Find your peak sun hours and design month
Peak sun hours (PSH) are the equivalent hours per day of full sun (1 kW per square metre) on your panels. Free government tools give monthly values for your exact location, and a stand-alone system must be sized for the worst month, not the yearly average.
How
- Open NREL PVWatts (worldwide coverage, strongest in the Americas) or the EU JRC PVGIS tool (Europe, Africa, Asia and more) and enter your coordinates.
- Read the monthly solar radiation in kWh per square metre per day on the tilted plane. This number is the same as PSH.
- Try three tilts: equal to your latitude, latitude plus 15 degrees and latitude minus 15 degrees.
- For each month, divide daily load (step power-3) by PSH. The month with the highest ratio is your critical design month, and the tilt giving the lowest worst-month ratio is your design tilt.
- In PVGIS, also run the off-grid tool with a trial array, battery and daily use. It reports the share of days the battery would be full or empty, which is a useful check on your sizing later.
- Record the design month PSH and chosen tilt.
- In snowy climates, enter a snow loss in PVWatts. NREL modelling put annual snow losses for fixed arrays in the contiguous US between 0 and about 16% (highest in Minnesota and Michigan), and suggested 4 to 7% for a 20 degree array in Nebraska. Use local figures where you can.
Do these first
Sources
- PVWatts Calculator. National Renewable Energy Laboratory, accessed 2026-11
- Photovoltaic Geographical Information System (PVGIS) online tool. European Commission Joint Research Centre, accessed 2026-11
- PVGIS 5 tools: off-grid PV systems. European Commission Joint Research Centre, accessed 2026-11
- Stand-alone system sizing (AE 868 Commercial Solar Electric Systems). Penn State University, John A. Dutton e-Education Institute, accessed 2026-11
- Integration, Validation, and Application of a PV Snow Coverage Model in SAM (NREL/TP-6A20-68705, 2017). National Renewable Energy Laboratory, accessed 2026-11