Improved solar radiation forecast

With direct normal irradiance and clear sky index interpolation

Forecasts for solar irradiance just got a bit more accurate and easier to use for photovoltaic energy predictions.

Direct and diffuse solar radiation are now using clear sky models to preserve the correct daytime curvature even after 4 or more days of forecast. Previously, after 4 days of weather forecast, the daily maximum of solar radiation was a bit off, which could lead to inaccurate photovoltaic energy predictions.

Weather models return only one value for every 3 hours after 4 days of weather forecast to save space. This makes sense, because after 4 days, the forecast is less accurate and 1-hourly values should be trusted less. For application development, this is challenging, because different time resolutions are tricky to handle. Open-Meteo APIs interpolate data from 3-hour to 1-hour resolution. For correct results, this requires physical modelling and not just “linear interpolation”.

A clear sky model is now used which calculates the maximum solar radiation on a clear sky day at very high resolutions (every minute).

The following graph shows shows the corrected shortwave radiation in Southern Africa with peaks of up to 1090 watts per square meter.

Additionally, Direct Normal Irradiance (DNI) is now available. DNI shows the direct solar radiation of an area directly facing the sun. Larger photovoltaic installations use “tracker” to align pannels to the sun. This greatly improves energy production in the morning and afternoon.

The following graph shows direct radiation on a horizontal plane and DNI. The difference at 6:00 UTC time is 450 W/m² to 150 W/m². This factor of 3 in solar radiation is only caused by the inclination of the panel to the sun.

Important to notice: All solar radiation weather variables at Open-Meteo are given as backwards averages over the preceding hour. The value at 6:00 UTC is therefore the average radiation between 5:00 and 6:00. The instantaneous value at 6:00 will be much higher. Especially for photovoltaic energy predictions this can make a huge difference!

Next steps for solar radiation forecasts?

Some regional weather models offer 15 minutes solar radiation forecasts. This could improve short-term predictions for 2 days. 15 minutes data will be added in the next month to Open-Meteo APIs!

Do you have any questions, ideas or suggestions to improve Open-Meteo? Please write it in the comments, contacts us or create a GitHub issue!