Kilowatt-hours to Joules Converter

A kilowatt-hour is what you get when 1,000 watts run for one hour. A watt is one joule per second and an hour is 3,600 seconds, so 1 kWh = 1,000 × 3,600 = 3,600,000 J. The factor is exact, with nothing to round.

That also gives you the easiest mental shortcut in energy conversion: kWh × 3.6 = megajoules. 2.5 kWh is 9 MJ, which the converter shows as 9000000 J.

Electricity is usually billed in kWh, while physics, engineering and many data sheets work in joules. Converting lets you put them side by side:

• A 2 kW heater running for 3 hours uses 6 kWh, which is 21600000 J.
• 0.001 kWh, one watt-hour, is 3600 J.
• 1 kWh is 860.42 kcal if you set the To menu to kilocalories.
• A 2,000 kcal day of food energy, run the other way, is 2.3244 kWh.

The last line puts the scale in perspective: a whole day's food energy is only a couple of kilowatt-hours, about what that 2 kW heater uses in a little over an hour.

Joule results get long quickly and are shown without thousands separators, so count the digits carefully: 10 kWh appears as 36000000. Once the answer reaches a billion joules, which happens from 277.78 kWh upward, the display switches to scientific notation. 1000 kWh shows as 3.6000e+9, meaning 3.6 times 10 to the ninth power.

For shorter numbers, set the To menu to kilojoules: 10 kWh is 36000 kJ. Megajoules and gigajoules are not in the menu, so for those, divide the joule result by a million or a billion.

Mixing the two is the classic mistake. kW is a rate, how fast energy is used, while kWh and joules are amounts of energy. You cannot convert kW to joules without a time: 1 kW for one second is 1,000 J, but 1 kW for an hour is 3,600,000 J. If a figure on a spec sheet has no hours attached, it is power, and this page is the wrong tool for it.