Watts to kWh
With watts, kw or kwh 1500, hours 3 h, price per kwh 0.17 USD, watts to kwh comes to 4.500 kWh — kilowatt-hours. It is reached in 6 steps, the last of which is 1.5 * 3, and each one is printed on the page with its numbers filled in. The formula is the one published by US EIA, not an approximation fitted to it.
Watts is a rate and kWh is an amount, so the conversion needs a duration. This gives the energy, the running cost and what the meter will show.
Formula and sources checked · How we check
I know the Power in watts — find the energy, Watts, kW or kWh 1500, Hours 3 h, Price per kWh 0.17
4.500 kWh
Kilowatt-hours for the example below. Editing a field recomputes the calculator below; this figure holds the answer the page was loaded with.
It is written into the HTML rather than drawn by a script, so a search engine reading this page without running JavaScript still finds an answer.
- Power
1 * (1500 / 1000) + 0 * 1500 + 0 * (1500 / 3)1.5 kW- Power
1.5 * 10001,500 W- Energy used
1.5 * 34.5 kWh- The same energy in megajoules
4.5 * 3.616.2 MJ- Cost at that rate
4.5 * 0.170.765 USD- Cost if that happened every day for a year
0.765 * 365279.225 USD
Worked example
A 1,500 W heater run for three hours uses 4.5 kWh and costs 77 cents at 17 cents a unit. Run every evening through a winter, that is $69 for the season and $279 if it ran all year.
How to work it out yourself
- 1.Give the hours. Watts is a rate, kWh is a quantity, and no conversion between them exists without a duration — this is the whole reason the question has to be asked rather than looked up.
- 2.For an appliance that cycles, use the hours it actually runs rather than the hours it is switched on. A fridge is on all day and its compressor runs perhaps eight hours of it.
- 3.kW to kWh needs no arithmetic beyond the multiplication: a 2 kW load for 30 minutes is 1 kWh. The conversion people search for is really the multiplication people forget to do.
- 4.Going the other way, kWh ÷ hours gives the average power — which is how a meter reading becomes a wattage.
Watts to kWh over three hours
| Watts, kW or kWh | Kilowatt-hours | Cost at that rate |
|---|---|---|
| 10 | 0.030 kWh | 0.01 USD |
| 50 | 0.150 kWh | 0.03 USD |
| 100 | 0.300 kWh | 0.05 USD |
| 200 | 0.600 kWh | 0.1 USD |
| 500 | 1.500 kWh | 0.26 USD |
| 800 | 2.400 kWh | 0.41 USD |
| 1000 | 3.000 kWh | 0.51 USD |
| 1200 | 3.600 kWh | 0.61 USD |
| 1500 | 4.500 kWh | 0.77 USD |
| 2000 | 6.000 kWh | 1.02 USD |
| 3000 | 9.000 kWh | 1.53 USD |
A kilowatt-hour is a thousand watts for one hour, so a 100 W bulb takes ten hours to use one. The cost column is at 17 cents, close to the 2025 US residential average of 17.45 cents reported by the EIA.
The formula
- Power
1 * (1500 / 1000) + 0 * 1500 + 0 * (1500 / 3) - Power
1.5 * 1000 - Energy used
1.5 * 3 - The same energy in megajoules
4.5 * 3.6 - Cost at that rate
4.5 * 0.17 - Cost if that happened every day for a year
0.765 * 365
Source: US EIA — average retail price of electricity to residential customers, BIPM SI Brochure — the joule, and the kilowatt-hour as 3.6 MJ, US EIA — Residential Energy Consumption Survey, measured appliance use
Questions people actually ask
- How do you convert watts to kWh?
- Multiply by the hours and divide by a thousand. A 1,500 W heater for three hours is 1500 × 3 ÷ 1000 = 4.5 kWh. Without the hours there is no answer: watts is a rate of use and kWh is an amount used.
- What is the difference between kW and kWh?
- kW is how fast energy is being used, kWh is how much has been used. A 2 kW kettle draws 2 kW whether it runs for a second or an hour; only the kWh changes. Bills are in kWh, nameplates are in kW.
- How many watts is 1 kWh?
- The question needs a time. One kWh is 1,000 W for one hour, 100 W for ten hours, or 40 W for 25 hours. Divide 1,000 by the hours to get the wattage.
- How much does it cost to run a 1500 W heater?
- 25.5 cents an hour at the 2025 US residential average of about 17 cents per kWh, so 77 cents for a three-hour evening. State prices range from roughly 11 cents in Utah to over 40 in Hawaii, which changes that figure by a factor of nearly four.
- What is a kWh in joules?
- 3.6 million — 3.6 MJ. A watt is a joule per second, so a kilowatt-hour is 1,000 joules per second for 3,600 seconds. The kilowatt-hour survives on bills because 3.6 MJ is an awkward number to price.
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