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Quotes and forums use two percentages that look similar but are not.

• Self-consumption (Eigenverbrauchsanteil): how much of the energy produced stays in the house instead of being fed into the grid. Battery losses count as energy that is not fed into the grid.
• Self-sufficiency (Autarkiegrad): how much of the energy consumed does not come from the grid.

With a system that is large compared with consumption, self-consumption stays low even when self-sufficiency is high: there is much more energy than the house can use.

One house, thirteen batteries

A house using 4,500 kWh in a year, 6 kWp facing south with 981 kWh/kWp, the BDEW standard load profile [1] and the PVGIS weather year [4]. The system produces about 5,886 kWh in a year.

What the table shows

• The first kWh are worth the most. The first kWh removes 300 kWh a year from what is taken from the grid; going from 11 to 12 kWh removes 21.
• Cycles go down. A small battery fills and empties almost every day; a large one does not fill up on days with little sun.
• The surplus stays high. Even with 12 kWh, about 2,447 kWh are fed into the grid: when the battery is full, the surplus for the rest of the day has nowhere to go.

In euros, at the example rates

With electricity at 37.0 ct/kWh, the BDEW average for households in 2026 [8], and a feed-in payment of 7.70 ct/kWh, the rate for systems up to 10 kW commissioned between August and December 2026 [10], this house’s electricity spending falls, compared with no battery, by about €349 a year with 5 kWh, €425 with 8 kWh and €456 with 12 kWh. This is the difference over one year, not a payback time: purchase price, battery lifetime and incentives are left out. Redo the sums with your data.