Calcylator
Kilowatt-hours to Joules

kWh to joules:
multiply by 3,600,000

Convert kilowatt-hours to joules for electricity bills, batteries and physics homework, with a chart, worked examples and common mistakes.

Calcylator Editorial Team

Updated · 5 min read

How many joules are in a kilowatt-hour?

One kilowatt-hour is exactly 3,600,000 joules, which is 3.6 megajoules. The figure comes from the definitions: 1 kW is 1,000 watts, 1 watt is 1 joule per second, and an hour is 3,600 seconds.

Multiplying 1,000 by 3,600 gives the 3.6 million. Because every part is an exact definition, the conversion has no rounding at all.

The kWh to joules formula

Kilowatt-hours to joules =J = kWh × 3,600,000
J:
energy in joules
kWh:
energy in kilowatt-hours
3,600,000:
joules in one kilowatt-hour (exact)
To go the other way, divide: kWh = J ÷ 3,600,000.

  • Energy

    1.5 kWh

  • Multiply

    1.5 × 3,600,000

Energy in joules

5,400,000 J

That is 5.4 megajoules.

In reverse, 45 megajoules is 45,000,000 ÷ 3,600,000 = 12.5 kWh. A quick check is that one megajoule is about 0.278 kWh, so a few megajoules is a fraction of a kWh.

  1. Write the energy in kWh as a decimal.
  2. Multiply by 3,600,000.
  3. For joules to kWh, divide by 3,600,000.
  4. Use megajoules (divide by a million) to keep the numbers short.

Chart: kilowatt-hours to joules from 1 to 1000

Large values are shown in scientific notation, so 1.8e+9 means 1,800,000,000.

Kilowatt-hours to joules
Kilowatt-hoursJoules
13600000
27200000
518000000
1036000000
2590000000
50180000000
100360000000
250900000000
5001.8e+9
10003.6e+9

A heater running at 1 kW for one hour uses one kWh, and 3.6 million joules of energy are released as heat.

Where kWh to joules comes up

Electricity bills are in kWh, but physics problems and engineering data sheets use joules. A household using 2.5 kWh of electricity for cooking has used 2.5 × 3,600,000 = 9,000,000 J, or 9 MJ.

Battery sizes work the other way round. A phone battery rated 15 Wh holds 0.015 kWh, which is 54,000 J, since a watt-hour is 3,600 joules.

Appliance makers who print kJ on a label can be compared with a kWh meter this way: 3 kWh is 10,800,000 J, or 10,800 kJ.

If you ever need the cost per joule, divide the tariff per kWh by 3,600,000. A tariff of 0.30 per kWh works out to roughly 0.00000008 per joule, which shows why bills use kWh instead.

Common mistakes converting kWh to joules

  • Mixing up kW and kWh. A kilowatt is power, in joules per second, and a kilowatt-hour is energy in joules.
  • Using 1,000 instead of 3,600,000. The factor of 1,000 turns kWh into Wh, and the hours need the extra 3,600.
  • Dividing when you should multiply. A joule is tiny, so a kWh is millions of joules.
  • Slipping a zero. 3.6 million has six zeros, and one more or less is a factor of ten.

Common questions

How many joules are in 1 kWh?

One kilowatt-hour is exactly 3,600,000 joules, or 3.6 megajoules. It is the energy used by a 1,000 watt device running for one hour, since a watt is one joule per second and an hour has 3,600 seconds.

How do I convert kWh to joules?

Multiply the kilowatt-hours by 3,600,000. For example, 2.5 kWh is 2.5 × 3,600,000 = 9,000,000 J, or 9 megajoules. To go back to kilowatt-hours, divide the joules by 3,600,000 instead.

How many kWh is 1 megajoule?

One megajoule is about 0.2778 kWh, found by dividing 1,000,000 by 3,600,000. Put the other way, 3.6 megajoules make exactly 1 kWh, which is why 3.6 is the number worth remembering.

Is a kilowatt-hour a unit of power or energy?

A kilowatt-hour is a unit of energy. A kilowatt is power, the rate of using energy. Multiplying power by time gives energy, so 1 kW for one hour is 1 kWh, equal to 3,600,000 joules.

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