Key numbers
- kWh = W × h ÷ 1,000
- 1 kWh: 100 W for 10 hours, or 1,000 W for 1 hour
- 18.2¢: cost of 1 kWh at the US residential average (EIA, August 2026)
- Watts measure power; kilowatt-hours measure energy
On this page
- How to use the Watts to kWh
- How to convert watts to kWh
- How many watts are in a kWh?
- Watts to kWh conversion table
- Converting kWh back to watts
- Tips for accurate results
- Step-by-step: estimate a device’s monthly kWh
- Watts vs kWh: what’s the difference?
- A short history of the watt
- References
- Do watts affect your electric bill?
- Frequently asked questions
How to use the Watts to kWh
- 1Enter your numbersFill in power (W) and time (hours). Every field starts with a typical value you can overwrite.
- 2Check the assumptionsWatts measure power (a rate); kWh measure energy (power × time). kWh = W × h ÷ 1,000.
- 3Read the resultsThe calculator shows energy, watt-hours, hours one kwh lasts at this power and cost at the us average price, and updates as you type.
- 4Use the resultCompare it with your bill, a quote or a product spec sheet before you buy or install anything.
Results update as you type. Back to the calculator ↑
Use this watts to kWh calculator to turn a device’s power rating into the energy it uses over a period of time. That is the number your utility bills you for. Enter the watts and the hours, and it returns kilowatt-hours, watt-hours, how long one kWh lasts and the cost at the US average electricity price.
How to convert watts to kWh
The W to kWh formula is:
kWh = watts × hours ÷ 1,000
Watts (W) measure power, the rate at which a device uses energy; one watt is one joule per second (NIST SP 811). A kilowatt-hour (kWh) is energy: 1,000 watts used for one hour, or 3.6 MJ. The hour is accepted for use with the SI (BIPM SI Brochure). Multiplying by time turns a rate into an amount, and dividing by 1,000 turns watt-hours into kilowatt-hours.
Worked example with the default values
- Power: 100 W
- Time: 10 hours
- Energy: 100 × 10 = 1,000 Wh; 1,000 ÷ 1,000 = 1 kWh
- One kWh lasts 1,000 ÷ 100 = 10 hours at this power
- Cost at 18.2¢/kWh: 1 × $0.182 = $0.18
The 18.2¢ default is the US residential average from the EIA Electric Power Monthly (18.16¢, August 2026).
How many watts are in a kWh?
There are 1,000 watt-hours in a kWh, but no fixed number of watts. A kilowatt-hour is 1,000 watts running for one hour, or any equal product of power and time: 100 W for 10 hours, 50 W for 20 hours or 2,000 W for 30 minutes. Watts are a rate; kWh are an amount.
| Power | Time to use 1 kWh |
|---|---|
| 50 W | 20 hours |
| 100 W | 10 hours |
| 500 W | 2 hours |
| 1,000 W | 1 hour |
| 2,000 W | 30 minutes |
Looking for watts in a kilowatt (power to power)? That is exactly 1,000; see the kW to watts converter.
Watts to kWh conversion table
Energy used per hour, per 8-hour day and per 24-hour day:
| Power | kWh per hour | kWh in 8 hours | kWh in 24 hours | Cost of 24 h at 18.2¢ |
|---|---|---|---|---|
| 10 W | 0.01 | 0.08 | 0.24 | $0.04 |
| 60 W | 0.06 | 0.48 | 1.44 | $0.26 |
| 100 W | 0.1 | 0.8 | 2.4 | $0.44 |
| 500 W | 0.5 | 4 | 12 | $2.18 |
| 1,000 W | 1 | 8 | 24 | $4.37 |
| 1,500 W | 1.5 | 12 | 36 | $6.55 |
| 3,000 W | 3 | 24 | 72 | $13.10 |
Converting kWh back to watts
Reverse the formula: watts = kWh × 1,000 ÷ hours. If a device used 6 kWh over 4 hours, its average draw was 6 × 1,000 ÷ 4 = 1,500 W. Utility bills and smart meters report kWh, so this tells you the average power of what was running.
Tips for accurate results
- Use real running watts. Nameplate ratings are maximums. Fridges, air conditioners and heaters cycle on and off, so their average draw is lower than the label.
- Count standby power. TVs, consoles and chargers draw a few watts all day; over 24 hours that adds up.
- Measure. A plug-in watt meter shows real watts and accumulated kWh.
- Kilowatts first? If the rating is in kW, multiply by 1,000 or use the kW to watts converter.
Pro tip
Kilowatt-hours are also how solar panels, wind turbines and home batteries are measured, so the same unit runs through everything on this site, from household bills to renewable energy production. For a plain-English explanation of the unit, read what is a kilowatt-hour.
Step-by-step: estimate a device’s monthly kWh
- Find the watts on the label, or measure with a plug-in meter.
- Estimate the hours it runs each day.
- Multiply watts by hours and divide by 1,000 for daily kWh.
- Multiply by 30 for a monthly figure, and by your price per kWh for the cost.
For example, a 60 W monitor used 8 hours a day uses 0.48 kWh a day, or about 14.4 kWh a month, costing about $2.62 at 18.2¢/kWh.
Watts vs kWh: what’s the difference?
Watts measure power, not energy. Kilowatt-hours measure energy, the power used over time. A watt is one joule per second, so a watt is a rate, like speed, while a kWh is an amount, like distance travelled.
| Unit | Measures | Example |
|---|---|---|
| Watt (W) | Power, rate of use | A 10 W LED bulb |
| Kilowatt (kW) | Power, 1,000 W | A 1.5 kW heater |
| Watt-hour (Wh) | Energy | A 74 Wh power bank |
| Kilowatt-hour (kWh) | Energy, 1,000 Wh | The unit on your bill |
A short history of the watt
The unit honours James Watt, the steam-engine pioneer. C. William Siemens proposed the watt as the unit of electrical power in 1882, and the 11th General Conference on Weights and Measures adopted it into the SI in 1960 (Watt (unit)). Billing in watt-hours followed the first recording watt-hour meters of 1889.
References
- BIPM (2019). The International System of Units (SI Brochure), 9th edition. Bureau International des Poids et Mesures. bipm.org
- Thompson, A. & Taylor, B. N. (2008). Guide for the Use of the International System of Units (SI). NIST Special Publication 811. doi:10.6028/NIST.SP.811e2008
Do watts affect your electric bill?
Only through the hours they run. Utilities charge for the total energy delivered over the billing period, not for how much power a device can draw at once. A 1,500 W hair dryer used for 10 minutes a day costs less than a 60 W monitor left on for 10 hours, because the dryer uses 0.25 kWh and the monitor 0.6 kWh. Always multiply by time.
FAQ
Frequently asked questions
How do I convert watts to kWh?
Multiply watts by the number of hours the device runs, then divide by 1,000. A 1,500 watt space heater running 4 hours uses 1,500 × 4 ÷ 1,000 = 6 kWh. Watts alone cannot be converted to kWh; you always need a time.
How much does 1,000 watts cost per hour?
1,000 watts for one hour is exactly 1 kWh. At the US average residential price of about 18.2 cents per kWh (EIA, August 2026), that is about 18 cents per hour. Multiply by your own bill rate for an exact figure.
What is the difference between W and Wh?
W (watt) is power, how fast energy is used at a moment. Wh (watt-hour) is energy, the total used over time. A 60 watt bulb uses 60 Wh each hour. 1,000 Wh equals 1 kWh, the unit on your electric bill.
How many kWh does a 100 watt device use per day?
Running all day, 100 watts × 24 hours ÷ 1,000 = 2.4 kWh per day, about 73 kWh per month. At 18.2 cents per kWh that costs about 44 cents a day. Most devices do not run continuously, so use real hours of use.
How do I work out the cost from watts?
Convert to kWh first, then multiply by your price per kWh. A 1,500 watt heater for 4 hours uses 6 kWh; at the US average of 18.2 cents per kWh (EIA, August 2026) that costs about $1.09. The electricity cost calculator does this per day, month and year.
How do I convert kWh to watts?
Multiply the kWh by 1,000 and divide by the hours. If something used 6 kWh in 4 hours, its average power was 6 × 1,000 ÷ 4 = 1,500 watts. Without a time period, kWh cannot be turned into watts.
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Last updated October 25, 2026. Part of our renewable energy toolkit. Found an error? Tell us.