What a kilowatt-hour is

If one thing is worth understanding before anything else on this subject, it is the kilowatt-hour. It is the unit your bill is counted in, the unit a solar quotation promises, and the unit every saving is made of, and almost nothing else on the subject makes sense without it.

Two different things, easily confused

There are two numbers, and they get muddled constantly.

A kilowatt (kW) is a rate. It is how fast something uses or makes energy, right now. A kettle is about 3 kW while it is boiling. A fridge is about 0.1 kW. A heat pump might be running at 2 kW on a mild afternoon.

A kilowatt-hour (kWh) is an amount. It is what you get if something runs at one kilowatt for one hour. That kettle at 3 kW, boiling for ten minutes, uses half a kilowatt-hour.

The comparison that usually lands: a kilowatt is like miles per hour, and a kilowatt-hour is like miles. One is how fast you are going; the other is how far you have gone. Nobody would confuse a speed with a distance, but the units here look so similar that people do it all the time.

A "unit" is a kilowatt-hour

Your electricity bill probably says "units". A unit is a kilowatt-hour. They are the same thing, and bills use both words, sometimes on the same page.

So when a tariff says 27p per unit, it means 27p for every kilowatt-hour. That is the number to hold on to, because it is what turns every energy figure on this site into money.

Getting a feel for the size of one

Roughly, one kilowatt-hour will do any of these:

• Boil a full kettle about six times
• Run a fridge for most of a day
• Run a washing machine through about one cycle
• Light a room with LED bulbs for a fortnight of evenings
• Drive an electric car about four miles
• Heat about a third of a bathful of water, if you are heating it with electricity

An average British household uses something like 2,700 kilowatt-hours of electricity a year, and rather more than that again in gas if it has gas heating. A family house with electric heating might use four or five times as much.

Where you will meet it

Once you have the unit, the numbers on a quotation stop being a wall.

"This system will generate 3,800 kWh a year." That is a quantity of electricity, roughly what an average household uses in a year. How much of it turns into money depends on how much the house is using at the moment it is generated, since the rest is sold back, but the kilowatt-hours themselves are a real, checkable number.

"Your house needs 11,000 kWh of heat a year." That is the heat, not the fuel. A boiler burning gas loses some of it up the flue, so the gas going in is more than the heat coming out.

"This heat pump has a seasonal efficiency of 3.5." That means three and a half kilowatt-hours of heat for every one of electricity. Multiply your heat demand by the electricity price and divide by 3.5, and you have a running cost.

One more place the two get confused

Solar panels are sold in kilowatts and generate kilowatt-hours.

A "4 kW system" describes the rate at which the panels would produce in bright, cool, ideal conditions — a maximum, essentially. What you actually care about is what it produces over a year, which is a kilowatt-hour figure, and in Britain a 4 kW system makes somewhere between about 3,000 and 4,000 kilowatt-hours depending on where you are and which way the roof faces.

So the kilowatt number is the size of the tap and the kilowatt-hour number is how much came out of it over the year. Both appear on quotations, and only the second one is worth money.

Heat pumps are the same. An "8 kW heat pump" is a rate: the most heat it can put out at once, which matters on the coldest day. How much it costs to run depends on the kilowatt-hours it uses over a whole winter.

Why this is worth ten minutes

Most of the confusion around home energy comes from these two numbers being swapped, sometimes carelessly and occasionally on purpose. A figure that sounds impressive as a rate can be unimpressive as an amount, and the other way round.

Once you can tell which one you are being shown, a quotation becomes something you can check, where before it was something you had to believe.