Calculating photovoltaic yield correctly
Formula, location factors and performance ratio at a glance
The annual solar yield results from three factors: system capacity in kWp, location-specific peak sun hours, and the performance ratio. The formula is: yield = kWp × peak sun hours × PR. A 10 kWp system in Bavaria with 950 peak sun hours and a PR of 0.80 produces exactly 7,600 kWh/year – enough for a well-supplied four-person household without a heat pump.
Peak sun hours by region: Germany has very different levels of solar irradiation depending on the region. Bavaria and Baden-Württemberg lead with 950–1,050 peak sun hours. Saxony and Thuringia reach 900–950 h, while Schleswig-Holstein and Hamburg trail with 800–870 h. The European Commission provides location-accurate figures via the PVGIS tool – free of charge and available down to postcode level.
Performance ratio in practice: The single most important factor for a PV system's efficiency is the inverter. Modern devices operate at 97–98% efficiency, older ones at only 90–93%. On top of that come temperature losses: silicon solar cells lose output at high temperatures (about –0.4%/°C above 25°C). A system on a dark flat roof without rear ventilation only reaches 75–77% PR in summer, while a well-ventilated pitched-roof system reaches 82–85%.
Economic viability: The payback period of a PV system is around 8–12 years as of 2026. For systems up to 10 kWp commissioned between 1 August 2026 and 31 January 2027, the grid operator pays 7.70 cents/kWh for surplus feed-in and 12.22 cents/kWh for full feed-in (Bundesnetzagentur, section 48 EEG 2023); up to 40 kWp the rates are 6.66 and 10.24 cents/kWh. Because a self-consumed kilowatt-hour is worth 30–35 cents – far more than the feed-in rate – battery storage pays off to raise the self-consumption share to 60–80%. For an accurate calculation, maintenance costs (0.5–1% of the investment per year) and module degradation (0.3–0.5%/year) should also be factored in.