A Morning in the Kitchen
It is seven o'clock on a grey morning. You fill the kettle, press the switch, and in the quiet of the kitchen the element starts to hiss. A little more than two minutes later there is a click, a cloud of steam, and a cup of tea is on its way. Nothing about this feels scientific. Still, that small ritual holds the two ideas that sit under every climate and energy headline you will ever read. One is how much. The other is how fast.
Turn the kettle over and look at the base. Most British kettles are stamped with something like 3,000 W, or 3 kW. That number is not an amount of anything. It is a rate. It tells you how quickly the kettle draws energy from the wall while it is switched on. This is power, and we measure it in watts.
The thing the kettle actually uses up is energy. Scientists measure energy in joules. One joule is tiny, roughly the effort of lifting an apple a metre off a table. A watt is simply one joule every second. So a 3 kW kettle swallows 3,000 joules each second it runs.
The Bathtub and the Tap
Picture a bathtub. Energy is the water sitting in the tub. Power is how far the tap is opened. A wide-open tap fills the bath quickly and a trickle fills it slowly, but given enough time the trickle can fill it just the same. That is why a small appliance left on all day can use more energy than a big one that runs for a moment.
Joules are too small for household bills, so electricity companies use a bigger bucket called the kilowatt-hour, written kWh. It is exactly what it sounds like: one kilowatt of power kept up for one hour. That works out to 3.6 million joules. Notice how the unit is built. Power multiplied by time gives energy. Remember that sum and most of this lesson follows from it.
Back to the kettle. Bringing a litre of cold water to the boil takes roughly a tenth of a kilowatt-hour. The kettle is powerful, at 3 kW, but it runs for only a couple of minutes, so it uses little energy. A 10-watt LED bulb is gentle by comparison, yet left burning for ten hours it uses the same tenth of a kilowatt-hour. Different taps, same bathful.
Climbing the Ladder of Prefixes
Headlines rarely stop at kilowatts. They climb a ladder of prefixes, and each rung is a thousand times the one below. A thousand watts make a kilowatt. A thousand kilowatts make a megawatt, a million watts. A thousand megawatts make a gigawatt, a billion watts. A thousand gigawatts make a terawatt. The same rungs work for energy, so you can step from kilowatt-hours to megawatt-hours, gigawatt-hours and terawatt-hours, written TWh.
The ladder is easier to hold if you hang a familiar picture on each rung. Start with the home. Britain's energy regulator, Ofgem, now treats about 2,500 kWh of electricity a year as typical for a medium-use household. Spread over the 8,760 hours in a year, that comes to an average draw of under 300 watts. That is far less than one kettle, because most of the time a house is just ticking over with a fridge humming and a router blinking.
Now climb to the power station. Hinkley Point C, the nuclear plant being built in Somerset, is designed to provide 3.2 gigawatts, about as much power as a million kettles all boiling at once. Then climb to the nation. In 2025 the United Kingdom generated roughly 290 terawatt-hours of electricity, about 290 billion kilowatt-hours. Divide by the hours in a year and you get an average flow of a little over 30 gigawatts. So the whole country, on an average moment, runs on about ten Hinkleys' worth of power.
The Headline Trap: Capacity Versus Generation
This is where careful readers earn their keep. When a news story says a wind farm is "1 gigawatt", it is giving the farm's capacity, the most power it can deliver flat out. Capacity is power. It is the width of the tap. What the farm actually delivers over a year is its generation, measured in energy, such as terawatt-hours. It is the water in the tub.
The two are linked by how often the tap is really open. Wind does not always blow at full strength, so over recent years Britain's offshore wind farms have produced, on average, a little under 40 percent of what they would make running flat out all year. Engineers call this ratio the capacity factor, or load factor. A 1 GW offshore wind farm therefore yields not the 8.76 TWh that nonstop full power would give, but something nearer 3.3 TWh. Divide that by 2,500 kWh per home and you get enough electricity for roughly 1.3 million typical households over a year.
The mistake runs both ways. A story might set a solar farm's gigawatts of capacity beside a gas plant's terawatt-hours of output as if they were the same kind of thing. Or it might say a new battery "stores 100 megawatts", which is a rate pretending to be an amount. When you see a number, ask whether it describes the tap or the bathful. If the unit has "-hour" on the end, it is energy. If it does not, it is power.
Tomorrow morning, when the kettle clicks off, you will know what just happened. A 3-kilowatt tap opened for two minutes, and about a tenth of a kilowatt-hour flowed through it. Every gigawatt and terawatt-hour in the news is the same story told at a bigger scale.
Key takeaways
- Energy is an amount you spend, measured in joules or kilowatt-hours; power is how fast you spend it, measured in watts.
- Power multiplied by time gives energy: 1 kW running for 1 hour is 1 kWh, or 3.6 million joules.
- Each prefix step (kilo, mega, giga, tera) is a factor of a thousand.
- A typical UK home uses about 2,500 kWh of electricity a year; Hinkley Point C is designed for 3.2 GW; the UK generated roughly 290 TWh in 2025.
- Capacity (GW) is the maximum power; generation (TWh) is the energy actually delivered, and the capacity factor links the two.
- A unit ending in "-hour" is energy; without it, it is power.