Solar battery sizing should start with the electricity you cannot use while the panels are generating. A battery mainly helps by moving surplus daytime solar into the evening or by supporting tariff shifting where the household has the right meter and tariff.
Do not compare batteries only by headline capacity. Usable capacity, inverter compatibility, charge and discharge rates, warranty terms and installation location all matter. A compact battery that matches the household load can make more sense than a larger battery that rarely cycles properly.
The simplest early check is to compare expected daytime surplus with evening usage. If the home already uses plenty of power during daylight hours, storage may have less work to do. If the home exports heavily and buys back electricity every evening, a battery deserves a closer look.
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