What size battery do I need? A simple way to work it out

Step 1: how much power do you use a day?

Your daily use is the kWh on your power bill for the billing period, divided by the number of days in it. If you already have solar, that figure is only what you bought from the grid, which is mostly your evening and overnight use: a useful number in itself. If you're not sure what kW and kWh mean, start with kW vs kWh, explained.

Step 2: estimate what you use after dark

A battery earns its keep by covering the hours when your solar isn't producing: evening cooking, TV and lights, overnight fridge, hot water on a timer, and the morning before the sun is up. Roughly, a home that's empty during the day uses 50 to 60% of its electricity after dark; a home with people in it all day, less. If your retailer offers an interval-data download or an app, you can see your actual evening use.

This is the same starting point our estimate uses. It rounds up to a common battery size and allows a little headroom, because a battery shouldn't be run completely flat every night.

Step 3: check your solar can fill it

A battery only saves money on days it fills up, from solar you'd otherwise export. On a winter day a 6.6 kW system may produce half what it does in summer, and your daytime use comes first. If your bill shows you export only a few kWh a day, a big battery will sit partly empty. The fix is a smaller battery or more panels, not a bigger battery. The government's own advice is that bigger isn't always better: a battery too large for your solar or your inverter limits the benefit.

Why does 14 kWh matter for the battery rebate?

The federal battery rebate pays the full rate on the first 14 kWh of usable capacity, 60% of it up to 28 kWh, and 15% up to 50 kWh. So the step from 14 to 20 kWh adds far less rebate than the first 14 kWh did. That's not a reason to avoid a bigger battery you need, but it's a reason not to buy one you don't. See how the battery rebate works.

When does a bigger battery make sense?

A bigger battery makes sense when you need more than your evenings covered:

• Blackout backup. If you want to ride through long outages, size for the essential loads you want to keep running, and check the battery's power output (kW), not just its capacity.
• An electric car. Charging at home in the evening can add 5 to 15 kWh a day. Many owners charge from solar in the middle of the day instead, which needs no battery at all.
• Going all-electric. Replacing gas heating, hot water or cooking moves more of your use into the evening.
• A virtual power plant. Some plans pay you to let them use part of your battery. That capacity isn't available to you at the same time, so read the terms.

Can I add more later?

Many batteries are modular, so you can add modules later, usually from the same product range. Check whether your model can be expanded, and by how much, before you buy. The federal rebate is one claim per property: you can only claim for added capacity if the original battery never claimed it, and you add at least 5 kWh. If you think you'll need more within a year or two, it can be cheaper to size for it now.

How do you compare battery prices?

Compare quotes on price per usable kWh after the rebate, not on the headline price. A cheaper battery that's too small to cover your evenings saves less than a slightly dearer one that fits. And don't compare a battery that includes blackout backup with one that doesn't as if they were the same thing.