Should I add more panels or buy a battery?

They fix different problems: panels add production, a battery adds value per kWh you already produce. If your bills are high because winter and evenings drain the meter, and roof space remains, extra panels are usually the cheaper kWh. If you already export heavily at poor rates while buying back expensive evening power, the battery attacks exactly that spread.

Why the answer differs per household

Per unit of yearly energy, added panels are hard to beat — hardware is cheap, and every extra kWh either offsets consumption or earns export income. But panels amplify a pattern you may already dislike: more midday surplus, exported at a rate typically two to three times below your import price, and no help at 8 pm.

A battery adds zero production but upgrades each shifted kWh from “sold cheap” to “not bought dear”. Its economics live and die on three numbers: the import-export spread, how many kWh you can genuinely cycle daily (see the sizing answer), and whether a time-of-use tariff lets it earn a second arbitrage income. In the UK, pairing a battery with tariffs like Octopus Flux or Agile materially shortens payback; similar TOU logic applies in Australia; flat-tariff households see the slowest returns.

What to do

1. Pull two numbers from your data: annual kWh exported and annual kWh imported during non-solar hours. Big export + big evening import = battery territory; small export + big total import = panels first.
2. Price the spread — (import price − export price) × realistically shiftable kWh × 365 is the battery’s honest annual ceiling; compare against its cost.
3. Check what extra panels would face — export caps or overvoltage tripping (see the K1 answers) can strand new capacity; a production forecast for the enlarged array shows what would actually reach the meter.
4. Sequence, don’t agonise — the common winning order is: fill the roof while install crews are mobilised cheap, add the battery when a good tariff makes the spread wide enough.