Using PVGIS weather data

PVGIS is a free solar radiation database provided by the European Commission. SunSolve integrates directly with PVGIS through the Search for weather data wizard on the Weather tab — type a location name, and SunSolve fetches a complete Typical Meteorological Year (TMY) weather file for that site. This is the quickest way to set up weather data for a new location.

PVGIS TMY files have a subtlety: timestamps are written as HH:00, but the irradiance values may correspond to a different time within the hour, depending on the underlying radiation database. PVGIS communicates this via a metadata field called irradiance_time_offset (in hours). SunSolve reads this field and automatically adjusts the solar position used for irradiance calculations — no user action is required.

The irradiance_time_offset field was added to PVGIS output headers in December 2024 to make the true irradiance timestamp explicit. Older PVGIS files did not contain this field, so modelling tools often had to infer the correct irradiance timing from the dataset documentation.

SunSolve currently uses PVGIS version 5.2.

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How it works

SunSolve computes the effective irradiance time as:

$$t_{\text{effective}} = t_{\text{timestamp}} + \text{irradiance\_time\_offset}$$

Solar position is evaluated at $t_{\text{effective}}$ for all irradiance-dependent calculations: plane-of-array transposition, incidence angles, horizon shading, and diffuse irradiance decomposition.

Internally this uses the Instantaneous observation period with a time offset, which applies the offset directly without the sunrise/sunset edge-case adjustments that SunSolve normally applies to conventional weather files (see time steps for details).

Offset by radiation database

The offset depends on which radiation database PVGIS uses for a given location:

DatabaseCoverageIrradiance meaningTypical offsetERA5, Global, Hourly average centred on mid-hour, 0.5 hours

SARAH2, Europe, Africa & parts of Asia (Meteosat), Instantaneous satellite observation, 0.1 – 0.2 hours (latitude-dependent)

NSRDB, Americas (GOES satellite), PVGIS-normalised timestamp representing the irradiance time directly, 0 hours

For example, with ERA5 data an irradiance value labelled 10:00 actually represents the hour centred on 10:30, so SunSolve evaluates solar position at 10:30. With SARAH2 data, the offset might be 0.1852 hours, shifting the effective time to approximately 10:11.

For NSRDB data, PVGIS already aligns the irradiance timestamp with the labelled hour, so the offset is 0 and no timing adjustment is required.

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What changed in March 2026

Prior to March 2026, SunSolve did not read the irradiance_time_offset field. Instead, it assumed all PVGIS timestamps represented the end of the hour — equivalent to an offset of −0.5 hours.

The offsets implied by this assumption compared with the offsets provided by PVGIS are:

DatabaseTrue offsetOld assumed offsetTiming differenceERA5, +0.5 h, −0.5 h, 1.0 hourSARAH2, +0.1 – 0.2 h, −0.5 h, 0.6 – 0.7 hourNSRDB, 0 h, −0.5 h, 0.5 hour

From March 2026 onward, SunSolve reads the irradiance_time_offset value provided by PVGIS and applies it directly when calculating solar position.

The impact of this change in solar position on annual energy yield depends on the location, system configuration, and other simulation inputs.

Users should reload the weather data for the site from PVGIS via the SunSolve wizard and rerun simulations to quantify the resulting change in modelled energy yield.