Battery sizing is where a lot of solar quotes over- or under-shoot. Bigger is not automatically better, because a battery you cannot fill or fully use is wasted money. Here is how to think about it honestly.
Capacity is measured in kWh
Battery size is given in kilowatt-hours (kWh) of usable capacity. The right number depends on three things: how much surplus solar you have to store, how much electricity you use in the evening and overnight, and your tariff. A battery that is too big for your generation and usage simply sits part-empty.
Match the battery to your evening usage
A useful starting point is your typical evening and overnight consumption, the energy you would otherwise buy at peak rates. If a battery covers most of that from stored solar, or cheap off-peak charging, it is doing its job. Going much larger only helps if you can actually fill and use it.
Tariffs change the maths
On a time-of-use tariff with a big peak/off-peak gap, a battery can earn its keep by charging cheap and discharging at peak, even beyond what your solar fills. Figures are indicative and vary by roof, energy use and tariff. The right size is specific to your home, so be wary of a quote that recommends the same battery for everyone.
Future-proofing vs over-spending
If you are planning an EV or a heat pump, slightly more capacity may make sense. But paying upfront for capacity you will not use for years rarely pays off. A good installer models this with your real usage rather than guessing.
Get it sized honestly
Sunready's free check flags whether a battery is likely to suit your home, and we match you with vetted MCS-certified installers who size it to your usage and tariff, not to the biggest sale.
Frequently asked questions
Can a battery be too big?
Yes. If your solar and usage cannot fill and use it, the extra capacity is wasted money. Right-sizing to your evening usage and tariff matters more than maximising kWh.
Can I add more capacity later?
Many systems allow expansion, but it depends on the battery and inverter. If you expect your needs to grow, mention it so the installer specifies something expandable.
Does a bigger battery mean blackout backup?
Not necessarily, because backup needs specific configuration, separate from capacity. If backup matters to you, raise it before you get quotes.
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