kWh vs kW: what the difference costs you

Kilowatts and kilowatt-hours are separated by one letter and by a difference in kind. One is a rate, the other is a quantity, and confusing them is why appliance labels feel uninformative and why battery and solar specifications are so easy to misread.

The distinction in one analogy

kWh = kW × hours

A 2 kW heater run for 30 minutes uses 1 kWh. Run for three hours it uses 6 kWh. Its kW rating never changed — only the time did. You are billed on the odometer, not the speedometer, which is why runtime matters more than the rating on the label.

Reading an appliance label

Rating plates state the maximum draw, not the average. A fridge labelled 150 W does not use 150 W continuously — its compressor cycles, so real consumption might average 40–50 W over a day. A kettle labelled 3 kW genuinely draws 3 kW, but only for the two minutes it is boiling.

Note the ordering. The kettle has the highest rating in the list and one of the lowest annual totals. The heater is the opposite. A high kW rating is only expensive if it runs for a long time.

Why heat dominates every bill

Anything whose job is to produce heat — heating, hot water, ovens, tumble dryers, kettles — is measured in kilowatts. Anything whose job is information — TVs, computers, phones, lighting — is measured in watts, a thousand times smaller.

This is physics rather than engineering laziness. Raising the temperature of matter takes an enormous amount of energy compared with switching transistors. It also means the standard advice to unplug idle electronics is largely theatre: a household's phantom load might be 30–50 W, roughly $50 a year, while a single hour less of electric heating a day saves three times that.

Batteries, solar, and the units that get mixed up

A 10 kWh battery rated at 5 kW output can run 5 kW of load for two hours, or 1 kW of load for ten. Both numbers are needed and neither implies the other. Buying on capacity alone leaves you unable to run the oven; buying on output alone leaves you out of power by midnight.

kWp on a solar quote is peak capacity in laboratory conditions, not output. A 4 kWp array in the UK produces roughly 3,400 kWh a year — about 850 kWh per kWp installed. In Arizona the same array produces nearly double. The ratio between them is the capacity factor, and it is what the quote should really be telling you.

Amp-hours need a voltage before they mean anything. A 100 Ah battery is 1.2 kWh at 12 V and 4.8 kWh at 48 V — same amp-hours, four times the energy. Compare batteries in kWh, never in Ah.

kVA and power factor on a commercial supply

Commercial customers meet a third unit. Apparent power (kVA) is what the supply has to carry; real power (kW) is what does useful work. The ratio is the power factor:

kW = kVA × power factor

Motors, transformers, and fluorescent ballasts draw current that lags the voltage, so some of the delivered current does no work but still has to be carried by the cables. A site at 0.8 power factor needs 125 kVA of supply to do 100 kW of work.

Domestic tariffs bill kWh only and ignore this entirely. Commercial tariffs frequently bill kVAh as well, or apply a penalty below a power factor threshold — which is why industrial sites install capacitor banks to correct it.

The √3 — about 1.732 — is why three-phase supplies move substantially more power through the same conductors, and why heavy equipment is specified for it.

Energy unit conversions

That last row is worth stating plainly, because it causes real confusion: when a supplier says "units", they mean kilowatt-hours. There is nothing else to convert.