kW vs kWh: How to Read a Battery Quote

Every home battery quote carries two numbers that look almost identical and mean completely different things. One is kWh, the other is kW. Confuse them and you can end up with a battery that holds plenty of energy but cannot deliver it quickly enough to be useful, or one that discharges hard but runs dry by ten in the evening.

Here is what each number controls, and the few other figures worth hunting for on the spec sheet.

kWh is the size of the tank

A kilowatt hour is a quantity of energy. It is the unit your electricity bill is priced in, currently around 24p per kWh from the grid, and it is what the headline figure on a battery refers to. A 10 kWh battery stores ten units of electricity.

That number decides how long you can run your house without buying anything. A typical household draws somewhere between 0.3 and 0.8 kW across an evening once the cooking is done, with spikes on top. Ten stored units will usually carry a family through the evening and overnight, which is why 10 kWh comes up so often on quotes.

Batteries stack from 5 kWh up to 60 kWh, so this is the number that scales with your usage. Bigger is not automatically better: capacity you never empty is capacity you paid for and are not using.

kW is how fast it pours

A kilowatt is a rate. The battery’s power rating says how much it can deliver at any one moment, regardless of how much is stored.

This matters more than people expect. Put a 5 kW shower, a kettle and an oven on at once and you are asking for something close to 10 kW. If the battery is rated at 3.7 kW, it supplies its 3.7 kW and the grid quietly makes up the difference. Nothing breaks, you simply buy more units than you expected to.

It matters again when charging. On a smart tariff with a cheap overnight window, the charge rate decides whether you can fill the battery inside that window. A 15 kWh battery charging at 3 kW needs around five hours; the same battery at 5 kW needs three. If the cheap rate lasts four hours, those are two very different outcomes.

And it is the number that governs backup. A battery that keeps the lights, the fridge and the router going is doing perhaps 0.5 kW of work. Running an electric shower through a power cut is a different specification entirely.

Usable capacity, not nominal

Spec sheets often quote nominal capacity, while the figure that matters is usable capacity. Lithium batteries hold a little back at the bottom to protect cell life, so a 10 kWh nominal unit might give you around 9.5 kWh in practice.

Manufacturers are reasonably consistent about publishing both, but quotes are not always as clear. It is a fair question to ask, and comparing one installer’s nominal figure against another’s usable figure is how two quotes end up looking closer than they are.

Round trip efficiency

Electricity in does not equal electricity out. Some is lost converting from AC to DC and back, plus a little to the battery’s own electronics. Round trip efficiency of somewhere in the region of 90% is normal for current home storage.

In practical terms, filling a 10 kWh battery draws a bit more than 10 kWh from the roof or the grid. It is not a reason to reject a system, just a reason to be sceptical of savings estimates that quietly assume nothing is lost.

What to ask before you sign

Ask for usable capacity in kWh, continuous power output in kW, and charge rate in kW, all three written on the quote rather than described in conversation. Ask what the system does in a power cut and which circuits stay live, because that is a design decision made at installation, not a setting changed later. And check the installer is MCS accredited, which governs both the standard of work and your Smart Export Guarantee eligibility, currently paying in the region of 12p per kWh for what you send back.

Two quotes with the same headline kWh can behave very differently in the same house. If you would like someone to look at your consumption, your tariff and where the unit could physically go, and set the numbers out plainly, our survey is free and carries no obligation.