Nominal capacity (battery)

Written and checked by the Energy Merchants desk · Reviewed 2 October 2026

Nominal capacity is the total energy a battery is designed to store, as quoted on marketing material. It is the number on the box and the name of the product: a “13.5 kWh battery” or a “10 kWh stack”.

It is not the number that counts for certificates. Batteries keep a reserve that cannot be used, to protect the cells and keep the system within its warranty. What remains is the usable capacity, and that is the figure on which battery STCs are calculated. See usable capacity.

An illustration

The figures below are made up to show the principle, not taken from a product.

If an installer claimed on nominal capacity, the claim would overstate the certificates by about four, and a reviewer would catch the mismatch against the product listing.

In practice

Nominal capacity shows up in conversations with customers. A customer wants a 10 kWh battery because it matches their evening consumption, and the installer needs to explain that the sizing, the quote and the STC claim may refer to different numbers. A good habit is to put both figures on the quote and say which one drives the discount.

On the claim, use the usable capacity from the approved product listing. Nominal capacity can still help with a sanity check: if the usable figure on your form is larger than the nominal figure, something has been entered wrongly.

Common confusion

Nominal capacity is not the same as the power rating, and it is also not the same as “total” capacity when several modules are stacked. Each module has its own nominal and usable figures, and totals should be built from the usable ones.

Some listings publish only one figure, and it may be unclear which. If in doubt, ask the manufacturer or check the approved product list rather than guessing.

For the process, see battery STCs and the battery submission guide. The wider vocabulary is in the certificate trading glossary.