kilowatt-hour
Energyexact by definition
The kilowatt-hour is one kilowatt sustained for one hour, exactly 3 600 000 joules or 3.6 megajoules. The relation is exact because the hour is exactly 3600 seconds. It is the unit electricity is sold in almost everywhere, and it is an energy, not a power.
Watch out: Kilowatt and kilowatt-hour are different quantities and the difference is the most common utility-bill confusion. A kilowatt is a rate, what an appliance draws while running. A kilowatt-hour is an amount, what it consumes over time. A 2 kW heater running for 3 hours uses 6 kWh. Writing kW/h for either is wrong, since that would be a rate of change of power.
| 1 kWh | 3,600,000 J |
About the kilowatt-hour
The kilowatt-hour survives because it fits human timescales. A joule is far too small for household accounting, and a megajoule is unfamiliar, but a kilowatt-hour is roughly what a kettle uses in an hour, or a fridge in a day, and it costs a recognisable amount of money. Multiplying a power in kilowatts by a duration in hours to get kilowatt-hours involves no conversion constants, which is why utilities never migrated to the joule.
The arithmetic is worth doing once. A 100 W bulb left on for 10 hours consumes 1 kWh. A 1500 W space heater runs for 40 minutes on the same kilowatt-hour. An electric car with a 60 kWh pack that uses 18 kWh per 100 km has a range near 330 km. Because 1 kWh is exactly 3.6 MJ, converting to fuel comparisons is easy: a litre of gasoline holds about 34 MJ, or about 9.4 kWh of chemical energy, though an internal combustion engine wastes most of it as heat while an electric motor does not.
Note the related unit on utility bills for demand charges, which is a kilowatt of peak draw rather than a kilowatt-hour of consumption. Commercial customers are often billed for both, one for how much energy they used and one for how hard they pulled at their worst moment.