Watts to Joules Converter

Energy is power multiplied by time

A watt describes how fast energy flows: one watt moves one joule every second. A joule is the energy itself. To find out how much energy a device has used, you multiply its power by the time it ran.

A 40 W light switched on for one second has used 40 J, and the same light left on for a minute has used 2,400 J. The wattage never changed; only the time did. This is why the converter needs both a power and a duration before it can produce a number in joules.

The watts to joules formula

energy (J) = power (W) × time (s)Equivalent to E = P × t.

The converter first expresses the power in watts and the time in seconds, then multiplies, then rescales to your chosen energy unit:

• Power: milliwatts ÷ 1,000, kilowatts × 1,000 and megawatts × 1,000,000 give watts.
• Time: minutes × 60 and hours × 3,600 give seconds.
• Energy: joules ÷ 1,000 give kilojoules, and joules ÷ 1,000,000 give megajoules.

How to use the converter

1. Enter the power and choose milliwatts, watts, kilowatts or megawatts.
2. Enter the time and choose seconds, minutes or hours.
3. Pick the energy unit for the answer. The result is calculated as you type.

A blank power or time box leaves the answer empty. A time of zero is accepted and simply gives zero joules.

Worked examples

Example 1. A 60 W bulb stays on for 8 hours. The time is 8 × 3,600 = 28,800 s, so energy = 60 × 28,800 = 1,728,000 J, which is 1.728 MJ. As a bill would show it, that is 0.48 kWh.

Example 2. A 1.5 kW space heater runs for 20 minutes. Power is 1,500 W and time is 1,200 s, so energy = 1,500 × 1,200 = 1,800,000 J, or 1,800 kJ.

Example 3. A sensor that draws 250 mW is active for 3 minutes. Power is 0.25 W and time is 180 s, so energy = 0.25 × 180 = 45 J.

Watts to joules table for a 100 W load

Joules, watt-hours and kilowatt-hours

Electricity is sold in kilowatt-hours rather than joules. One watt-hour is 3,600 J (1 W for 3,600 s), and one kilowatt-hour is 3,600,000 J, or 3.6 MJ. To turn the joules from this converter into kilowatt-hours, divide by 3,600,000: the 1,728,000 J of the bulb example is 1,728,000 ÷ 3,600,000 = 0.48 kWh.

Rated power is what a device draws while working at full load. Real consumption over a day is usually lower, because most devices cycle on and off or idle part of the time.

Related tools

Use the joules to watts converter to find the average power from an energy and a time. If you only know voltage and current or resistance, start with the Ohm's law calculator to get the power in watts. Smaller scale changes are covered by the kilojoules to joules converter, and the rest of the family is on the physics converters page.