The short answer

Start with the demand between charging opportunities, then check usable capacity, output power and reserve requirements. There is no single correct battery size for every home with the same annual bill.

Describe the period you want to cover

List the loads normally used after useful solar generation ends and before the next charging opportunity. Use measured consumption if available; otherwise keep the estimate visibly provisional.

Separate daily use from occasional high demand. A once-a-month event may not justify sizing the everyday battery around it.

Work through an illustrative evening

Assume the home needs 6 kWh during the chosen period and the discharge path delivers 90% of energy removed from the battery to the loads. The battery would need about 6 ÷ 0.90 = 6.67 kWh available above any reserve.

This example uses a discharge-efficiency assumption, not a complete round-trip efficiency. Use the correct equipment data and avoid applying the same loss twice. It is a calculation example, not a size recommendation.

Check power separately

If simultaneous appliances require 4 kW, enough stored kWh does not prove the system can supply that power. Confirm the discharge rating and any restrictions for the proposed battery and inverter combination.

Also ask how quickly the battery can recharge within the available tariff or solar window. More capacity may be underused if the charging opportunity is too short.

Compare the additional capacity

Request forecasts for two realistic sizes with the same panels, demand and tariffs. Compare the extra installed cost with the extra benefit, keeping reserve assumptions consistent.

SolarIQ’s displayed battery sizes are comparison scenarios. They should be refined through an equipment and household assessment rather than treated as an automatically optimised choice.

Before your assessment

Bring these questions with you.

  • Energy needed between charging opportunities.
  • Usable capacity above the chosen reserve.
  • Charge and discharge power for the complete system.
  • Additional cost and benefit of the next battery size.