If you’ve decided a battery is worth it, the next question is how big — and it’s an easy one to get wrong in an expensive direction. Battery salespeople sometimes nudge you toward a large unit “to be safe”, but an oversized battery is money sunk into capacity you’ll rarely use. The right size follows a simple principle: a battery exists to carry your evening and overnight load on stored daytime solar, so you size it to that load — not to your whole day’s usage, and not to the size of your panel array. Here’s how to land on the right number.
Start with your after-dark usage
The single most useful figure is how much electricity you use from sunset to sunrise — because that’s the gap a battery is there to fill. During the day, your panels power the house directly; the battery only matters once the sun’s gone.
- Estimate your evening and overnight kWh. Roughly how much power do you use from dusk until the panels wake again — the evening cooking and heating peak, plus overnight base load like the fridge, standby, and hot water? For many homes that’s in the region of a handful up to around ten kWh.
- Size the usable capacity to it. Batteries quote two numbers: a total capacity and a usable capacity (you can’t safely drain a battery to absolute zero). Always size on the usable figure — that’s what you actually get to use each night.
- Don’t oversize. A battery bigger than your evening load spends its life half-empty. You’ve paid for capacity you never cycle, which is pure waste.
Typical New Zealand sizing
For most family homes, a single ~10 kWh battery comfortably covers the evening peak and a good chunk of overnight base load — which is exactly why it’s the common choice. Larger homes, or households wanting more outage backup, go bigger; smaller or low-use households often need less. The “right” size is genuinely personal: it tracks your evening habits, not a one-size-fits-all number.
Match the battery to your panels
There’s a second constraint people forget: a battery is only useful if it can actually fill up on a normal day. That means your panel array has to produce enough surplus — generation beyond your daytime use — to charge it. If your array is small, or you already self-consume most of your daytime solar, there may be little left over to fill a large battery, so it would sit part-charged and under-used. Panels and battery need to be sized together, not in isolation. A bigger battery may call for more panels to keep it fed.
Factor in backup, if that’s a goal
If outage backup is part of why you’re buying a battery, sizing gains an extra wrinkle. Many systems reserve a portion of the battery purely for backup — it’s held in case the grid fails, so it’s not available for everyday savings. Check how much is reserved and which circuits the backup actually powers (often just essentials like lights, fridge, and internet, not the whole house). If you want both meaningful daily savings and real backup, you may need a little more capacity to cover both jobs.
The verdict
Size a battery to your evening and overnight load — not your whole day, and not your panel array — and base it on the usable capacity, not the headline figure. For most New Zealand homes a single ~10 kWh unit hits the mark, but the right size depends on your after-dark usage, whether your panels can keep it charged, and how much you want reserved for backup. The goal is a battery that fills up most days and empties most nights; one that’s always half-full is one you over-bought.
Get a free assessment and we’ll size storage to your real evening load.
Sources: Battery sizing, usable-capacity, and self-consumption guidance per NZ industry references (2026). Figures vary by home and product.