About the kWh Calculator
A kilowatt-hour is the unit your electricity meter counts, and it is simply power multiplied by time. This calculator works both ways. Enter the power of a device and how long it runs, and it gives the energy in kWh, Wh, MWh, megajoules and BTU, plus what the same daily use adds up to in a week, 30 days and a year. Or enter an amount of energy and the time it was used over, and it returns the average power in watts and kilowatts. It is a pure maths tool that runs on your device, and it needs no prices or rates.
Updated September 2026
How to use the kWh Calculator
- 1Choose Energy to find kWh, or Power to find watts or kW.
- 2For energy, type the power and pick W, kW or MW, then type how long it runs in minutes, hours or days.
- 3For power, type the energy in Wh, kWh or MWh and the time it was used over.
- 4Read the main result and the other units underneath.
- 5To turn kWh into money, take the result to the Electricity Cost Calculator.
The one formula behind every conversion
Energy equals power times time. If you use watts and hours, the answer is in watt-hours, and dividing by 1,000 gives kilowatt-hours. A 1,500 W heater running for 2 hours uses 3,000 Wh, which is 3 kWh. A 100 W bulb left on for 10 hours uses exactly 1 kWh.
Going the other way, power equals energy divided by time. If a battery delivers 12 kWh over 4 hours, the average power is 3 kW, or 3,000 W. This is the direction to use when you want to size a generator or an inverter from a known amount of energy.
A common mix-up is kW versus kWh. A kilowatt measures how fast energy is used right now. A kilowatt-hour measures how much energy was used over time. A 2 kW oven uses 2 kWh each hour it runs at full power.
Reading the other units
- Wh and MWh are the same energy written in smaller and bigger steps: 1 kWh is 1,000 Wh and 0.001 MWh.
- Megajoules are the SI form. One kWh is 3.6 MJ.
- BTU is used for heating and cooling. One kWh is about 3,412 BTU.
Working out the energy of a real appliance
Use the watts on the appliance label as a starting point, but remember that many devices do not draw that power all the time. A fridge compressor runs in cycles, so its daily energy is well below its label power times 24 hours. For a true figure, use a plug-in energy meter.
When your appliance runs for less than a day, the tool shows week, 30-day and year totals on the assumption that you repeat the same use daily. It leaves those out when the time entered is longer than a day, because that would not be a daily figure. Once you have the kWh, multiply by your price per unit, or use Electricity Cost Calculator to see the cost. To turn the energy into money, use the Electricity Bill Calculator, and use the Energy Converter to move between kWh and other units.
Features
- Watts, kW and MW with minutes, hours or days
- Energy in kWh, Wh, MWh, MJ and BTU
- Average power from energy and time
- Week, 30-day and year totals for daily use
- Clear messages for empty or zero values
Frequently asked questions
How do I convert watts to kWh?
Multiply the watts by the hours and divide by 1,000. A 60 W fan running for 5 hours uses 60 × 5 ÷ 1,000 = 0.3 kWh.
How do I convert kW to kWh?
Multiply the kilowatts by the hours of use. A 3 kW heater on for 2 hours uses 6 kWh. The power is given in kW, so no further division is needed.
How do I convert kWh to watts?
You cannot convert directly, because kWh is energy and watts is power. Divide the energy by the time it was used over. Switch to Power mode, enter the kWh and the hours, and the average watts appear.
What is the difference between kW and kWh?
A kW is a rate of using energy, like speed. A kWh is an amount of energy, like distance travelled. Running 1 kW for one hour uses 1 kWh.
How many kWh is 1,000 watts for one hour?
It is exactly 1 kWh. That is how the unit is defined: 1,000 W running for 1 hour.
Why does my appliance use less than watts × hours?
Many appliances cycle on and off or run at lower power most of the time. The label gives the maximum, so the real energy is lower. A plug-in meter shows the actual use.
How many BTU is a kWh?
One kWh is about 3,412 BTU. The tool shows the BTU and megajoule equivalents of every result.