Usable vs Total Battery Capacity: What the kWh Number Really Means

What is the difference?

• Total (or nominal) capacity is the energy the cells can store in theory.
• Usable capacity is the energy you can actually use. Batteries hold back a little at the top and bottom of the charge to protect the cells.

The usable figure is the one that matters for your home.

Why does it matter when comparing quotes?

Two batteries can be advertised as similar sizes but give your home different amounts of energy if one quote uses total capacity and the other uses usable capacity. When you compare prices, compare the usable kWh. Our quote checklist covers what else to look for.

Does it affect the rebate?

Yes. The federal Cheaper Home Batteries Program works out certificates from usable capacity, tapered by size. See our battery rebates page for how the calculation works.

Are there other numbers to watch?

• Power (kW). How much the battery can deliver at once. A big battery with low power can't run several big appliances together.
• Depth of discharge. How much of the battery can be used each cycle; part of the usable figure.
• Modules. Many batteries stack in modules, so capacity can be added within limits.

How big should usable capacity be?

That depends on your evening and overnight use and your solar surplus. Read what size battery do you need for how we size it.

A real example

We installed a 49 kWh GoodWe battery in Russell Lea for a busy household that wanted to rely on the grid as little as possible. With a battery that size, how much of the capacity is usable is exactly the question worth checking on the datasheet before signing.

What to ask

Ask any installer, us included: "What is the usable capacity, and what is the power rating?" A clear answer makes the rest of the comparison easy.