Monthly solar output calculator

For the DACH region, guides quote 900–1,100 kWh per kWp per year; computed site by site (roof mount, 35° south) the German cities in the table below span roughly 950 (Hamburg) to 1,100 kWh/kWp (Munich), with Alpine and western Swiss sites up to ~1,200. A 10 kWp system therefore delivers around 10,000 kWh per year on a good south roof — but only some 200–450 kWh of that in December (about 2–4% of the annual total; winter as a whole contributes 10–15%). If a heat pump or EV must run through winter, plan with the December figure and the P90 band, not the annual average.

In Berlin at 35° tilt, about one fifth of the June value: an 8 kWp roof system delivers a good 1,000 kWh in June and just over 200 kWh in December. That is exactly why this calculator computes every month individually instead of dividing the annual yield by twelve — the annual average exists in no real month, and off-grid systems are sized for December, not for the mean.

The usual calculation is sun hours × system size × performance ratio — one flat factor for all losses. This calculator instead simulates each of the 8,760 hours with the PVGIS power chain: module temperature from air temperature, wind and irradiance (Faiman), the cell's low-light behaviour (Huld) and reflection loss at shallow incidence (Martin & Ruiz). The rule of thumb still runs as a comparison line — the 3–8% annual difference is itself an insight.

Measured against the hourly model: in summer. A fixed performance ratio ignores that modules work worse in the heat — in June it overestimates Berlin's yield by around 8%. In December the errors nearly cancel: the missing temperature penalty offsets the low-light and reflection losses. The common belief that the simple formula fails mainly in the dull winter is not confirmed by the simulation.

A real 3–4% of annual yield — 3.4% measured in Berlin, identical to PVGIS's own calculation. A roof-mounted or integrated module runs about 13 Kelvin hotter in full sun than a ventilated one, because the back sheds hardly any heat. Almost no free calculator asks about mounting; here it is a visible input.

Your system reaches that value in 9 out of 10 years — computed from the 19 real weather years (2005–2023) in the dataset, scaled to your configuration. For sizing and financing, P90 is the conservative, dependable number; the mean (P50) is missed every other year.

From the committed TMY hourly grid (PVGIS-SARAH3, 2005–2023, 1,155 grid points across the DACH region) with terrain horizon, calibrated to the multi-year mean. Cross-validated against PVGIS PVcalc at four reference sites, per month and for both mounting types: annual total within 1.7%, individual months within 6%. The remainder is explained — PVcalc adds a spectral correction whose underlying data is not freely available; our numbers are therefore deliberately slightly conservative.

Not yet. The hourly grid currently covers the DACH region (1,155 grid points at 0.25°). For other regions the official tools are the best choice: NREL PVWatts for the United States, or PVGIS by the European Commission (worldwide except polar regions) — this calculator is cross-validated against PVGIS. A US grid built on NSRDB hourly data is in preparation, with the same monthly resolution and P50/P90 bands.