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A simple rule circulates in German-language guides: 1.0 to 1.5 kWh of battery capacity for every 1,000 kWh of consumption in a year, attributed to HTW Berlin [14]. We did not find it in an HTW document. On forums the answers are even wider: in two 2023 discussions on energiesparhaus.at they range from 5 to 12 kWh, with some advising against a battery altogether [12] [13].

We compared the rule with the calculator: the same BDEW standard load profile [1], the same PVGIS weather year facing south with 981 kWh/kWp [4], six combinations of consumption and power.

The comparison

“Where the curve flattens” runs from the last capacity at which each extra kWh still adds at least 2 points of self-sufficiency to the last at which it adds at least 1. The rule with 1 kWh per 1,000 kWh is rounded to the whole kWh.

What the table shows

• The rule falls short of the point where the curve flattens, in all six houses: with the rule, self-sufficiency is 62 to 71%; at the end of the range, 69 to 78%. With 1.5 kWh per 1,000 kWh the rule comes closer to the range: for the 4,500 kWh house with 6 kWp that is 7 kWh, within the 6 to 8 range.
• The system’s power matters. With the same consumption of 4,500 kWh, going from 6 to 10 kWp moves the range from 6–8 to 7–9 kWh: more surplus, more energy to shift to the evening. The rule, which looks only at consumption, does not see this.
• No capacity gives complete self-sufficiency. In winter production is not enough even during the day, and a bigger battery stays half empty: for the 4,500 kWh house with 6 kWp, even with 12 kWh self-sufficiency is 72%.

What it does not tell you

The comparison uses a standard profile and a PVGIS weather year from Kassel: for your house the numbers change, and the calculator redoes them with your data. It does not say which capacity to buy: the range is a reading of the curve, not purchasing advice, and the price per kWh of batteries is not part of the calculation.