kWh Calculator

This kWh calculator turns power and time into energy. Enter a device’s watts, how many hours it runs each day and over how many days, and it returns the total kilowatt-hours (kWh) along with watt-hours, megawatt-hours and the cost at your electricity price. It is the quickest kilowatt hour calculator for checking appliances, solar output or generator loads.

How kWh is calculated

A kilowatt-hour (kWh) is the energy used by one kilowatt of power for one hour. It is not itself an SI unit, but the BIPM SI Brochure accepts the hour for use with the SI, and one kWh equals exactly 3.6 MJ. The formula is:

kWh = watts × hours ÷ 1,000

1. Energy per day = watts × hours per day ÷ 1,000
2. Total energy = energy per day × number of days
3. Watt-hours = kWh × 1,000; megawatt-hours = kWh ÷ 1,000
4. Cost = kWh × price in ¢/kWh ÷ 100

Worked example with the default inputs

Defaults: 1,000 W, 5 hours a day, 30 days, 18.2¢/kWh.

• Energy per day: 1,000 × 5 ÷ 1,000 = 5.000 kWh
• Over 30 days: 5 × 30 = 150.00 kWh
• In watt-hours: 150,000 Wh
• In megawatt-hours: 0.15 MWh
• Cost: 150 × 18.2 ÷ 100 = $27.30

kWh conversion reference table

How long common power levels take to use one kWh, and what that kWh costs at the US average of 18.2¢ (EIA, August 2026).

Power	Time to use 1 kWh	kWh per 8-hour day	Cost per 8-hour day
10 W (LED bulb)	100 hours	0.08	$0.01
100 W	10 hours	0.8	$0.15
500 W	2 hours	4	$0.73
1,000 W (1 kW)	1 hour	8	$1.46
1,500 W	40 minutes	12	$2.18
3,500 W	about 17 minutes	28	$5.10

Energy units compared

Unit	Equals
1 kWh	1,000 Wh
1 MWh	1,000 kWh
1 kWh	3,412.14 BTU (NIST SP 811)
1 kWh	3.6 megajoules

What affects the result

Actual versus rated watts. The label shows maximum draw. Thermostat-driven devices cycle, so their real daily kWh is lower. A plug-in meter measures the true figure.

Hours of use. Small errors in hours scale the result directly. Estimate across a typical week, not a single day.

Days per month. This calculator uses the days you enter. Our other electricity calculators use the average month of 30.42 days.

Standby. Devices in standby still use a few watts around the clock, which adds up across a home.

How to find watts when the label shows amps

Many chargers, tools and appliances list only volts (V) and amps (A). Multiply the two to get watts: a device rated 120 V and 5 A draws up to 600 W. A 240 V, 30 A circuit can carry up to 7,200 W. For motors and some electronics, the real power is a little lower because of the power factor, so the result is a ceiling rather than the typical draw. The amps to kW converter handles single-phase and three-phase circuits and power factor for you.

Common kWh questions answered quickly

• Is a kWh the same as a kW per hour? No. A kWh is power multiplied by time, not a rate of change.
• Does a 2,000 W device always use 2 kWh per hour? Only if it runs flat out. Heaters with thermostats cycle and use less.
• Can I use this for solar or generator output? Yes. Enter the average output in watts and the hours it runs. To size a whole solar array, use the solar panel calculator.
• Can I use it for a battery? For stored energy, multiply the battery’s volts by its amp-hours and divide by 1,000 (12 V × 100 Ah = 1.2 kWh), or use the Ah to kWh converter. For how long it will run a load, use the battery runtime calculator.

Putting kWh in context

The average US home used 855 kWh a month in 2023 (EIA Table 5A), about 28 kWh a day. Your 150 kWh example is roughly 18% of that. To see what each appliance contributes, use the electricity usage calculator. For cost per day, month and year, use the electricity cost calculator. For a single power-and-time conversion, the watts to kWh converter is the shortest route.

History of the watt and the kilowatt-hour

The watt is named after the Scottish engineer James Watt. C. William Siemens proposed it as the unit of electrical power in August 1882, in his presidential address to the British Association for the Advancement of Science (Watt (unit)). The same year, Edison’s Pearl Street Station opened, and the new utilities needed a way to bill customers for energy. By 1889 the Ganz Works (Ottó Bláthy’s design) and Elihu Thomson had produced recording watt-hour meters (electricity meter history), and the kilowatt-hour became the commercial unit of electricity. The watt joined the SI in 1960, and today’s definitions are set out in the BIPM SI Brochure.

Year	Milestone
1882	Siemens proposes the watt; Pearl Street Station opens
1889	First commercial watt-hour meters (Bláthy, Thomson)
1960	11th CGPM adopts the watt as the SI unit of power

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

Want the background? Read what a kilowatt-hour is, with kW versus kWh examples. Knowing your kWh is also the first step to sizing renewable energy such as solar panels for your home.