Capacity is only useful when it matches an energy job
A 5 kWh-class battery can be enough for evening self-consumption or a deliberately small backup load, while a 10 kWh-class battery can cover more overnight consumption or provide a longer reserve. But nominal capacity does not tell you usable energy, and energy does not tell you power. Start with measured household consumption and the loads you actually want the battery to support.
Solar surplus determines how often extra capacity can be filled
If a PV system rarely exports more than a few kilowatt-hours after daytime consumption, doubling battery capacity may leave the larger battery partly empty on many days. Conversely, a larger array with consistent surplus can justify more storage. Seasonal differences matter too. Use several months of interval data where possible rather than sizing from one unusually sunny day or one high electricity bill.
Backup sizing adds a second constraint
For outages, list essential loads and estimate both their energy use and starting power. A larger battery can extend runtime, but it will not start a motor that exceeds the inverter or battery power limit. Reserve settings also reduce the capacity available for everyday tariff cycling. The essential-loads vs whole-home guide and runtime calculator help separate those questions.
Compare upgrade paths before buying the smaller system
A 5 kWh system that can accept an identical module later may be a sensible first step, but expansion rules can depend on model generation, battery age, firmware and allowed module combinations. A 10 kWh system can avoid future installation work, yet oversizing has a cost if the energy is rarely used. Compare warranty throughput, minimum and maximum module counts, usable energy, continuous power and the mixed-age expansion checklist before treating capacity as the only decision.