Estimate how many years a home battery takes to pay back from its net cost after incentives, your yearly bill saving and a yearly change in that saving.
This calculator gives a simple payback: how many years of bill savings it takes to recover what you paid after incentives. It is a screening test, not a financial forecast.
How to estimate the yearly saving. Work out how many kWh the battery shifts each day (usually the smaller of its usable capacity, your spare solar, and your evening and overnight use), then multiply by the difference between the import rate you avoid and the feed-in tariff you give up, and by 365. Add any VPP payments you are confident of.
Worked example (illustrative). A 13.5 kWh battery shifts about 9 kWh a day. Avoided import 35 c/kWh minus a 5 c/kWh feed-in tariff = 30 c, adjusted for about 10% round-trip losses: 9 × $0.30 × 0.9 × 365 ≈ $887 a year. With a $14,000 quote and $3,200 of incentives the net cost is $10,800, so simple payback is about 12 years at a flat saving.
The yearly change box. A positive figure models rising tariffs; a negative figure models capacity fade or a falling peak rate. Try both.
Limits. No discounting, finance interest, replacement costs or inverter failures are included. Compare the payback with the warranty term (often 10 years) before deciding. See the payback worked example for the full method.
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