Solarposition.org

Methodology · updated 2026-10-15

How every number is made

Every angle, time and kilowatt-hour on this site comes from published, peer-reviewed methods and open data. Here is the whole chain, with its known limits.

Sun position: NREL SPA

Positions come from the Solar Position Algorithm of the US National Renewable Energy Laboratory (Reda & Andreas, 2004, revised 2008). It computes the Earth's heliocentric position from the VSOP87 planetary theory, applies nutation, aberration, the equation of the equinoxes, topocentric parallax and atmospheric refraction, and is accurate to ±0.0003° for the years −2000 to 6000. ΔT (the difference between terrestrial and universal time) follows the Espenak–Meeus polynomials.

The same implementation runs on the server (Pascal) and in your browser (JavaScript); both are checked against the reference example in the NREL publication and NOAA values at 29 control points on both hemispheres, including polar cases — the largest deviation is 0.00001°.

Sunrise, sunset and twilight

Events are found by scanning the geometric elevation of the sun through the local calendar day and refining every crossing by bisection to about one second. Thresholds: −0.833° for sunrise and sunset (refraction plus the solar radius), −6°, −12° and −18° for civil, nautical and astronomical twilight, and +6° for the end and start of the golden hour. Scanning handles polar day, polar night and days with two crossings without special cases. Local time uses the IANA time-zone database, including daylight saving time.

Sunlight: NASA POWER

Monthly averages of global horizontal irradiation, its diffuse part and 2 m air temperature come from NASA's Prediction Of Worldwide Energy Resources (POWER) climatology, derived from satellite observations and reanalysis. We store them on a 1° grid (latitudes 60° S to 74° N) and interpolate bilinearly to your point. Beyond that range the sun data is exact but yield figures are not shown. The data was obtained from the National Aeronautics and Space Administration (NASA) Langley Research Center's Prediction Of Worldwide Energy Resources (POWER) project funded through the NASA Earth Science Division; we use the POWER monthly climatology retrieved in August 2026.

From sunlight to kilowatt-hours

  1. For a representative day of each month (the 15th), the Ineichen–Perez clear-sky model gives the shape of the day in 20-minute steps; it is scaled so that the day's total global and diffuse irradiation match the NASA monthly means.
  2. Direct light follows from the difference between global and diffuse. Light on the tilted panel is computed with the Hay–Davies anisotropic sky model, with Brandemuehl–Beckman effective angles for diffuse and ground-reflected light.
  3. Reflection losses at the glass use the ASHRAE incidence-angle modifier (b0 = 0.05) for each component separately.
  4. Module temperature uses the Faiman model (U0 = 25, U1 = 6.84) with the monthly mean temperature plus 3 °C for daytime and 1.5 m/s wind; power falls by 0.38% per °C above 25 °C.
  5. A logarithmic low-light correction and 8% for wiring, mismatch and soiling complete the DC yield.

The chain is tested against the European Commission's PVGIS 5.3 for five locations from Cairo to Stockholm: annual yields agree within 3% (Warsaw −2.1%, London −2.4%, Madrid −0.6%, Stockholm −1.7%, Cairo −1.5%, with PVGIS's own climate as input). For off-grid sizing we add the charge controller (97%), the battery round trip (95% LiFePO4, 92% sodium-ion) and the inverter (92%).

Snow. Where a month's mean temperature is near or below freezing, ground albedo rises from 0.2 to 0.4 (−2 to 0.5 °C) or 0.65 (below −2 °C). Snow lying on the panels is not modelled — a real, and for shallow tilts large, extra loss.

Forecasts: DWD ICON-EU

The 48-hour solar forecast for about 650 European cities uses the ICON-EU model of the German Weather Service (Deutscher Wetterdienst), published as open data four times a day. Our own collector downloads every run, extracts direct and diffuse radiation, cloud cover, temperature and wind for each city and keeps an archive. The forecast is turned into panel output with the same transposition and module model as above. Data basis: Deutscher Wetterdienst (DWD), ICON-EU open data under CC BY 4.0 (see the sources below); for each city and hour we combine the newest usable values of several runs, reject outlier steps and convert the radiation into panel output.

Places and maps

Place names, coordinates, elevations and time zones come from GeoNames (CC BY 4.0): every town of 15,000 people or more, about 31,700 places. Maps are © OpenStreetMap contributors (ODbL), served by the OpenStreetMap Foundation's tile servers.

Data sources and licences

Every number on this site is computed from the sources below. Where a source asks for a particular credit, it is quoted here as the source asks for it; the footer of every page and the notes next to the data point to this list.

  • Sun position. NREL Solar Position Algorithm (Reda & Andreas, Solar Energy 76, 2004), a published algorithm in our own implementation. NREL SPA
  • Climate. The data was obtained from the National Aeronautics and Space Administration (NASA) Langley Research Center's Prediction Of Worldwide Energy Resources (POWER) project funded through the NASA Earth Science Division. We use the POWER monthly climatology, retrieved in August 2026, and interpolate it to each place. NASA makes it available free of charge without a licence of its own. NASA POWER
  • Weather forecast. Data basis: Deutscher Wetterdienst (DWD), ICON-EU open data, licensed under Creative Commons Attribution 4.0 (CC BY 4.0). Changes: for each city and hour we combine the newest usable values of several model runs, reject outlier steps and convert the radiation into panel output. Deutscher Wetterdienst · DWD Open Data · CC BY 4.0
  • Places. GeoNames, licensed under Creative Commons Attribution 4.0 (CC BY 4.0). Changes: we use the towns of 15,000 people or more and add web addresses. GeoNames · CC BY 4.0
  • Time zones. IANA time zone database (tz), in the public domain. IANA time zones
  • Map. © OpenStreetMap contributors; the map data is available under the Open Database License (ODbL) and the map tiles come from the OpenStreetMap Foundation's servers. The credit also stays in the corner of the map. OpenStreetMap copyright and licence
  • Fonts and icons. Big Shoulders Display, Instrument Sans and Instrument Serif under the SIL Open Font License 1.1; Material Symbols under the Apache License 2.0. SIL Open Font License · Apache License 2.0

Known limits

  • Monthly climate is an average: a real December can be half or double it. Autonomy exists for that.
  • Local horizon (mountains, buildings, trees) is not included in yield figures — check it on the sun map.
  • Elevations of places are for the town centre; the sun position changes only negligibly with altitude, but clear-sky light does.