In brief
For every minute of a reference year (365 days), the position of the sun and the strength of its direct radiation under a clear sky are calculated. The sun position comes from the NREL Solar Position Algorithm (SPA) in the pvlib library. All times are given in standard time of the project time zone, without daylight saving time.
V.1.1 Time axis
The sun path is calculated for a reference year without a leap day (365 days) from 1 January 00:00 to 31 December 23:59 in steps of 1 minute. This time series is the time axis of the entire simulation; every further check takes place at one of these instants. Only minutes in which the apparent solar elevation is above 0° are considered.
All times are given in the project's time zone as standard time, with a fixed offset from Coordinated Universal Time (UTC); half-hour time zones are possible. Daylight saving time is not applied. During daylight saving time, local clock time equals the stated time plus one hour.
V.1.2 Solar position
Solar azimuth and elevation are calculated by the Solar Position Algorithm (SPA) of the National Renewable Energy Laboratory (NREL), as implemented in the pvlib library. The authors state an uncertainty of ±0.0003° for the algorithm. The apparent solar elevation including atmospheric refraction is used; this matters at low sun.
The solar position is calculated once per simulation, for the centre of all corner points of the PV areas, and applies to all areas and observation points. Azimuth is counted clockwise from geographic north: 0° north, 90° east, 180° south, 270° west.
V.1.3 Direct irradiance under clear sky
For every minute, the direct normal irradiance (DNI) in W/m² is calculated with the clear-sky model of Ineichen and Perez. Atmospheric turbidity enters as a climatological Linke turbidity factor, which pvlib obtains for the site from a worldwide database: monthly values, interpolated to the individual day. In Ch. V.5, the DNI determines the brightness of the reflection; a low sun therefore produces a weaker reflection than the midday sun.
The calculation uses the site altitude: the mean terrain height of all corner points of the PV areas, one altitude per simulation. Direct irradiance rises with altitude because the light crosses less air. For 47.5° north on 21 June, the model gives about 7 % more at midday, about 17 % more at 15° sun elevation and about 45 % more at 5° at 1,000 m than at sea level. This affects luminance and illuminance; the glare duration stays practically the same. Simulations before calculation state 2026.10.11-5 were calculated for sea level.
Deep dive: algorithm, time step, refraction
| Quantity | Value |
|---|---|
| Algorithm | NREL SPA (Reda and Andreas 2004), pvlib, method nrel_numpy |
| Time step | 1 minute, 1 January to 31 December of a year without a leap day |
| Time reference | fixed UTC offset of the project time zone, without daylight saving time |
| Solar elevation | apparent elevation with refraction; minutes with elevation above 0° are calculated |
| Location of the solar position calculation | centre of all corner points of the PV areas |
| Direct irradiance | clear-sky model Ineichen and Perez (2002), climatological Linke turbidity (monthly values, interpolated to the day), site altitude = mean terrain height of the PV corner points |
The extent of a project is irrelevant for the location of the solar position calculation: a shift of 1 km changes the solar position by less than 0.01°.
V.1.4 Relation to the guidelines
| Topic | Guideline | PV-GlareCheck |
|---|---|---|
| Temporal resolution | Key points paper of the Federal Highway Authority: year-round calculation with a resolution of at least 5 minutes (ch. 4, p. 7). FAA, Interim Policy 2013: entire calendar year in 1-minute intervals (78 FR 63277). LAI, OVE R 11-3 and the EnergieSchweiz/Swissolar guide state no temporal resolution. | 1 minute |
| Solar elevation | LAI: sun from sunrise to sunset (Annex 2 No. 4, p. 24). OVE R 11-3 and the EnergieSchweiz/Swissolar guide state no minimum solar elevation; the guide counts glare only at a direct normal irradiance above 120 W/m² (p. 38). | calculated down to 0°; a minimum elevation can be set in the report (Ch. V.8); the report offers no threshold for direct irradiance |
| Sun | LAI and OVE R 11-3: point-shaped (p. 24 and section 9, p. 14) | disc of 0.53° |
| Weather | astronomically maximum or cloudless (Ch. V, section "Overview") | clear sky |