Is a home battery worth it with solar?

A battery lets you use more of your own solar power in the evening instead of buying from the grid, and avoids exporting it for a low buy-back rate. The saving is the gap between the retail power price and the export rate, on the energy you shift, weighed against the battery cost.

How is the battery saving calculated?

It is the energy the battery lets you self-use each year, multiplied by the difference between what you would pay to buy power and what you would get for exporting it. The bigger that gap, the faster the battery pays back.

Do batteries last long enough to pay back?

Batteries are warrantied for around 10 years. If the payback is well under that, it stacks up; if it is close to or beyond the battery life, the economics are marginal, though backup power has value in outages.

Solar Battery Payback Calculator

This New Zealand calculator estimates the payback period on a home solar battery. A battery lets you store your own solar power and use it in the evening, instead of exporting it for a low buy-back rate and buying power back at the retail price. You enter the battery cost, the energy it lets you self-use, and your power and export rates, and the tool shows the yearly saving and payback.

Battery cost (installed)

Self-used kWh per year

Power price (per kWh)

Export buy-back (per kWh)

battery payback period

Saving per year$0

Saving per kWh shifted$0

The saving is the gap between the power price and the export buy-back rate, on the energy the battery shifts to evening use. Compare the payback with the battery's warranty (often around 10 years). Backup power in outages is a benefit beyond the dollars. Estimate only.

How it works

The calculator finds the saving per kWh as the difference between your power price and the export buy-back rate, since each kWh the battery lets you self-use both avoids buying at retail and would otherwise only have earned the low export rate. It multiplies that by the self-used kWh a year for the annual saving, then divides the battery cost by it for the payback period.

Worked example

Self-using 2,500 kWh a year, with a 30 cent power price and a 12 cent export rate, the saving is 18 cents a kWh, about $450 a year. A $12,000 battery would take around 27 years to pay back on that alone, which shows why battery economics are often marginal in NZ today.

Related calculators

Solar Payback Calculator: solar panels payback.
Electricity Cost Calculator: power costs.
EV Home Charger Payback: charger payback.
Heat Pump Savings Calculator: heating savings.

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