Watt-Hours Calculator
A 60-watt light bulb running for 2 hours uses 120 watt-hours of energy, which is 0.12 kWh. This watt-hours calculator converts between power (watts), time (hours), and energy (watt-hours) so you can estimate electricity use, battery capacity, or operating cost for any device. Enter any two of the three values and the tool solves for the missing one, plus it shows the kilowatt-hour equivalent for utility-bill comparisons. Watt-hours are the standard way to measure electrical energy over time. Your utility bill charges you per kilowatt-hour, your phone battery is rated in watt-hours, and your solar panel output is measured in kilowatt-hours per day. This calculator connects all three so you can move between device power ratings, run times, and energy totals without doing the multiplication by hand.
Quick answer
Watt-hours = watts x hours. A 100 W device running for 5 hours uses 500 Wh.
What this tells you
- •Watt-hours = watts x hours. A 100 W device running for 5 hours uses 500 Wh.
- •Kilowatt-hours = watt-hours / 1,000. Your utility bill uses kWh.
- •To find power: watts = watt-hours / hours.
- •To find time: hours = watt-hours / watts.
- •Battery capacity in watt-hours tells you total energy, not just how fast it can deliver power.
- •The calculator works in three modes: find energy, find power, or find time from the other two values.
How to Use
- 1Choose the mode that matches what you need to find. Most people use the default energy mode to find watt-hours.
- 2Enter the two values you know. For energy mode, enter the device's power in watts and the run time in hours.
- 3Click Calculate to see the energy in watt-hours and kilowatt-hours, along with all three quantities.
- 4For battery runtime, switch to time mode and enter the battery capacity in watt-hours and the device power in watts.
- 5To estimate cost, multiply the kilowatt-hour result by your utility rate per kWh.
How It Works
Formula
Wh = W x hours
kWh = Wh / 1,000
W = Wh / hours
hours = Wh / WThe core relationship is that energy equals power multiplied by time. A watt is a rate of energy use (joules per second), and multiplying by time in hours gives total energy in watt-hours. One kilowatt-hour is 1,000 watt-hours, which is the unit your electric utility uses for billing. The calculator can rearrange this relationship to solve for any one of the three variables. In energy mode, it multiplies watts by hours. In power mode, it divides watt-hours by hours. In time mode, it divides watt-hours by watts. All results display to 2 decimal places for watt-hours and 4 decimal places for kilowatt-hours.
Calculation note: values are processed in the order shown above, using the current input units.
Worked Examples
LED light bulb (60 W for 2 hours)
Multiply 60 watts by 2 hours to get 120 watt-hours. Dividing by 1,000 gives 0.12 kWh. At $0.15 per kWh, that costs about 1.8 cents to run.
Space heater (1,500 W for 3 hours)
1,500 watts times 3 hours is 4,500 watt-hours, or 4.5 kWh. Space heaters are among the highest-power household appliances, and this example shows why they add noticeably to a utility bill.
Phone battery capacity (find power draw)
A 15 Wh phone battery that lasts 10 hours means the phone draws an average of 1.5 watts. This helps you understand how different usage patterns affect battery life.
Battery runtime for a laptop (60 Wh battery, 30 W draw)
Divide 60 watt-hours by 30 watts to get 2 hours. This is the simplest way to estimate how long a battery will last under a known constant load.
Common Device Energy Use
Approximate wattage and energy consumption for everyday devices running for 1 hour, with kilowatt-hour equivalents.
| Device | Power (W) | Energy in 1 hour (Wh) | Energy in 1 hour (kWh) |
|---|---|---|---|
| LED light bulb | 10 | 10 | 0.01 |
| Laptop | 50 | 50 | 0.05 |
| Refrigerator (average) | 150 | 150 | 0.15 |
| Window AC unit | 1,000 | 1,000 | 1.0 |
| Electric water heater | 4,500 | 4,500 | 4.5 |
| EV charger (Level 2) | 7,200 | 7,200 | 7.2 |
Values are approximate averages. Actual power draw varies by model, settings, and usage pattern. Check the device label or specification sheet for exact wattage.
Common mistakes
- Confusing watts with watt-hours. Watts measure the rate of power use at one moment. Watt-hours measure total energy consumed over time. A 100 W bulb uses 100 Wh in one hour and 200 Wh in two hours.
- Using minutes instead of hours without converting. If a device runs for 30 minutes, enter 0.5 hours, not 30 hours.
- Forgetting to divide by 1,000 for kilowatt-hours. 5,000 Wh is 5 kWh, not 5,000 kWh.
- Assuming a device's rated wattage is its constant draw. Many devices cycle on and off (refrigerators, AC units) or vary by setting (fans, heaters), so actual energy use may differ from the rated value.
- Comparing battery capacities in amp-hours without considering voltage. Amp-hours times voltage equals watt-hours. Two batteries with the same amp-hour rating but different voltages store different amounts of energy.
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