Use the overnight average, not only rated watts
An air conditioner can draw more while the compressor runs and less when it cycles off or modulates. Multiplying the highest input by the hours creates a constant-power scenario, not necessarily an overnight average.
If a suitable meter or built-in report shows 4 kWh from bedtime to morning across eight hours, the period-average input was 500 W: 4 ÷ 8 × 1,000. For the cost, you can multiply the measured 4 kWh directly by the applicable electricity rate.
Calculate one night and a 30-night month
At 500 W average for eight hours, energy is 0.5 × 8 = 4 kWh. At $0.18/kWh, the night costs $0.72. Repeating that exact night 30 times produces 120 kWh and $21.60.
That monthly result is a sensitivity scenario, not a weather forecast. A mild night may use less energy and a hotter or more humid night may use more. A timer that switches the unit off changes the schedule; a thermostat that cycles changes the average input.
When a tariff changes by time of day, split the measured energy across the relevant price periods if your data supports that detail. Do not apply one off-peak price to the whole night when part of the session is billed differently.
| Average input | Energy/night | Cost/night at $0.18 | 30 equal nights |
|---|---|---|---|
| 300 W | 2.4 kWh | $0.43 | $12.96 |
| 500 W | 4.0 kWh | $0.72 | $21.60 |
| 800 W | 6.4 kWh | $1.15 | $34.56 |
Keep comfort and safety outside the cheapest row
A lower set point, blocked airflow, dirty filter or poorly sealed installation can change runtime and comfort. Follow the appliance instructions for installation, filter care and operating limits. Do not choose an unsuitable overnight setting only because one calculated row costs less.
People, pets and health conditions can have different heat risks. An electricity estimate cannot set a safe indoor temperature or replace appropriate public-health and product guidance.
Create a three-night record before projecting a season
Record start and end time, starting room conditions, set point, fan mode, outdoor conditions and kWh. Repeat on several nights that reflect the range you expect. Separate weekday and weekend schedules when they differ.
Use low, middle and high overnight energy cases in a seasonal budget rather than treating one night as every night. The record also makes it easier to explain why the whole electricity bill changed by a different amount from the AC-only estimate.
Choose your next step
Continue with the question that matches your next decision:
Sources & further reading
Numerical examples are illustrative unless explicitly described as measured. This guide does not claim hands-on product testing. Read the methodology and editorial disclosure.