Is a home battery worth it? The honest calculation

What a battery actually earns

A common error is to assume the battery saves the full electricity price. It does not. The kilowatt hour it releases at night would not have needed storing during the day — it would have been exported and would have earned the feed-in tariff.

The battery therefore only earns the difference. At 35 cents retail and 7.86 cents feed-in, that is about 27 cents per kilowatt hour. Everything the battery costs has to come out of that spread.

The cost per stored kilowatt hour

Do the arithmetic: purchase price divided by usable capacity, divided by the guaranteed cycle count, divided by the round-trip efficiency.

An example: 8 kWh of usable capacity for €4,400, 6,000 cycles, 90 percent efficiency. That gives 4,400 ÷ (8 × 6,000 × 0.9) = 10.2 cents per stored kilowatt hour. Against a 27-cent spread there is a surplus — the battery carries itself.

The catch: the cycle count is only reached if the battery really is fully cycled daily. In practice, household batteries manage 200 to 280 full cycles a year. After 20 years that is around 5,000 cycles — calendar ageing becomes the limiting factor rather than cycling.

Why self-sufficiency saturates

A battery can only move one day's surplus into the following night. Once it is large enough to bridge an average night, additional capacity only helps on a handful of unusually productive days.

That is exactly what the saturation curve in the storage calculator shows. The first four kilowatt hours typically lift self-sufficiency from 30 to 55 percent. The next four add about ten points. The third four add three.

Winter self-sufficiency is unreachable with household-scale storage. December in Germany yields roughly a tenth of June — that would require a seasonal store, not a daily one.

When a battery is worth it anyway

Not every purchase is a return-on-investment decision. A battery with backup power has a value that does not express itself in cents per kilowatt hour. Households with a heat pump or an electric car shift considerably more energy and reach their cycles more easily.

Conversely: if the choice is between more modules and a battery, more modules almost always win. An extra kilowatt peak costs a third of a kilowatt hour of storage and generates fresh electricity every year, instead of merely moving electricity that already exists.