Joules to Watts Calculator

You cannot convert joules to watts

Not because it is difficult, but because they measure different kinds of thing. A joule is an amount of energy. A watt is a rate — energy per unit of time. Asking how many watts are in a joule is like asking how many miles per hour are in a mile.

1 W = 1 J / s

What connects them is time. Divide an amount of energy by how long it took and you get the average power over that period. That is the calculation the tool above does: fill in any two of energy, time and power, and it works out the third.

A worked example. A 60 W bulb left on for an hour uses 60 joules every second for 3,600 seconds, which is 216,000 joules. Run the same bulb for two hours and the energy doubles while the power stays at 60 W. Power describes how fast; energy describes how much.

The working, step by step

Every line updates with the numbers above.

Common uses and tips

Where this comes up

• Electricity bills. You are billed for energy in kilowatt-hours, while appliances are rated in watts. A 2 kW heater running for three hours uses 6 kWh.
• Batteries. Capacity is in watt-hours or amp-hours — an amount. Draw is in watts — a rate. Dividing one by the other gives runtime.
• Physics coursework, where the distinction between work done and power delivered is the point of the question.
• Exercise and cycling, where a power meter reads watts and the session total comes out in kilojoules.

Units that look like power but are energy

• Kilowatt-hour (kWh) — energy. One kilowatt sustained for an hour: 3,600,000 J, or 3.6 MJ.
• Watt-hour (Wh) — energy. 3,600 J.
• Watt (W) — power. A rate, not an amount.

The hyphen is doing a lot of work there. A watt-hour is a watt multiplied by an hour, which turns a rate back into an amount.

Tips worth knowing

• If a question gives you joules and asks for watts, it must also give you a time. If it does not, the question is incomplete.
• Power is the derivative of energy. Average power is total energy divided by total time; instantaneous power is the rate at a particular moment.
• To get kWh from joules, divide by 3,600,000.

Quick reference

Energy	Over	Average power
3,600 J	1 second	3,600 W
3,600 J	1 minute	60 W
3,600 J	1 hour	1 W
216,000 J	1 hour	60 W
3,600,000 J (1 kWh)	1 hour	1,000 W
8,368,000 J	1 day	96.9 W

The last row is a person's daily food intake spread over 24 hours, which works out at roughly the power of a bright old-fashioned light bulb.

Questions people ask

How do I convert joules to watts?

You cannot, without a time. Watts are joules per second, so divide the energy in joules by the number of seconds it took.

What is the formula?

P = E ÷ t. Power in watts equals energy in joules divided by time in seconds.

How many joules in a watt?

The question has no answer as asked. One watt is one joule per second, so a watt sustained for one second delivers one joule.

What is a kilowatt-hour in joules?

3,600,000 J, or 3.6 MJ. A kilowatt-hour is energy, not power.

Is a watt-hour the same as a watt?

No. A watt is a rate; a watt-hour is that rate sustained for an hour, which is an amount of energy — 3,600 J.