How home battery size is estimated

Home battery marketing often jumps straight to product names and kilowatt-hour stickers. The question most households actually need answered first is simpler: roughly how much energy do you want the battery to cover when the solar system is not producing?

This calculator turns a short set of inputs into a capacity range. It is designed for a quick, transparent estimate — not a full system design or a payback model.

1. Start from daily energy use

We average your winter and summer daily usage (kWh/day) from your electricity bills. Using two seasons reduces the chance of sizing only for the best or worst week of the year. That average is the base household load the rest of the math builds on.

Optionally, you can add appliance-level loads (EV, air conditioning, pool equipment, oven, and other circuits). When those are enabled, we treat them as a second view of daily energy and show a low–high range on the results. Large gaps between bill-based and appliance-based totals are a signal to double-check inputs, not a reason to silence the range.

2. Decide how many hours the battery should cover

Cover hours is the length of the day you want storage to support, typically the hours outside solar production. By default we set this from your solar start and stop times: twenty-four hours minus the solar window. You can override it — for example if you only care about evening peaks — but the default keeps the model honest about “non-solar” periods.

Battery energy for those hours is estimated as daily energy multiplied by coverHours ÷ 24. That assumes a fairly even spread of load across the day. Real homes peak in the evening; treat the output as an order-of-magnitude capacity, not a minute-by-minute profile.

3. Add a reserve (minimum state of charge)

Batteries are rarely run from 100% to empty. Installers leave a reserve for longevity, grid outages, or warranty requirements. We apply reserve as a simple uplift:

recommended kWh ≈ (usage × coverHours / 24) ÷ (1 − reserve%)

A 10% reserve means the usable band is 90% of nameplate capacity, so the battery must be a little larger than the pure energy number. Raising reserve increases size; lowering it does the opposite — with trade-offs this tool does not optimise for you.

What this tool does not include

• Tariffs, feed-in rates, or a full financial payback analysis
• Inverter / battery power limits (kW), only energy capacity (kWh)
• Shading, orientation, degradation, round-trip losses in high detail
• Installer constraints, switchboard capacity, or local regulatory rules

Use the result as a briefing for an accredited designer. For bill inputs, see reading your electricity bill. For solar figures, see solar generation and export.