kWh per Month Calculator
A 1,500 W space heater used 4 hours a day for 30 days burns 180 kWh, which is 27 at a price of 0.15 per kWh.
Quick answer: Multiply the watts by the hours per day and the days, then divide by 1,000. For example, 1,500 W for 4 h a day over 30 days is 180 kWh, 60 W for 5 h a day over 30 days is 9 kWh and 2,000 W for 6 h a day over 30 days is 360 kWh.
The kWh per month formula
Monthly energy in kWh equals watts times hours per day times days, divided by 1,000. Multiply the result by your price per kWh to get the cost.
This is the same three step method that the US Department of Energy uses for appliances (Energy.gov: Estimating Appliance and Home Electronic Energy Use).
Take 1,500 W for 4 h a day over 30 days. 1,500 × 4 × 30 ÷ 1,000 gives 180 kWh.
Take 60 W for 5 h a day over 30 days. 60 × 5 × 30 ÷ 1,000 gives 9 kWh.
Watts, hours and days
Watts tell you how fast a device uses energy. Multiply by the hours it runs to get watt-hours, and divide by 1,000 to get kWh.
Do this per day first, then multiply by the days in the month. That keeps the numbers easy to check.
How to convert kWh per month step by step
- Find the watts on the label or with a watt meter.
- Estimate the hours of use per day.
- Multiply the watts by the hours and the days.
- Divide by 1,000 to get kWh, then multiply by your price for the cost.
Example 1: 1,500 W for 4 h a day over 30 days
- Start with 1,500 W for 4 h a day over 30 days.
- 1,500 × 4 × 30 ÷ 1,000 = 180.
- The monthly energy is 180 kWh.
1,500 W for 4 h a day over 30 days = 180 kWh
Example 2: 60 W for 5 h a day over 30 days
- Start with 60 W for 5 h a day over 30 days.
- 60 × 5 × 30 ÷ 1,000 = 9.
- The monthly energy is 9 kWh.
60 W for 5 h a day over 30 days = 9 kWh
Example 3: 2,000 W for 6 h a day over 30 days
- Start with 2,000 W for 6 h a day over 30 days.
- 2,000 × 6 × 30 ÷ 1,000 = 360.
- The monthly energy is 360 kWh.
2,000 W for 6 h a day over 30 days = 360 kWh
The last column shows the kWh per day, which helps you compare devices on the same scale.
kWh per month conversion table
| Device and use | Energy per month | Per day |
|---|---|---|
| 60 W for 5 h a day over 30 days | 9 kWh | 0.3 kWh per day |
| 100 W for 8 h a day over 30 days | 24 kWh | 0.8 kWh per day |
| 200 W for 24 h a day over 30 days | 144 kWh | 4.8 kWh per day |
| 800 W for 2 h a day over 30 days | 48 kWh | 1.6 kWh per day |
| 1,000 W for 3 h a day over 30 days | 90 kWh | 3 kWh per day |
| 1,500 W for 4 h a day over 30 days | 180 kWh | 6 kWh per day |
| 1,800 W for 1 h a day over 30 days | 54 kWh | 1.8 kWh per day |
| 2,000 W for 6 h a day over 30 days | 360 kWh | 12 kWh per day |
| 3,500 W for 0.5 h a day over 30 days | 52.5 kWh | 1.75 kWh per day |
| 5,000 W for 3 h a day over 30 days | 450 kWh | 15 kWh per day |
The highlighted row is the default of the calculator. The last column gives kWh per day.
A quick mental shortcut for kWh per month
Work per day first. Watts times hours divided by 1,000 gives kWh per day. Then multiply by 30.
A 1,000 W device that runs 1 hour a day uses about 30 kWh in a month.
The three step formula
Energy.gov gives the method as watts times hours per day, divided by 1,000, then times the days, then times the rate. The page works an electric kettle example: 1,500 W for 1 hour a day (Energy.gov: Estimating Appliance and Home Electronic Energy Use).
That gives 1.5 kWh a day, and 45 kWh over 30 days.
Cost at different prices
At 0.10 per kWh, 180 kWh costs 18. At 0.15 it costs 27, and at 0.30 it costs 54.
Check your bill for the price you pay, since it may change by tier or time of day.
Standby power
Many devices draw a small amount of power when switched off. Energy.gov calls this a phantom load, and each device adds only a little (Energy.gov: Estimating Appliance and Home Electronic Energy Use).
A device that draws 5 W all month uses 3.6 kWh, so many of them add up.
Devices that cycle
A fridge or an air conditioner switches on and off, so its real use is lower than its label watts times 24 hours. Energy.gov notes a watt meter is helpful for devices that cycle (Energy.gov: Estimating Appliance and Home Electronic Energy Use).
Measure over a day or more and use the kWh reading.
Month length
A month may have 28 to 31 days. A 1,200 W device used 2 hours a day uses 67.2 kWh in 28 days and 74.4 kWh in 31 days.
Use the real number of days in your billing period.
Going the other way
To find the watts that a monthly figure implies, multiply the kWh by 1,000 and divide by the total hours of use.
For 180 kWh over 120 hours, the power is 1,500 W.
Check your answer
Divide your monthly kWh by the days, then by the hours per day, then multiply by 1,000. You should get the watts back.
For 180 kWh over 30 days and 4 hours a day, the result is 1,500 W.
Quick conversion answers
| Conversion | Result |
|---|---|
| 10 W for 24 h a day over 30 days | 7.2 kWh |
| 40 W for 6 h a day over 30 days | 7.2 kWh |
| 120 W for 10 h a day over 30 days | 36 kWh |
| 500 W for 3 h a day over 30 days | 45 kWh |
| 1,200 W for 2 h a day over 31 days | 74.4 kWh |
| 2,500 W for 1 h a day over 28 days | 70 kWh |
Where this conversion is used
Households find which appliances drive the electricity bill.
Landlords and renters estimate the cost of a heater or air conditioner for a season.
Small businesses budget the energy use of shop equipment.
Common mistakes to avoid
- Using hours per month as hours per day. Enter the hours per day, because the calculator multiplies by the days.
- Leaving out the divide by 1,000. Watt-hours are 1,000 times bigger than the kWh answer.
- Assuming full power all day. Fridges and heaters cycle, so their real use is lower.
- Forgetting standby loads. A few watts all month add up across many devices.
Related converters
Frequently asked questions
How do I calculate kWh per month?
Multiply watts by hours per day and by days, then divide by 1,000.
How many kWh is a 1,500 W heater for 4 hours a day?
It is 180 kWh over 30 days.
How do I find the cost?
Multiply the kWh by your price per kWh.
What is a phantom load?
It is power a device draws while switched off or on standby.
Why is my real use lower than the answer?
Many devices cycle on and off, so they do not run at full watts all the time.
Can I measure the real use?
Yes. A plug-in watt meter records the kWh of 120 V devices over time.
Sources
- Energy.gov: Estimating Appliance and Home Electronic Energy Use (energy.gov)
- Wikipedia: Watt (en.wikipedia.org)
