Do you actually need a battery with your solar?

Batteries are exciting, and they’re also the part of a solar system most likely to be over-sold. Sometimes a battery is a brilliant addition; sometimes it’s thousands of dollars that would never quite pay for itself. The right answer depends entirely on what you want it to do.

First, decide what the battery is for

There are three different reasons to add a battery, and they lead to three different sizings:

• Bill savings: storing cheap daytime solar to use at night instead of buying from the grid.
• Backup: keeping the lights, fridge and internet running through a power cut.
• Self-sufficiency: getting as close as possible to running your home on your own power.

Let’s talk actual numbers

Here’s a rough, honest illustration — your figures will differ. Say you have 8 kWh of surplus solar going to the grid on an average day. Exported at a typical buy-back of around 10c/kWh, that’s about 80c a day, or roughly $290 a year. Store and use that same 8 kWh in the evening instead of buying it back at, say, 30c/kWh, and it’s worth about $2.40 a day — roughly $875 a year. So the battery’s job is to turn cheap exports into avoided expensive imports: here, a swing of around $585 a year. Against a home battery that might cost $10,000–$15,000 installed, that’s a long payback — which is exactly why the maths has to be done, not assumed.

The gap is what matters: the bigger the difference between what you pay for power and what you’re paid to export, and the more surplus you have, the better a battery looks.

The 2026 peak export rebate changes the sums

From 1 April 2026, lines companies must pay a rebate for power exported at peak times, and from 1 July 2026 large retailers must offer plans that reward peak export. A battery lets you deliberately hold power back and release it into those peak windows, which can tilt the maths further in a battery’s favour than it was a year ago. How much depends on your network’s rates and whether your retailer passes them on — so it’s worth checking both before you buy.

When a battery makes sense

A battery tends to stack up when you have plenty of surplus daytime solar going cheaply to the grid, high evening usage, a retail plan with a big gap between what you pay and what you’re paid for export, or you simply place real value on backup during outages.

When it probably doesn’t (yet)

If you already self-consume most of your solar, or your export rate is healthy, the financial case gets thin fast. In that situation the honest advice is often to install solar now, size it well, and revisit batteries later as prices keep falling.

Safety isn’t optional

Home batteries store a lot of energy, and in New Zealand they’re governed by AS/NZS 5139, which became mandatory in November 2025 (with a transition to November 2026). It sets real rules on where and how a battery can be installed — placement, ventilation, protection — so this is firmly a job for a qualified installer working to the standard, not a DIY project.

Sources

• Peak export rebates decision — Electricity Authority
• Battery energy storage & AS/NZS 5139 — EWRB