kW vs kWh: how to think about battery size

The simple version

kWh (kilowatt-hours) is energy: how much the battery can store. kW (kilowatts) is power: how much it can supply at any one moment.

Think of a water tank. kWh is how much water is in the tank. kW is how wide the tap is. A big tank with a narrow tap lasts a long time but cannot run many things at once.

Usable capacity, not headline capacity

Look for usable capacity on the datasheet. Some batteries keep part of their storage in reserve to protect the cells. Comparing usable figures is the fair comparison.

Sizing capacity from your evenings

A battery mostly earns its keep between sunset and sunrise. To get a starting size:

• Use interval data or your smart meter app to find typical use from about 5 pm to 7 am.
• Check whether your solar has enough surplus during the day to fill the battery, especially in winter.
• Consider what is coming: an EV charged overnight changes the picture a lot.

Why bigger is not always better

A battery that never fills, or never empties, is storage you paid for and do not use. If your solar system is small, a large battery may only reach full charge on the best summer days. Sometimes the better move is more solar first.

Power: what can run at once

Continuous power (kW) limits how many appliances the battery can supply together. An oven, a kettle and ducted air conditioning at the same time can exceed a single battery's output, and the grid will supply the rest. That is normal day to day.

It matters more for backup. In a blackout there is no grid to make up the difference, so the battery's power rating and the circuits you choose to back up need to match. Peak or surge ratings for motor starts are a separate figure.

Reading a datasheet

Ask for these four numbers for any battery you are quoted:

• Usable capacity (kWh)
• Continuous power output (kW)
• Peak or surge output and for how long
• Warranted throughput or retained capacity at end of warranty