Solar Self-Use & Battery Payback Calculator

Electricity you use while the panels are producing, added up over the year. It is the part the panels can feed directly. A yearly total cannot see the hours, so if your home is empty all day, use a lower figure than your meter suggests: smart-meter data split into day and evening is the best source.

The battery is filled from the surplus that daytime use leaves, up to its usable size times the number of days a year it is filled and emptied (250 is a reasonable start for a sunny climate, fewer for a cloudy one). It then covers evening and night use, and the round-trip efficiency, about 90 %, is lost on the way. Whatever is left over is exported.

A battery earns the difference between the price you pay for grid power and the price you would otherwise get for exporting that energy, on only a few hundred kWh to a couple of thousand kWh a year. When the export price is close to the buying price, or the battery costs a lot per kWh, payback can run past the battery's warranty, and the tool says so instead of showing a number.

It is a simple payback: cost divided by the first year's saving, with prices held constant. Panel degradation, price changes, inverter replacement, financing, grants, tax rules, standing charges and time-of-use tariffs are not included, so treat it as a first comparison, not a quote.

The production splits three ways: used at home (straight from the panels or later from the battery), lost in the battery, and exported. The three shares add to 100 %. Self-sufficiency is a different number: it is the share of everything the home uses that came from the panels.