What size battery do I need for my existing solar panels?

The right battery size depends on when you use electricity and what you want the battery to do. The number of solar panels on your roof is only part of the picture.

A useful quote should explain the proposed size using information about your home. Before comparing battery models, gather a few records and decide which problems you want to solve.

Start with the electricity you use after sunset

A battery can store surplus solar electricity for later use. Energy Saving Trust also explains that some households charge batteries on a time-of-use tariff. These are different charging opportunities, so tell your installer which approach you want assessed. Read Energy Saving Trust’s battery-storage guide.

Download your electricity-use history if your supplier provides it. Half-hourly data can help show your daily pattern. If you do not have it, bring bills and describe a normal weekday and weekend: when people are home, when you cook and when larger appliances run.

An annual total is useful background, but ask the installer how they have accounted for the times when you actually need electricity.

Look at solar surplus, not just panel output

Bring generation records from your solar app or meter, plus export data if available. Ask the installer to distinguish electricity already used directly at home from the surplus available to charge a battery.

Include examples from different seasons. A proposal should not leave you guessing whether its battery recommendation is based on a sunny summer day, a typical day or a full year of records.

If records are missing, ask which assumptions have been used and how the survey will check them.

Compare capacity and power separately

Capacity, measured in kilowatt-hours (kWh), describes how much energy a battery can store. Power, measured in kilowatts (kW), describes the rate at which the system can supply it. A larger capacity does not, by itself, tell you which appliances can run together.

Ask for the usable capacity and the charging and discharging power limits of the complete proposed system. Have the installer explain those figures against the appliances you care about, rather than comparing product names alone.

Include future plans and backup needs

Tell the installer about a planned heat pump, electric car or change in working patterns. Ask for today’s needs and possible future needs to be shown separately, with the assumptions clearly labelled.

If power-cut protection matters, make that a specific part of the brief. Ask what backup equipment is included, which circuits it would serve and how any reserve setting affects the energy available for everyday use. Do not assume that a bigger battery automatically provides backup.

Ask why this size is being recommended

A good comparison is between explained options. Ask the installer to show what changes if you choose a smaller or larger system: the installed cost, expected use of the extra capacity and any limits that remain.

Useful questions include:

• Which consumption and generation records did you use?

• How often do you expect this capacity to be useful?

• What happens during lower-generation periods?

• Is grid charging assumed, and on which tariff?

• Can capacity be added later, and under what conditions?

Energy Saving Trust notes that there is no exact sizing formula that fits every home. Treat a recommendation as something the installer should explain in your circumstances.

Bring the right information to your quote

Our checklist for adding a battery to existing solar panels covers the details to gather before an assessment.

Explore Evoflo’s solar battery-storage service and get in touch with your inverter model, electricity bills and any generation or export records. That gives the discussion a useful starting point.