OpenWeather’s Solar Irradiance and Solar Panel Energy Prediction services provide solar radiation data and estimates of photovoltaic energy generation for projects of any size. They combine established scientific methods with OpenWeather models and data through APIs designed for straightforward integration.

Solar irradiance and solar irradiation are closely related, but describe different quantities:

Solar irradiance · W/m²

Solar power received per unit area. The API provides average and maximum irradiance for the requested interval.

Solar irradiation · Wh/m²

Total solar energy received per unit area over a specified period.

Multiply the average irradiance over an interval by the interval’s duration in hours:

Example at 400 W/m² average irradiance:

Our models provide three key components of solar radiation:

• Direct Normal Irradiance (DNI). Solar irradiance received directly from the sun by a surface perpendicular to the sun’s rays. DNI is particularly relevant to concentrating solar technologies and systems that track the sun.
• Diffuse Horizontal Irradiance (DHI). Solar irradiance received by a horizontal surface from sunlight scattered by the atmosphere and clouds, excluding direct sunlight.
• Global Horizontal Irradiance (GHI). Total solar irradiance received by a horizontal surface. GHI combines DHI with the horizontal component of direct solar irradiance and is widely used in photovoltaic performance modelling.

These components can also be expressed as irradiation when accumulated over a specified period. We provide GHI, DNI and DHI for both Clear Sky and Cloudy Sky models.

Our models build on established methods, including Simplified Solis and DIRINT, with adaptations developed for the OpenWeather service and evaluated against reference datasets.

The OpenWeather Clear Sky model estimates the solar radiation reaching the Earth’s surface under cloud-free conditions. It accounts for the position of the sun, site elevation and relevant atmospheric conditions to calculate clear-sky GHI, DNI and DHI.

The model builds on Simplified Solis, with adaptations developed for the OpenWeather service. These adaptations address the timing of calculations within each reporting interval and the treatment of solar angles around sunrise and sunset.

Clear-sky estimates provide a baseline for understanding the solar resource at a location. They also serve as the starting point for the OpenWeather Cloudy Sky model, which estimates the effect of clouds on solar radiation.

The OpenWeather Clear Sky model uses the following data to calculate clear-sky Global Horizontal Irradiance (GHI), Direct Normal Irradiance (DNI) and Diffuse Horizontal Irradiance (DHI):

• Date and time (required input) — determine the sun’s position for the requested period.
• Geographical coordinates (required input) — identify the location and support solar geometry calculations.
• Sunrise and sunset times (retrieved automatically from OpenWeather data) — define the daylight period and support adjustments near the horizon.
• Height above sea level (retrieved automatically from OpenWeather data) — specifies the elevation of the requested location.
• Surface pressure (retrieved automatically from OpenWeather data) — describes atmospheric pressure at the location.
• Near-surface air temperature (retrieved automatically from OpenWeather data) — represents local near-surface atmospheric conditions.
• Solar zenith angle (retrieved automatically from OpenWeather data) — describes the angle between the sun’s position and the local vertical, affecting how sunlight reaches a horizontal surface.

OpenWeather adaptations

Interval timing. Solar radiation varies throughout each reporting interval as the sun moves across the sky. A calculation made only at the beginning or end of an interval may therefore be less representative of conditions during that period.For hourly calculations, OpenWeather applies a 30-minute backward shift to use the solar position at the interval midpoint. This helps align the solar geometry used in the calculation with the period represented by the returned data.

Sunrise and sunset. Irradiance calculations are particularly sensitive when the sun is close to the horizon. At these low solar elevations, small differences in solar geometry can produce unrealistic calculated values.OpenWeather uses sunrise and sunset times to adjust the treatment of solar angles during these periods. These corrections help limit anomalous values as the sun rises above or falls below the horizon.

The model returns clear-sky GHI, DNI and DHI, representing the global horizontal, direct normal and diffuse horizontal components of solar radiation under cloud-free conditions.

The OpenWeather Cloudy Sky model estimates the solar radiation reaching the Earth’s surface with the effect of clouds included. It combines clear-sky radiation estimates with cloud information and atmospheric conditions to calculate cloudy-sky GHI, DNI and DHI.

The model uses a cloud-transmittance correction developed by OpenWeather, together with a modified DIRINT approach for estimating direct normal irradiance from global horizontal irradiance.

Clear-sky estimates provide the baseline, while the cloud correction accounts for the reduction in solar radiation caused by clouds.

The OpenWeather Cloudy Sky model uses the following data to calculate cloudy-sky Global Horizontal Irradiance (GHI), Direct Normal Irradiance (DNI) and Diffuse Horizontal Irradiance (DHI):

• Clear-sky GHI (calculated automatically by the Clear Sky model) — provides the baseline before cloud attenuation is applied.
• Dew-point temperature (retrieved automatically from OpenWeather data) — provides information about atmospheric moisture.
• Cloud cover (retrieved automatically from OpenWeather data) — supports the calculation of cloud transmittance.
• Date and time (required input) — determine the sun’s position for the requested period.
• Geographical coordinates (required input) — identify the location and support solar geometry calculations.
• Height above sea level (retrieved automatically from OpenWeather data) — specifies the elevation of the requested location.
• Surface pressure (retrieved automatically from OpenWeather data) — describes atmospheric pressure at the location.
• Near-surface air temperature (retrieved automatically from OpenWeather data) — represents local near-surface atmospheric conditions.
• Solar zenith angle (retrieved automatically from OpenWeather data) — describes the angle between the sun’s position and the local vertical, affecting how sunlight reaches a horizontal surface.

As in clear sky model, most of the input data is taken from the OpenWeather datasets and products.

OpenWeather adaptations

Cloud-transmittance correction. Cloud cover, expressed as a percentage, is converted into a coefficient representing the estimated transmission of solar radiation through clouds. This coefficient is applied to clear-sky GHI to calculate GHI with cloud attenuation included.

Higher cloud cover generally produces greater attenuation. In cloud-free conditions, the cloud correction does not reduce clear-sky GHI.

Direct and diffuse components. After cloud attenuation is applied, the model uses a modified DIRINT approach to estimate DNI from GHI. Solar geometry, surface pressure and dew-point information support this calculation. The direct and diffuse components describe how the radiation reaching the surface is distributed between direct sunlight and scattered light.

The model returns cloudy-sky GHI, DNI and DHI, representing the global horizontal, direct normal and diffuse horizontal components of solar radiation with the estimated effect of clouds included.

We conducted training and corrected our models by using reference data. As a result, we managed to achieve a sufficiently high accuracy from the product. As reference data, we used data from the ECMWF meteorological agency and the Copernicus programme (the European Union’s Earth Observation Programme). Thus we not only took data from weather models but also measured historical data from sensors. We used ERA5 as a weather model source and the CAMS programme data as a sensor’s real data. The ERA5 model has a resolution of 0.36 degrees and contains only historical data with a one-week delay. You can learn more about ERA5 here.

More information on data from the CAMS programme can be found here.

Most of the input data to the OpenWeather Cloudy Sky and Clear Sky models is taken from the OpenWeather datasets and products.

Please let us know if you have any questions about the Solar Irradiance API, and our technical support team will be happy to help you.

OpenWeather provides solar irradiance forecasts up to 15 days ahead, combining numerical weather prediction data, local correction methods and satellite-based short-term forecasting.

General forecast

The general forecast uses numerical weather prediction data together with the OpenWeather solar irradiance models to estimate GHI, DNI and DHI.

Improved correction methods, informed by local site data, help reduce systematic forecast errors and better represent local irradiance conditions.

Short-term satellite-based forecast

For supported locations across Europe, the Middle East and Africa, satellite observations further improve forecasts up to three hours ahead. Successive satellite images are used to estimate cloud movement and forecast its effect on solar irradiance.

These short-term forecasts are refreshed every 15 minutes as new satellite observations are processed. Longer-range forecasts continue to use numerical weather prediction data with the corrections described above.

The 15-minute refresh frequency applies to the satellite-based short-term forecast. Refresh frequency and satellite observation age are separate: imagery may have been acquired earlier because data delivery and processing take time.

Updates to the Solar Irradiance API’s models, coverage and forecast behaviour.

Three release entries are reserved below. Dates, final ordering and the third release are still to be supplied. The first two summaries are proposed copy for the updates discussed.

Release date to be added Satellite-based forecasts up to three hours aheadDraft release entry

Added satellite-based solar irradiance forecasts up to three hours ahead for supported locations across Europe, the Middle East and Africa. Short-term forecasts are refreshed every 15 minutes using updated Meteosat observations.

See Short-term solar irradiance forecasts for details.

Original rollout date to be addedSatellite observations added to the Cloudy Sky modelDraft historical entry

Integrated Meteosat satellite observations into the Cloudy Sky model to refine the representation of cloud conditions in supported regions when calculating GHI, DNI and DHI.

See Satellite observations for the current model description.

Confirm the coverage at the original rollout independently of today’s coverage. Keep a separate entry only if this integration was released separately from the short-term forecast.

Release date to be addedThird latest release — title to be addedReserved entry

Release summary to be added.

Record the customer-visible change, its affected coverage or forecast range, and any action required from API users. Use the actual release date.