Watt-hours to joules:
multiply by 3,600

How many joules are in a watt-hour?

One watt-hour equals exactly 3,600 joules. The number is exact because a watt is one joule per second and an hour has 3,600 seconds, so a device using one watt for an hour uses 3,600 joules.

Watt-hours are the practical unit on power banks, laptop batteries and small solar kits. Joules are the SI unit used in physics and engineering. The conversion is a plain multiplication, with no rounding involved.

The Wh to joules formula

The logic is easy to remember: Wh is power times hours, and the 3,600 turns hours into seconds. Multiply the watt-hours by 3,600 and you have joules.

Wh to J conversion chart

These rows follow directly from the factor of 3,600.

Battery and device examples

Many airlines limit lithium batteries carried in cabin baggage to 100 Wh, which is 360,000 J. A 50 Wh camera battery holds 180,000 J, and a 250 Wh portable power station holds 900,000 J.

The reverse is useful in lab and physics work. A result of 90,000 J is 90,000 ÷ 3,600 = 25 Wh, and 1,000,000 J is about 277.78 Wh. A kilowatt-hour is 1,000 Wh, so it is 3,600,000 J.

A second worked example from a phone and a bulb

Everyday devices make good practice. A phone battery of about 12 Wh holds 12 × 3,600 = 43,200 J, and a 40 Wh laptop battery holds 144,000 J. A 20 Wh tablet battery is 72,000 J. All of these are small next to a kilowatt-hour of mains electricity.

A second worked example uses power and time. A 10 W LED bulb that runs for half an hour uses 10 × 0.5 = 5 Wh, which is 18,000 J. Check it the other way: 10 watts is 10 joules per second, and 30 minutes is 1,800 seconds, so 10 × 1,800 = 18,000 J.

If a pack lists only milliamp-hours, find the watt-hours first. A 5,000 mAh pack at 3.7 V is 5 × 3.7 = 18.5 Wh, so it holds 18.5 × 3,600 = 66,600 J. Quote the joule figure only after you have the voltage, because mAh alone does not describe energy.

Bulbs, phones and laptops in joules

A 10 W LED bulb that stays on for 1 hour uses 10 Wh, which is 36,000 J. Left on for 5 hours it uses 50 Wh, or 180,000 J.

A phone battery of 4,000 mAh at 3.85 V holds about 15.4 Wh, which is 55,440 J. A laptop battery of 50 Wh holds 180,000 J, or 180 kJ.

The factor is simply the seconds in an hour, because a joule is one watt for one second. A kilowatt-hour is 1,000 Wh, so it equals 3,600,000 J, which is why electricity is billed in kWh rather than in joules.

Common mistakes when converting Wh to joules

• Using 60 instead of 3,600. That gives watt-minutes, not joules.
• Confusing watt-hours with watts. A watt is a rate, a watt-hour is an amount of energy.
• Mixing up Wh with mAh. Battery packs labelled in mAh need the voltage first: Wh = mAh ÷ 1,000 × volts.
• Forgetting that kWh is 1,000 Wh, so a kWh is 3,600,000 J, not 3,600 J.