The 3,600 That Connects Batteries to Physics
Energy is energy, whether a battery label calls it watt hours or a textbook calls it joules. The watt hour is defined through the joule: one watt is one joule per second, so one watt sustained for 3,600 seconds (one hour) is exactly 3,600 joules. That makes the conversion a definition, not a measurement — and it is why the numbers get big fast. A phone battery at 15 Wh is 54,000 J; an e-bike pack at 500 Wh is 1.8 MJ; a home battery at 13.5 kWh is 48.6 MJ. The joule is the SI unit every downstream calculation — thermodynamics, mechanical work, heat — expects, and this page supplies it.
joules = watt hours × 3,600
1 Wh = 3,600 J exactly
Definitional — from 1 W = 1 J/s and 1 hour = 3,600 s, zero uncertainty.
Battery Sizes in Joules
| watt hours | joules | What it powers |
|---|---|---|
| 15 Wh | 54,000 J | Smartphone battery. |
| 37 Wh | 133,200 J | The 10,000 mAh power bank. |
| 100 Wh | 360,000 J | Airline carry-on ceiling. |
| 500 Wh | 1,800,000 J (1.8 MJ) | E-bike and camping power pack. |
| 13,500 Wh | 48,600,000 J (48.6 MJ) | Home battery — a Tesla Powerwall class. |
The 100 Wh row is the one with legal weight: airlines cap carry-on batteries at 100 Wh, which is 360,000 J — and that joule figure is what airport and shipping regulations use when they rate batteries for transport. The 13,500 Wh row shows why the watt hour exists at all: 48.6 MJ is the honest physics number for a home battery, but nobody wants to shop for "48.6 megajoules of storage". The watt hour keeps consumer numbers legible; the joule keeps engineering honest; this page is the two-line bridge, and it plugs straight into the energy hub's Wh to kWh and kWh to joules chain.
Engineering Context
The reverse direction is joules to Wh. The watt hour lives at the center of the energy hub: Wh to kWh and kWh to Wh scale it to the electricity bill, kWh to joules / joules to kWh apply the same 3,600 factor at kilowatt scale, and Wh to Ah / Ah to Wh on the electric hub connect it to battery capacity. The mechanical work a joule represents — lifting, braking, heating — is a force and torque question. All roads lead to the energy hub.
More: J to Wh · Wh to kWh · kWh to J · Energy Hub
Related Unit Converters
Frequently Asked Questions
How do I convert watt hours to joules?
Multiply by 3,600: joules = watt hours × 3,600. A 500 Wh e-bike battery is 500 × 3,600 = 1,800,000 J = 1.8 MJ. The factor is exact: a watt is one joule per second, and an hour is 3,600 seconds, so 1 Wh = 1 W × 3,600 s = 3,600 J.
Why do batteries use watt hours instead of joules?
Because watt hours connect directly to the power ratings people actually use — a 100 W device draws 100 Wh in an hour, so a 500 Wh battery runs it for five hours. Joules are the SI energy unit but produce unmanageably large numbers at battery scale (500 Wh = 1.8 million J). Both are the same energy; the watt hour is the consumer-friendly name, the joule is the physics unit.
How many joules are in a kilowatt hour?
Exactly 3,600,000 J = 3.6 MJ, because 1 kWh = 1,000 Wh and each Wh is 3,600 J. This is the electricity-bill number in physics units: a typical US home uses about 30 kWh per day, which is 108 MJ — the energy of about 26 kg of gasoline burned. The kWh-to-joules conversion is the same factor scaled by 1,000.
More: joules to wh · wh to kwh · Energy Hub