Introduction
A watt hour (Wh) tells you how much energy a device uses. It is power multiplied by time. If you run a 100-watt light for 10 hours, it uses 1,000 watt hours, or 1 kilowatt hour (kWh). That kWh number is what your power company charges you for.
This Watt Hour Calculator does the math for you. Type in the power of your device and how long it runs. You get the answer in watt hours, kilowatt hours, and megawatt hours right away. Add your electric rate, and it also shows what the device costs to run. If you prefer to work straight in kilowatt hours, our kWh Calculator handles the same math on a larger scale.
You can pick a common device from the list, like an LED bulb or a kettle, if you do not know the wattage. You can enter power in watts, kilowatts, horsepower, or BTU/h, and time in seconds, minutes, hours, days, or weeks. Not sure of the wattage at all? The Wattage Calculator and the Amps to Watts Calculator can help you work it out from the label. You can also switch the answer to joules, BTU, or other energy units.
Below the results you will see the steps, a chart of energy building up over time, and a table showing what the same run would cost each week, month, and year. Use it to compare devices, plan battery or solar panel needs, or find what is driving up your power bill.
How to use our Watt Hour Calculator
Enter how much power your device uses and how long it runs. The calculator shows the energy used in watt-hours (Wh), kilowatt-hours (kWh), and megawatt-hours (MWh), plus the cost to run it, a step-by-step solution, and a chart.
Select a Device (Optional): Pick a common device, like an LED bulb or a microwave, to fill in a typical wattage for you. Choose "Custom" to type your own number.
Power: Type the running watts of your device. You can find this on the label, the power brick, or the manual. Use the drop-down to switch units to mW, kW, MW, hp, or BTU/h. If your label lists horsepower instead, the Horsepower Calculator shows how that converts to watts.
Time: Type how long the device runs. Pick seconds, minutes, hours, days, or weeks from the drop-down next to the box. To count exact run hours between two clock times, try the Hours Calculator.
Electricity Cost ($/kWh): Type the rate you pay per kilowatt-hour. It is on your power bill. Leave it blank if you only want the energy answer. For a full monthly bill breakdown across several appliances, use the Electricity Cost Calculator.
Show energy in another unit: After you calculate, pick a unit like joules, kilojoules, BTU, kilocalories, or foot-pounds to see the same energy a different way. Our Unit Converter Calculator covers other unit swaps you may need.
Click Calculate to see your results, or Reset to clear all the boxes and start over.
What Is a Watt Hour?
A watt hour (Wh) is a unit of energy. It tells you how much power a device used over time. One watt hour means a device pulled 1 watt of power for 1 hour.
Watts measure power, which is how fast energy is used right now. Watt hours measure energy, which is the total amount used. Think of watts as speed and watt hours as distance traveled. The Power Calculator works the other direction, solving for watts when you already know the energy and the time.
The Watt Hour Formula
The math is simple:
Watt hours = Watts × Hours
A 60 W fan running for 5 hours uses 60 × 5 = 300 Wh of energy.
Watt Hours vs. Kilowatt Hours
A kilowatt hour (kWh) is just 1,000 watt hours. Power companies bill you in kilowatt hours, not watt hours. To switch between them, divide watt hours by 1,000. The kW Calculator is handy when you are working with larger loads measured in kilowatts.
So 300 Wh = 0.3 kWh. A megawatt hour (MWh) is 1,000 kWh, or one million watt hours. Big buildings and power plants use that unit.
How to Find the Cost
Multiply your kilowatt hours by the price your utility charges per kWh. If you use 0.3 kWh and pay $0.16 per kWh, the run costs about 5 cents.
Your rate is printed on your power bill. In the U.S., most homes pay somewhere between $0.10 and $0.30 per kWh. To see how a whole house adds up, the Power Consumption Calculator totals several devices at once.
Where to Find a Device's Wattage
Look at the label on the back or bottom of the device, or check the power brick. It may list watts directly. If it only lists volts and amps, multiply them: Volts × Amps = Watts. The Ohms Law Calculator covers the full relationship between volts, amps, ohms, and watts, and the Watts to Amps Calculator goes the other way if you need current draw for wiring or breakers.
Use the running wattage, not the startup or surge number. Motors and fridges pull extra power for a second when they turn on, but that short burst adds very little energy. Surge watts do matter when you size backup power, so check the Generator Sizing Calculator for that job.
Typical Device Wattage
- LED bulb: about 10 W
- Laptop: about 50 W
- Refrigerator: about 150 W (but it cycles on and off)
- Desktop computer: about 200 W (size a supply with the PSU Calculator)
- Microwave: about 1,000 W
- Electric kettle: about 2,000 W
Other Energy Units
Watt hours can be written in other units too. One watt hour equals 3,600 joules (J). One BTU is about 0.293 Wh, and the BTU Calculator is useful for heating and cooling loads. Batteries often list watt hours to show how much charge they hold, and airlines use that number to set carry-on limits.
Why It Matters
Knowing watt hours helps you compare devices, size a solar system or battery bank, and see which appliances raise your power bill the most. High-wattage items that run a long time cost the most. A 10 W bulb left on all day uses less energy than a 1,500 W heater running one hour. If you want to see how that electricity use translates into emissions, the Carbon Footprint Calculator puts a number on it.