Energy unit conversions
Energy management runs on the kilowatt-hour: electricity is billed in it and UK gas bills convert to it, so once everything is in kWh, energy, cost and carbon all line up. These are the conversions for everything else you will meet.
Last reviewed July 2026. Figures are indicative working values for first-pass estimates; confirm anything compliance- or design-critical against the cited source.
Conversions to kWh
The one worth memorising: 1 kWh = 3.6 MJ, because 1 kW for 3,600 seconds delivers 3,600,000 joules.
| Unit | In kWh | Notes |
|---|---|---|
| 1 kWh | 1 | The everyday unit; billed on every meter |
| 1 MJ (megajoule) | 0.278 | 1 kWh = 3.6 MJ |
| 1 GJ (gigajoule) | 277.8 | Common in district heating and industry |
| 1 therm | 29.31 | Older gas unit (100,000 Btu) |
| 1 Btu | 0.000293 | 1 Btu ≈ 1.055 kJ |
| 1 toe (tonne of oil equivalent) | 11,630 | Large energy totals and national statistics |
SI prefixes
Prefix errors are the classic factor-of-a-thousand mistake. Sanity-check any figure's scale against something you already know.
| Prefix | Symbol | Factor | Example |
|---|---|---|---|
| kilo | k | ×1,000 | A 3 kW kettle |
| mega | M | ×1,000,000 | A 500 MWh annual site total |
| giga | G | ×10⁹ | A 2 GWh factory; 1 GJ of heat |
| tera | T | ×10¹² | National statistics (TWh) |
Power vs energy
kW is a rate (never 'per hour', that is already baked in); kWh is a quantity. Energy = power × time: a 10 kW heater for 3 hours is 30 kWh.
| Quantity | Unit | Analogy |
|---|---|---|
| Power | kW | Speedometer: the rate right now |
| Energy | kWh | Odometer: the total that accumulates |
| Capacity (electrical) | kVA | The supply's carrying capacity, incl. reactive power |