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.