kW and kWh for a home battery: power and energy explained
Battery quotes lead with kWh. The kW figures further down the specification sheet decide just as much about what the battery can do, and they also decide which network paperwork applies.
In short
kWh is energy: how much a battery holds, like the size of a tank. kW is power: how fast energy can flow, like the width of a tap. A battery can be full and still unable to run everything at once if the load in kW exceeds its power rating. The network rules (G98 or G99) are set in kW, not kWh.
The tank and the tap
A kilowatt-hour is the energy used by something drawing one kilowatt for one hour. A battery's capacity in kWh is how much energy it can hold. Its power rating in kW is how much it can deliver at any moment. A large tank with a narrow tap holds a lot but empties slowly; a small tank with a wide tap empties quickly.
| kWh (energy) | kW (power) | |
|---|---|---|
| What it measures | An amount | A rate |
| On a battery | Capacity: usable and nominal | Continuous and peak output; charging rate |
| On your bill | What you are charged for | Not usually shown |
| Limits | How long the battery can run things | How much it can run at the same time |
| Network paperwork | Does not decide the route | Decides G98 or G99 (3.68 kW is the line) |
Why a full battery can still draw from the grid
Suppose the oven, the kettle and an electric shower all run at the same moment. Together they ask for more power than many home batteries can supply. The battery delivers its maximum, and the grid supplies the rest, even though the battery still has plenty of energy stored. The kWh were there; the kW were not.
That is why the continuous power rating deserves a look alongside the capacity. A household with a big evening peak may care as much about kW as about kWh. You can find the power of appliances on their rating plates, usually in watts (1,000 W is 1 kW).
Power in a power cut
For backup, the relevant number is the backup output in kW, which can be lower than the normal output. If the circuits you want to keep on ask for more than that, the system cannot run them all. The estimator on backup in a power cut checks your loads against that limit before it works out hours.
kW and the network rules
The network operator cares about how much power could flow onto the street, so the notification route is set in power. G98 covers systems up to and including 16 A per phase, about 3.68 kW on a typical single-phase home; MCS says storage over 3.68 kW needs a G99 application before installation. Panels and a battery on one connection add together, which is why combined systems so often take the G99 route. G98 and G99 covers the detail.
Charging power
Power works in the other direction too. A battery's charging rate in kW limits how fast it can soak up a burst of midday surplus or fill during a short cheap-rate window. If your tariff's cheap window is short, check that the battery can take in its usable capacity within it.
Reading kW and kWh together
Dividing usable kWh by continuous kW gives the hours a battery could run at full power. A battery with 10 kWh usable and a 5 kW output could, in principle, run flat out for about two hours; at a steady 0.5 kW it would last about twenty (our arithmetic, before any reserve). Neither number alone tells you whether a battery suits your home; together, with your own loads, they do. Comparing specifications shows where both appear on a datasheet, and what size battery works out the kWh side.
Putting numbers on a typical evening
Appliance rating plates give power in watts. A kettle is commonly rated at around 3,000 W (3 kW) and an oven element can be similar; lights, a fridge, a router and a television together may come to a few hundred watts. Running the kettle for three minutes uses about 0.15 kWh; running a 300 W load for four hours uses 1.2 kWh. The short, high-power moments test the battery's kW rating; the long, low-power hours use up its kWh. Reading your own rating plates and the hours things run is the most direct way to see which limit your home is likely to meet first (figures here are illustrative, our arithmetic).
Phases and the 3.68 kW figure
The G98 limit is written as 16 A per phase. On a typical single-phase home supply that works out at about 3.68 kW, which is the figure you see in MCS guidance. Some larger homes have a three-phase supply, where the limit applies to each phase; if yours does, ask how that affects the route for your system.
In short: kWh tells you how long, kW tells you how much at once, and the network cares about kW.
Both figures appear on every specification sheet, usually near the top; if a quote gives only one of them, ask for the other.
What this page is based on
Questions about storage in your own home?
Tell us a little about your home and what you want a battery to do. Your message reaches SEO-DONS LTD, which publishes this guide, and we answer by email. It costs nothing to ask and commits you to nothing.
What happens next
- SEO-DONS LTD, the publisher of this storage guide, reads your message.
- We answer by email, using the same sources as the guide.
- What you do next, if anything, is up to you.