How to size a home battery

How much it holds and how fast it can deliver are two different specifications. A battery can be adequate on one and useless on the other.

Batteries are quoted in kilowatt-hours, which is energy — how much they hold. They also have a power rating in kilowatts — how fast they can deliver it. Sizing on capacity alone is the classic error.

Step-by-step

1. Enter your evening and overnight consumption.
2. Enter your peak simultaneous load.
3. Read both the capacity and the power required.

Why power matters as much as capacity

A 10 kWh battery rated at 3 kW output holds plenty of energy and cannot run a 4 kW shower at all. Add a kettle and an oven and it cannot keep up regardless of how full it is.

Work out what might run simultaneously and size the power rating for that. Capacity determines how long; power determines whether it works at all.

Usable capacity is not nameplate capacity

Batteries reserve some capacity to protect their lifespan. A 10 kWh battery may offer 9 kWh usable, sometimes less. Compare usable figures, since that is what manufacturers are least consistent about.

Round-trip efficiency also costs something: about 10% of what goes in does not come back out.

Bigger is not automatically better

A battery larger than your overnight consumption sits partly unused every night, and you have paid for capacity that never cycles. In winter there may not be surplus generation to fill it at all.

Size for a typical evening and overnight period, not the worst case. The economics of a battery are marginal in many places, and oversizing is what tips them from marginal to poor.

Frequently asked questions

What size battery do I need?

Enough capacity for a typical evening and overnight period, and enough power output for what might run at once. The second is what people forget.

Why can my battery not run the shower?

Its power rating is too low. Capacity in kilowatt-hours says how much it holds; the kilowatt rating says how fast it can deliver, and a high-draw appliance can exceed it whatever the state of charge.

Will a battery keep my lights on in a power cut?

Only if it is specified for it. Many grid-tied systems disconnect entirely when the grid goes down, for safety. Backup capability is a separate feature to ask about.