The formula
kWh = J × 2.777777778 × 10⁻⁷
Where the factor comes from — each unit by its definition, then its size in SI:
1 J is the SI unit of energy.1 kWh = 1000 × Wh = 3,600,000 J
Dividing the two SI sizes gives the factor: 2.77777777778 × 10⁻⁷ (rounded for display; the underlying ratio is fixed by the definitions). The reverse factor is 3,600,000.
The SI unit of energy is the joule. Food labels use kilocalories, electricity bills kilowatt-hours, heating specs BTU and particle physics electronvolts — the same quantity in five costumes.
Worked examples
- A day's food (2000 kcal): 8,368,000 J = 2.32444 kWh — 8,368,000 × 2.777777778 × 10⁻⁷ = 2.32444
- A 1 kW heater run for one hour: 3,600,000 J = 1 kWh — 3,600,000 × 2.777777778 × 10⁻⁷ = 1
- Lifting 1 kg through 1 m at standard gravity: 9.80665 J = 2.72407 × 10⁻⁶ kWh — 9.80665 × 2.777777778 × 10⁻⁷ = 2.72407 × 10⁻⁶
- A visible-light photon (2 eV): 3.20435 × 10⁻¹⁹ J = 8.90098 × 10⁻²⁶ kWh — 3.20435 × 10⁻¹⁹ × 2.777777778 × 10⁻⁷ = 8.90098 × 10⁻²⁶
Frequently asked questions
How many kilowatt-hours is 1 J?
1 J = 2.77777777778 × 10⁻⁷ kWh.
How do I convert kilowatt-hours back to joules?
Divide by 2.77777777778 × 10⁻⁷ — or multiply by 3,600,000: 1 kWh = 3,600,000 J.