kW vs kWh: Power and Energy Explained for Home Batteries
- kWh is the size of the tank. kW is how fast it can flow.
- Runtime is roughly usable kWh divided by the kW you draw, before losses.
- Usable capacity is often lower than the headline capacity.
- A battery can have plenty of kWh and still be too weak in kW for a big appliance.
- UK grid connection thresholds are set in kW, not kWh.
A kilowatt (kW) measures power, how fast electricity is used or delivered. A kilowatt-hour (kWh) measures energy, the total amount. A home battery has both numbers, and you need to check both before you buy.
The tank and the tap
The simplest picture is a water tank with a tap. The tank holds a volume, which is the kWh. The tap sets how quickly water can come out, which is the kW.
Take an example battery with 10 kWh of usable energy and a 5 kW output. It could deliver 5 kW for two hours, or 1 kW for ten hours, ignoring losses. These are illustrative numbers, not a specific product.
Your electricity bill is in kWh because you pay for energy over time. A 2 kW heater running for three hours uses 6 kWh. While it runs, it needs 2 kW of power the whole time.
Why both numbers matter
Capacity tells you how long a battery can keep going. Power tells you what it can run at once. A battery with a continuous output of 2.5 kW could run lights, a fridge and a router for many hours, but not a 3 kW appliance on top. Depending on the system, it may then limit its output or draw the rest from the grid.
Datasheets often list more than one power figure:
- Continuous power (kW): what the battery can deliver for hours. This is the figure for everyday use.
- Peak power (kW): a short surge, useful for motors starting up, but not something you can rely on for long.
- Backup output (kW): sometimes lower than the grid-connected figure. Check it if power cuts are your main reason for buying.
Rule of thumb
Read kWh as how long and kW as how much at once. Ask installers for both, in usable and continuous terms, and write the figures into your quote.
Usable capacity and losses
Many datasheets give a total or nominal capacity and a smaller usable capacity. Only the usable figure is energy you can draw. Some of it is also lost in the battery and inverter, which is why how a home battery saves money depends on more than the label.
For a rough runtime, divide usable kWh by the average kW you expect to draw, then leave a margin. As an example, 9 kWh usable and an average draw of 0.5 kW gives about 18 hours on paper. Real life will be shorter if you switch on bigger loads. To see how much storage typical homes use, browse our battery capacity pages, for example 10 kWh.
Where kW shows up in the UK
Grid connection rules look at power, not stored energy. Electricity North West, a network operator, says the G98 process covers installations up to 16 A per phase, which is 3.68 kW for single-phase and 11.04 kW for three-phase, and that anything larger needs a G99 application. For inverter systems it says the rating that counts is the inverter's continuous output. Your installer should handle this, but it is worth asking about, and the details are on Electricity North West's page.
Solar panels use a similar pair. They are rated in kWp, the peak power under test conditions, so an array rarely produces its full rating. What you generate over a day, in kWh, is what actually fills a battery.
Common questions
Is a bigger kWh number always better?
No. More capacity only helps if you have surplus solar or cheap-rate electricity to fill it, and evening demand to use it. A battery bigger than your needs costs more without saving more. See battery capacity for sizing.
How do I find my own kWh figures?
Your supplier app, your bills and a smart meter display all show kWh. Enter them into the home battery calculator to get a first estimate of the size that fits.
Can a battery run an electric shower or oven?
Only if its continuous kW rating is high enough for that appliance plus everything else running. Check with your installer before assuming it can, and ask about the backup output if you want it for power cuts.
What is the difference between kW and kWp?
Both are power. kWp is the peak output of solar panels under standard test conditions. kW on a battery is how fast it can charge or discharge.
Related guides
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