Calculate monthly kWh from watts and usage, or solve power, daily hours, or days per month with energy-cost, daily, and annual breakdowns.
kWh Per Month Formula
The main formula for monthly energy use is:
- kWh_month = kilowatt-hours used per month
- P_kW = power in kilowatts
- H_day = hours used per day
- D_month = days used per month
- N = number of identical appliances (defaults to one)
If power is entered in watts, the calculator first converts watts to kilowatts:
The calculator can also rearrange the same relationship to solve for a missing value. Enter the identical appliance count in every mode; the examples below use one appliance. An optional USD rate estimates energy charges only. Daily energy is per used day; the annual projection repeats 12 identical months.
- Calculate kWh per month: enter power, hours per day, and days per month.
- Calculate power: enter monthly energy use, hours per day, and days per month.
- Calculate hours per day: enter monthly energy use, power, and days per month.
- Calculate days per month: enter monthly energy use, power, and hours per day.
Common Power and Energy Unit Conversions
Use these conversions when checking the units in your result.
| Quantity | Conversion | Example |
|---|---|---|
| Watts to kilowatts | 1 kW = 1,000 W | 500 W = 0.5 kW |
| Kilowatt-hours to watt-hours | 1 kWh = 1,000 Wh | 2.4 kWh = 2,400 Wh |
| Megawatt-hours to kilowatt-hours | 1 MWh = 1,000 kWh | 0.02 MWh = 20 kWh |
| Minutes to hours | hours = minutes / 60 | 30 minutes = 0.5 hours |
| Device or Load | Illustrative Input Power | Monthly Use at 3 Hours/Day, 30 Days |
|---|---|---|
| LED light bulb | 10 W | 0.9 kWh |
| Laptop | 60 W | 5.4 kWh |
| Television | 100 W | 9 kWh |
| Space heater | 1,500 W | 135 kWh |
Example Problems
Example 1: Calculate kWh per month
You use a 1,500 W space heater for 4 hours per day on 20 days in a month.
The heater uses 120 kWh per month.
Example 2: Calculate hours used per day
A device uses 18 kWh per month. Its power is 300 W, and it is used 30 days per month.
The device is used for 2 hours per day.
FAQ
How do you calculate kWh per month from watts?
Convert watts to kilowatts, then multiply by hours used per day and days used per month. For example, a 500 W device is 0.5 kW. If it runs 6 hours per day for 30 days, the monthly use is 0.5 × 6 × 30 = 90 kWh.
Is kWh the same as kW?
No. A kilowatt, or kW, is a measure of power. A kilowatt-hour, or kWh, is a measure of energy used over time. A 1 kW device running for 1 hour uses 1 kWh of energy.
Should you use 30 or 31 days for a month?
Use the number of days the device is actually used within a calendar month, up to 31. For a rough monthly estimate, 30 days is common. Split longer billing periods into months. Annual results repeat the entered month 12 times and do not assume 365 operating days.
