Free Solar Battery Payback calculator with clear step-by-step results.
A home battery earns the difference between the import price it avoids and the export tariff it gives up, not the full import price - a distinction that changes payback dramatically. Round trip efficiency reduces delivered energy, and dividing warranted cycles by annual cycling shows whether payback arrives before the warranty expires.
Annual saving
Saving = usable capacity x round trip efficiency x cycles x (import price - export price)
Warranty life
Years = warranted cycles / cycles per year
Energy stored in the battery is energy you did not export. The saving is only the spread between what importing costs and what exporting would have paid.
Often marginally. Payback improves sharply with time-of-use tariffs where you can arbitrage cheap overnight import against expensive evening peaks.