Look at usable capacity, charging opportunities and evening demand together. Bigger is not automatically more useful.
Measure the gap you want to fill
Identify demand after solar production falls and before it resumes. Separate routine evening use from occasional peaks. A large annual electricity bill is not enough to size a battery because much of that demand may occur when the battery cannot recharge.
Read both capacity and power
Capacity in kWh describes stored energy. Charge and discharge power in kW describe how quickly that energy can move. Ask for usable rather than headline capacity and confirm whether an emergency reserve reduces everyday availability.
Check the whole-year pattern
In summer, a battery may fill regularly; in winter, solar charging can be limited. Ask for seasonal modelling, including losses, warranties and expected replacement. Compare a smaller option and a no-battery option using the same assumptions.
Build an evening energy budget
Add the energy used between the end of useful solar production and the next charging opportunity. For a calculation example, an average 0.4 kW demand lasting five hours uses 2 kWh. Another 1 kWh overnight brings the total to 3 kWh. That is an energy requirement, not a recommended battery size: charging losses, reserve settings and available solar surplus still need to be accounted for.
Check how much surplus can actually charge it
A battery cannot store electricity that the panels do not produce or that the home has already used. Request a comparison of daily surplus and later demand, not just annual generation. A large unit may rarely fill during darker months. Conversely, a small battery may fill early on sunny days and leave further surplus to be exported. The relevant question is what the extra capacity changes over a full year.
Compare the next size up
Ask for the additional installed cost and additional annual discharge of the larger option. If extra capacity costs more but rarely gets used, a lower price per kWh of capacity can be misleading. Check the conditions for later expansion: compatible modules, age limits, installer labour and available space. Obtain the warranty’s capacity-retention and usage conditions rather than comparing warranty years alone.
Your planning checklist
- Evening and overnight demand, including seasonal changes.
- Usable capacity after the chosen backup reserve.
- Maximum charge and discharge power.
- Extra annual benefit and cost of the next size up.
Common questions
Can a large battery cover the whole winter?
A household battery normally shifts electricity between parts of a day. It does not turn summer production into a winter supply. Check whether winter charging would come from solar, the grid or a combination.
Why might the house import while the battery still has charge?
The combined appliance load may exceed the battery’s discharge power, or its controls may retain a reserve. Ask the installer to show how the proposed system handles those situations and how the monitoring display reports them.