Calculate peak sun hours

Peak sun hours are energy: kWh/m² per day, expressed as hours of full reference irradiance (1000 W/m²). Weather-report "sunshine hours" measure something else: the time during which direct irradiance exceeds 120 W/m² (WMO definition) — a pale winter hour just above the threshold counts in full, exactly like a blazing June hour carrying many times the energy. Sizing only cares about energy. Berlin at 35° tilt: about 5.7 PSH in the best month, just over 1 in December — an annual average hides exactly that difference.

It simulates your module hour by hour through a typical meteorological year (TMY; 8,760 hourly values from PVGIS SARAH3, 2005–2023): solar position, angle of incidence, Perez transposition of the diffuse fraction, ground reflection, and the terrain horizon of your grid point. That is why it works for any azimuth — east-west or façade included. In cross-validation against PVGIS, the tilt gain deviates by less than 2% (up to 60° tilt) and 3% at the façade.

The typical year is an average; real years scatter by several percent. From the 19 individual years in the dataset, the calculator derives two bands: P50 is the median — half of all years exceed it. P90 is the value reached in 9 out of 10 years — the conservative number for sizing and financing. Plan with P50 and you will be disappointed every other year.

The terrain horizon (mountains, ridges) of your nearest grid point is included — the 0.25° grid is never more than ~17 km away. What is NOT included: shading from nearby buildings, trees, or the neighbour's gable; that comes on top, mostly in winter. Snow on the modules is not modelled either — the calculator warns explicitly for months where it is likely.

There is no minimum below which panels stop working — the question is how much energy you need. At 35° facing south, a module plane in the DACH region collects between about 3.4 (Hamburg) and 4.4 (Geneva) kWh/m² per day as an annual average — 1,250 to 1,600 kWh/m² per year (see the city table). Rule of thumb: panel wattage × daily peak sun hours ≈ ideal daily output before system losses; a 400 W panel at 3.6 PSH yields roughly 1.4 kWh per day. For off-grid sizing, design for the worst month and its P90 value, not the annual average — December in Berlin drops to about 1 PSH at 35° tilt.

No, but they are directly linked. Peak sun hours describe the solar resource per square metre of module area (kWh/m² per day); specific yield describes the output per installed capacity (kWh/kWp per year — also called full-load hours). The bridge: at reference irradiance one kilowatt-peak delivers exactly one kilowatt — the annual PSH total is therefore the ideal specific yield before real system losses (PVGIS assumes a flat 14%). Berlin at 35° south: about 1,300 kWh/m² per year works out to roughly 1,100 kWh/kWp.

Not yet. The hourly grid currently covers the DACH region: 1,155 grid points at 0.25° resolution, so the nearest point is never more than ~17 km away. For other regions the official tools are the best choice: PVGIS by the European Commission (worldwide except polar regions), or PVWatts by the US lab NLR (formerly NREL) for the United States. A US grid built on NSRDB hourly data is in preparation — same calculation path and the same P50/P90 bands as here.