Battery sizing: how to choose between 10, 13.5 and 16+ kWh

The governing principle

A battery earns its keep by cycling - filling with your surplus solar and emptying into your evening load, ideally daily. Capacity beyond what you can routinely fill and empty is money spent on kilowatt-hours that mostly sit idle. The right size is set by two ceilings: your daily solar surplus, and your overnight consumption. The battery should fit under both.

Work them out roughly: surplus is generation minus daytime use; overnight load is daily use minus daytime use. Whichever ceiling is lower is your sizing anchor.

Reading the common sizes

Around 10 kWh suits moderate evening loads and mid-sized arrays - very often the sweet spot for a typical family home. The 13.5 kWh class (the Powerwall 3's fixed capacity) fits higher-usage homes, larger arrays, and EV-adjacent futures. 16 kWh and beyond belongs to big consumers: ducted aircon evenings, pools, multiple EVs, or a deliberate whole-home backup strategy.

Modular ranges (FoxESS, Sungrow, AlphaESS and others) let you land between these marks and expand later; fixed-block products trade that granularity for integration benefits. Both are legitimate - the design question is which trade suits you.

Three adjustments

Backup changes the maths: capacity reserved for outages is capacity not cycling for savings, so whole-home backup ambitions push size up beyond pure economics. The federal rebate (applying up to 50 kWh) shifts the marginal cost of extra capacity and is worth modelling at current rates rather than assuming. And winter matters: surplus shrinks when generation drops, so a battery sized to summer surplus alone will under-fill for months - we size against the shoulder seasons, not the January peak.

The design tool runs exactly this logic against your bill; and if your surplus genuinely can't fill a battery worth buying, we'll tell you that too.