What a kilowatt-hour actually costs you

A kilowatt-hour is watts times hours divided by a thousand, so a 1,000 W heater running three hours a day uses 3 kWh — 91 a month and 1,096 a year, which at 28p a unit is about £307. The arithmetic is trivial and the useful part is the horizon: hourly costs are easy to dismiss and annual ones are not.

A device costing 3p an hour and left on permanently costs £263 a year. Stated as 3p, nobody acts on it.

What does a unit actually buy?

One kilowatt-hour runs quite different amounts of quite different things, which is what makes the unit hard to have intuition about.

The spread across that table is a factor of several hundred, which is why "turn the lights off" is worth much less than most people assume and "run the heating an hour less" is worth much more.

Why is the bill higher than the calculation?

Because rated wattage is not running wattage. Most appliances do not draw their plate rating continuously, and the ones that do are the exception rather than the rule.

• A fridge is rated at its compressor power and runs perhaps a third of the time.
• A washing machine draws most of its energy heating the water and very little for the rest of the cycle.
• A television rated at 120 W might average half that on ordinary content.
• Anything thermostatic — heating, cooling, kettles, ovens — cycles, and the duty cycle swamps every other variable.

That is what a duty cycle is, and it is the difference between an estimate and a measurement. A 150 W compressor running eight hours a day all year uses 438 kWh, which is a realistic fridge-freezer figure despite the 150 W plate rating suggesting 1,314.

It also means the same appliance uses different amounts in different places. A freezer in a cold garage runs a fraction as often as the same freezer in a warm kitchen.

What rate should you use?

Your own, from a bill, including standing charges only if you are comparing whether to own the appliance at all. For comparing two appliances, the unit rate alone is the right figure — the standing charge is the same either way.

Time-of-use tariffs change the question rather than the arithmetic. Where a rate varies by hour, the calculation runs per period and the interesting decision becomes when to run something rather than whether to.

Does standby power matter?

Less than it used to and not nothing. Modern regulations cap standby at a fraction of a watt for most equipment, so a device drawing 0.5 W costs about £1.20 a year at 28p.

What still matters is the things that are not really on standby: a games console in a fast-resume mode, a set-top box that never sleeps, a desktop left running. Those draw tens of watts continuously, and 30 W left on all year is 263 kWh, or £74.

The other half of a tariff comparison is what the rate is attached to. A cheap unit rate with a high standing charge suits a heavy user and penalises a light one, and the crossover is arithmetic rather than opinion: divide the difference in standing charge by the difference in unit rate to get the annual consumption at which the two tariffs cost the same.

How do you measure it properly?

A plug-in energy monitor, left in place for a week. It integrates the actual draw over time and removes every assumption about duty cycle in one step, which no amount of arithmetic can do for a cycling appliance.

For anything hard-wired, the meter itself is the instrument: read it, run nothing else for an hour, and read it again. It is crude and it is a direct measurement of the thing you are billed for.

Questions people ask

How much does it cost to run a 2 kW heater? Four hours a day at 28p is £2.24 daily, £68 a month and £818 a year. Electric resistance heating is expensive because it is the only form of heating with no multiplier — every unit in is a unit of heat out, where a heat pump moves three or four.

Do appliances use their rated power constantly? Almost none of them. Rated power is the maximum, and for thermostatic appliances the average is a fraction of it.

How do I find real running hours? Estimate first, then measure with a plug monitor if the answer matters. Estimates of running hours are the largest source of error in any of these calculations.

Is LED lighting really cheaper? For the fitting, yes and dramatically — a 10 W LED replacing a 60 W incandescent cuts that fitting by 83 per cent. For the household, modestly, because lighting is rarely the largest load. A 48 W strip run five hours a day uses 87.6 kWh a year, about £25.

Work in years rather than hours and the decisions become obvious. The kWh calculator converts watts and hours into units, the electricity cost calculator turns those into money at your rate, the energy consumption calculator handles duty cycle over a year, and the LED power calculator sizes a strip and its driver.