Watt-hours to Joules Converter
Common Conversions
| Wh | J |
|---|---|
| 0.001 | 3.6 |
| 0.01 | 36 |
| 0.1 | 360 |
| 1 | 3600 |
| 5 | 18000 |
| 10 | 36000 |
| 100 | 360000 |
| 500 | 1800000 |
| 1000 | 3600000 |
| 5000 | 18000000 |
| 10000 | 36000000 |
Why this conversion matters in chemistry
Lithium-ion cell-chemistry calculations is the usual setting. A 21700-format cell rated at 15,000 mWh = 15 Wh on the datasheet converts to 54,000 J — entered into a specific-energy calculation (J/g or J/kg) when comparing NMC-811 chemistry against LFP against the theoretical Li-metal anode limit. The constant of 3600 J per Wh is exact through the SI definitions. The conversion lets the cell-level Wh rating land in the joule form Gibbs-energy and Pourbaix-style electrochemistry calculations actually use.
Formula
Where the factor comes from
Two definitions and one multiplication. The watt is the coherent SI unit of power, one joule per second; the hour is not an SI unit but is accepted alongside them, and it is exactly 60 × 60 = 3600 seconds. A watt sustained for an hour therefore delivers exactly 3600 J, with nothing measured anywhere in the chain. The same product has a charge-based reading that matters more at an electrochemistry bench: energy is charge times potential, so 1 Wh is 1 V × 1 Ah, and 1 Ah is 3600 C on that identical time factor. Divide the charge by the Faraday constant, 96485.33 C/mol, and a watt-hour delivered at one volt corresponds to 0.037311 mol of electrons — the bridge from a datasheet rating to a stoichiometric quantity.
Precision and significant figures
3600 contributes nothing, so every digit in the answer arrived with the rating — and cell ratings are nominal. A watt-hour figure on a datasheet is a typical value under one discharge condition; pull harder or run colder and the delivered energy moves by percent-level amounts, which puts two or three significant figures at the honest limit. 15 Wh becomes 54,000 J, and only the 5 and the 4 mean anything. Where the number genuinely tightens is instrumentation: a meter integrating power over a measured interval holds four or five figures, and a coulometric charge measurement more still. Match the digits to whatever produced the watt-hours, and keep the factor out of that accounting entirely.
Worked Examples
The conversion anchor — 1 watt for 1 hour.
About a typical 18650 cell at 3.7 V × 2 Ah.
1 kWh in joules — about a household-scale energy unit.
About the energy budget of a coin cell.
Common mistakes
Watt-hours and watts are different quantities
A watt is a rate and a watt-hour is an amount. An instrument drawing 60 W has not consumed 60 Wh until it has run for an hour; over a twenty-minute measurement it consumes 20 Wh, or 72,000 J. Multiplying a power rating by 3600 with no time in hand produces joules per hour dressed as joules, and the size of the error tracks however long the run actually was.
Milliamp-hours are charge, not energy
A cell marked 2000 mAh states charge. Multiplying by 3600 gives 7200 C, which is correct and is not joules. Reaching energy needs the potential: 2 Ah at a nominal 3.7 V is 7.4 Wh, or 26,640 J. Skipping the voltage step leaves the result roughly 3.7 times too small for this chemistry, and differently wrong for any other.
Delivered joules are not −ΔG
The joules a cell actually hands over sit below the thermodynamic ceiling set by nFE°, because working voltage falls under load through polarization and internal resistance. Converting a rated 54,000 J and setting it straight against a Gibbs energy computed from standard potentials compares a real discharge with a reversible one. That gap belongs to the electrochemistry, not to the unit factor.