Batteries are the most emotional purchase in home energy. They're also the one where the arithmetic is easiest to check — you only need two numbers from your existing system.
The two numbers
- How much you export per day — that's the energy a battery could capture instead.
- How much you import after sunset — that's the energy a battery could replace.
The useful figure is whichever is smaller. A battery can't store what you don't export, and it can't save you money on electricity you were never going to buy.
A worked example
Say you export 12 kWh a day and import 8 kWh each evening. A battery can realistically shift about 8 kWh. At a 30c retail rate you'd avoid $2.40 of purchases, minus the ~5c per kWh you were being paid to export — call it $2 a day, or roughly $730 a year. Against a $10,000 installed battery, that's a payback near thirteen years before you account for degradation.
Change the inputs and the answer changes fast. Higher evening usage, a time-of-use tariff with an expensive peak, or a state battery subsidy can pull that under eight years. A household that exports 4 kWh a day will never get there.
What doesn't show up in the arithmetic
- Blackout resilience. Worth real money if you're on a fringe-of-grid line, worth little if outages are rare.
- Virtual power plant payments. Some retailers pay you to let them discharge your battery on peak days. Read the cycling limits.
- Future EV charging. Changes your evening usage profile completely.
Sizing without overspending
Match usable capacity to your evening consumption, not to your solar size. For most households that's 8–13 kWh. Bigger batteries spend more of their life partly empty, which is the most expensive way to own one.
Warranty is the real spec
Compare warranties on throughput (total kWh cycled) and on retained capacity at end of term — usually around 70%. A battery that guarantees 70% at ten years is making a much stronger claim than one that guarantees ten years and nothing else.