COP) of 3 draws only about 1.17 kW of electricity.
The principle
Electrical energy is power multiplied by time. One kilowatt running for one hour uses one kilowatt-hour (kWh), the unit on your bill. Dividing watts by 1,000 converts to kilowatts; multiplying by hours gives kWh per day, and by days gives the monthly total. An air conditioner is a heat pump: it moves heat out of the room rather than creating cold, so its cooling output is usually two to four or more times its electrical input. That ratio, shown as EER, COP or seasonal ratings such as SEER, is why two units of the same cooling size can cost very different amounts to run.
Worked example
- Inputs: 1,200 W, 8 hours a day, 0.25 per kWh, 30 days (USD).
- Energy per day: 1,200 ÷ 1,000 × 8 = 9.6 kWh.
- Per month: 9.6 × 30 = 288 kWh.
- Monthly cost: 288 × 0.25 = $72.00.
- Yearly cost: 72 × 12 = $864.00.
| Power (W) | Monthly cost |
|---|---|
| 300 | $18.00 |
| 600 | $36.00 |
| 900 | $54.00 |
| 1,200 | $72.00 |
| 1,500 | $90.00 |
| 1,800 | $108.00 |
| 2,400 | $144.00 |
How to read the result
The calculation assumes the unit draws its full rated power every hour, which is the worst case. Inverter units slow their compressor once the room reaches the set temperature, so their average draw is often well below the rating. The yearly figure also assumes every month is like this one; for seasonal cooling, multiply the monthly cost by the number of months you actually use it. In tiered tariffs, extra kWh may be billed at a higher rate than your average price.
Common mistakes
- Entering cooling capacity (BTU/h or kW of cooling) instead of electrical input watts.
- Using the average bill price when the extra kWh falls in a higher tier.
- Forgetting standby power and fan-only operation.
Frequently asked questions
Does raising the set temperature save electricity?
Yes. A higher set point reduces the heat that must be removed, so the compressor runs less.
Is it cheaper to leave the air conditioner on all day?
Usually not. Turning it off when a room is empty for hours generally saves energy, although short breaks matter less with inverter units.
How do I convert BTU/h to kW?
Divide by about 3,412. A 12,000 BTU/h unit has about 3.5 kW of cooling capacity.
Sources: U.S. Department of Energy, Energy Saver: Estimating Appliance and Home Electronic Energy Use · ISO 16358-1, Air-cooled air conditioners and air-to-air heat pumps — Testing and calculating methods for seasonal performance factors