What Is a kWh?
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Understanding one simple unit — the kilowatt-hour — unlocks almost everything about solar. It is the unit on your electricity bill, the unit your panels generate and the unit you are paid for when you export. Here it is, explained without the jargon.
The simplest definition
A kilowatt-hour (kWh) is the amount of energy used by a 1,000-watt (one kilowatt) appliance running for one hour. So a 1,000-watt electric heater on for one hour uses one kWh. A 100-watt item would take ten hours to use the same one kWh. It is simply power multiplied by time, and it is the standard way electricity is measured and billed in the UK — your supplier charges you per kWh, and under the Ofgem price cap that has recently been around 24 to 25 pence per unit.
Watts, kilowatts and kilowatt-hours
People often muddle these, so here is the distinction:
- Watt (W) and kilowatt (kW) measure power — the rate at which energy is used or generated at a given moment. 1 kW = 1,000 W.
- Kilowatt-hour (kWh) measures energy — power used over time. It is what actually appears on your bill.
An analogy: power (kW) is like the speed of a car, while energy (kWh) is like the distance travelled. Driving fast for a short time can cover the same distance as driving slowly for longer — and likewise a powerful appliance used briefly can use the same energy as a small one left on for hours.
Where kWp comes in
Solar systems are sized in kilowatts-peak (kWp). This is the power output of the panels under standard test conditions — essentially their maximum rating. A "4kW system" means 4kWp of panels. The actual energy it produces over a year, measured in kWh, depends on daylight, weather, direction and shading. In the UK, each kWp of well-sited panels generates roughly 850 to 1,000 kWh a year, so a 4kWp system produces around 3,400 to 4,000 kWh annually.
Putting it together for solar
Once you are comfortable with kWh, the economics of solar become clear:
- Your panels generate a number of kWh each year.
- Every kWh you use yourself saves you the import price (around 24–25p).
- Every kWh you export earns you a Smart Export Guarantee payment (anything from a few pence to over 30p at peak on the best tariffs).
This is why using more of your own generation is so valuable: a kWh used at home is worth the full import price you avoid, which is usually more than you would earn exporting it. A battery helps by storing daytime kWh for use in the evening.
A quick everyday guide
To make kWh tangible, here are rough figures for common activities: boiling a full kettle uses around 0.1 kWh; a typical washing machine cycle around 1 kWh; an hour of television well under 0.2 kWh; and charging an electric car might use 40 to 70 kWh for a full battery. Knowing roughly how many kWh your home uses each day — your bill or smart meter will tell you — helps you size a solar system and battery sensibly rather than over- or under-buying.
How kWh appears on your bill
Your electricity bill has two parts: a daily standing charge (a fixed daily fee regardless of use) and a unit rate charged per kWh. Solar reduces the unit-rate part, because you buy fewer units from the grid; it does not remove the standing charge, which you pay simply for being connected. When you read your bill or smart meter, the kWh figure is the one to watch — it is what your generation and battery directly reduce, and what your export earns against. Understanding this split helps you set realistic expectations: solar cuts your usage charges substantially, but a small fixed cost remains.
Using kWh to size your system
Knowing your annual consumption in kWh is the starting point for sizing a system sensibly. Divide your annual usage by twelve for a rough monthly figure, and note how much falls in daylight versus evening hours. A household that uses a lot during the day can use more of its generation directly; one that uses most power in the evening benefits more from a battery. A good installer will look at your kWh history and design around it rather than simply fitting the largest array that will fit on the roof.
kWh, batteries and export
Battery capacity is also measured in kWh — a 5kWh battery stores up to five kilowatt-hours. Matching that capacity to your evening and overnight kWh demand is how you avoid paying for storage you never fill. And because the Smart Export Guarantee pays per kWh exported, the unit ties the whole system together: generate kWh, use or store as many as you can, and export the rest for payment. Once the unit clicks into place, every other solar decision becomes easier to reason about.
One unit, the whole picture
The kilowatt-hour really is the key that unlocks solar. Bills, generation, battery capacity and export payments are all measured in it, so once the unit makes sense, you can weigh up system sizes, savings and tariffs with confidence rather than relying on a salesperson's summary. Keep an eye on your daily kWh figure on your smart meter for a week or two before you buy, and you will have a far clearer idea of what size of system and battery genuinely suits your household.
As a closing thought, do not be put off by the technical-sounding language some installers use. Every meaningful number in a solar quote ultimately comes back to kilowatt-hours — how many you will generate, how many you will use, and how many you will export. If a salesperson cannot translate their pitch into those plain terms, ask them to; the clarity will help you compare quotes and spot any optimistic assumptions.
