Battery: when it pays, and when it doesn't
A battery does not generate electricity. It stores daytime solar you would otherwise export, so you can use it in the evening instead of buying grid electricity. Its value is therefore not the full price of that electricity — it is the difference between the price you pay and the feed-in tariff you would otherwise have received. That single point explains most battery payback results.
When a battery is more likely to help
- You have real night-time use to shift — evening cooking, heating or cooling, hot water, or electric vehicle charging — and that use is currently bought from the grid.
- Your price gap is large: the price you pay is much higher than your feed-in tariff, so each stored kWh saves more.
- You regularly export a lot of solar, so there is surplus to store on most days.
- The battery cost, after any rebate you actually received, is low enough that the extra yearly value repays it within the battery's warranted life.
When a battery is more likely to hurt payback
- You are rarely home in the evening, or your night use is small, so the battery sits partly unused.
- Your exported solar is small, so there is little surplus to store.
- The battery cost is high relative to the extra value it creates each year.
Worked example (example inputs only, not market prices)
Take the calculator's earlier example: 6.6 kW, $6,000 solar cost, 20 kWh/day use, 50% self-use, 35 cents buying price, 8 cents feed-in, 4.2 kWh per kW per day. Solar-only saving is $2,175.33 a year and payback is 2.76 years. Now add a battery with 5 kWh/day of night use to shift, 10 kWh usable capacity, 90% round-trip efficiency, and an $8,000 battery cost. Extra self-use is 4.5 kWh/day, or 1,642.5 kWh a year, worth $443.48 a year at the 27-cent price gap. Total saving rises to $2,618.80 a year, but total cost rises to $14,000, so payback with the battery is 5.35 years — longer, not shorter — and the battery cost alone would take 18.04 years to repay from its extra value. That does not mean batteries never pay; it means you must run your own figures, not assume one will.
Use the battery section on the calculator with your own night use, capacity, efficiency, and cost, and compare the "with battery" payback with the solar-only payback before deciding.
Estimates only, not financial advice. See the Disclaimer.