Physics · Glossary

What is The kilowatt-hour?

Also known as: kilowatt-hour

Definition 68.6 Primary & Middle School Physics · Chapter 68 — Electric Power and Energy

Household energies make ungainly joule-counts, so the meter speaks kilowatt-hours ( $\mathrm{kWh}$ ): the energy of one kilowatt sustained for one hour,

1\,\mathrm{kWh} = 1000 \times 3600 = 3.6 \times 10^{6}\,\mathrm{J}

— three point six million joules per unit on the bill. With $P$ in kilowatts and $t$ in hours, $E = P \times t$ delivers kilowatt-hours directly: the formula the meter lives by.

Energy as area: power times time. The kettle’s tall sliver and the television evening’s long strip can enclose comparable areas — comparable kilowatt-hours.

Examples

Example 68.8 (Reading the bill)

The meter by the front door counts every kilowatt-hour that enters the house; the bill is its month’s difference times the tariff. A typical family’s $250$ monthly units decompose by audit: water heating and radiators first (the kilowatt club), then the cold appliances — modest watts, but running always — then cooking, washing, light and electronics. The audit’s power: it finds the real levers. Replacing ten old $60\,\mathrm{W}$ bulbs with $5\,\mathrm{W}$ ones saves more than unplugging every charger in the house a thousand times over.

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Definition 9.17 High School Physics · Chapter 9 — Energy: Forms and Conservation

The kilowatt-hour ( $\mathrm{kW}\,\mathrm{h}$ ) is the energy of a $1\,\mathrm{kW}$ flow running for one hour: $1\,\mathrm{kW}\,\mathrm{h} = 1000 \times 3600\,\mathrm{J} = 3.6\,\mathrm{MJ}$ — the unit electricity meters count: power in $\mathrm{kW}$ times hours.

An energy ladder: each rung is a factor of 100. Everyday life spans ten orders of magnitude. — An energy ladder: each rung is a factor of $100$ . Everyday life spans ten orders of magnitude.

Examples

Example 9.18 (Reading the bill)

A bill is a subtraction and a multiplication: new meter reading minus old gives the kilowatt-hours; times the unit price (about $0.25$ euros per $\mathrm{kW}\,\mathrm{h}$ ), plus a fixed subscription, gives the total. A $2\,\mathrm{kW}$ heater running $2.5\,\mathrm{h}$ nightly uses $5\,\mathrm{kW}\,\mathrm{h}$ a day: $38$ euros a month on its own.

Example 9.19 (Orders of magnitude)

Worth knowing by heart: lifting an apple one metre, about $1\,\mathrm{J}$ ; a phone battery, $10\,\mathrm{W}\,\mathrm{h} \approx 4 \times 10^{4}\,\mathrm{J}$ ; a cereal bowl, $1.5\,\mathrm{MJ}$ ; your food for a day, $10\,\mathrm{MJ}$ ; a kilogram of gasoline, $45\,\mathrm{MJ}$ — almost exactly a household’s daily electricity ( $\approx 12\,\mathrm{kW}\,\mathrm{h}$ ); a lightning bolt, $5\,\mathrm{GJ}$ : a hundred household-days, delivered in a flash.

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Examples

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