What battery size do I need for my solar and usage?

Size the battery from what you consume between sunset and sunrise, not from how many panels you have. For most homes that lands at roughly 1–1.5 kWh of usable capacity per 1000 kWh of annual consumption — so a 4000 kWh household is well served by 5–7 kWh, unless a heat pump, EV or a time-of-use tariff argues for more.

Why panel-based sizing goes wrong

The battery’s daily job is to shift your solar surplus into the evening and night. If it is bigger than your typical overnight consumption, the extra capacity sits full and idle for most of the summer (see the answer on a battery that is always full) — and in winter there is often not enough surplus to fill even a small one. Oversizing therefore buys capacity that cycles rarely, and a battery that does not cycle does not pay for itself.

Two things legitimately justify going bigger: large shiftable loads (an EV charged overnight, a heat pump) and cheap-rate windows on time-of-use tariffs (Octopus Go/Flux-style in the UK, similar TOU plans in Australia), where the battery earns a second income arbitraging grid prices, not just storing sunshine — covered in the overnight-charging answer.

What to do

1. Measure your sunset-to-sunrise consumption for a couple of typical weeks — a smart meter, energy monitor or Home Assistant makes this a five-minute read.
2. Match usable capacity to that figure — batteries quote usable vs nominal differently; compare usable kWh and round up modestly, not aggressively.
3. Check surplus reality against a forecast — a production forecast for your array shows how many days a year actually generate enough surplus to fill the candidate size; a battery you can only fill 60 days a year is a poor investment.
4. Add capacity for the tariff, not for pride — if you plan grid-charging in cheap windows, size for the arbitrage cycle; otherwise stay modest and spend the difference on panels (see: more panels or a battery?).