Quick answer
Battery size should follow evening and overnight consumption, backup goals and the amount of excess solar available to recharge it.
What matters most
- Half-hourly smart-meter data is more useful than total quarterly usage.
- Reserve capacity for outages reduces the energy available for daily bill shifting.
- A larger battery is not automatically more economical.
Battery decision framework
Start with three numbers: how much energy you use after solar production falls, how much excess solar is available to charge a battery, and the difference between your import tariff and export tariff. Then add non-financial goals such as backup power.
How to calculate it with SunnyWatly
Use the relevant calculator with your own bill, quote or vehicle information. Defaults are examples only and can become stale as tariffs, rebates and hardware change.
Do not rely on a headline figure alone. Ask what assumptions sit behind the estimate and whether they match your property, tariff and usage pattern.
What can change the result
- Electricity usage and time of day.
- Retail import and feed-in tariffs.
- Roof, shading, inverter or charger constraints.
- Hardware efficiency and usable capacity.
- Network limits and future tariff changes.
Primary public sources used by SunnyWatly