I have some hourly data of surface solar radiation downwards (SSRD) measured in J/m^2 from the ERA5 dataset (ECMWF - European Centre for Medium-Range Weather Forecasts).

How can I convert these to global horizontal irradiance (GHI) in kWh/m^2? Is it just simply SSRD/3600000?

I know the formula for GHI is:

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DNI = direct normal irradiance

DHI = diffuse horizontal irradiance

z = solar zenith angle

Is SSRD sufficient for calculating GHI or do I need other data?

1 Answer 1


As written in the ERA5 description, variable SSRD ...

...is the model equivalent of what would be measured by a pyranometer (an instrument used for measuring solar radiation) at the surface.

Therefore it is equivalent to GHI.

Your formula for units conversion (J/m2 to kWh/m2) is correct (SSRD/3600000).

Just maybe the better term here is irradiation (because in this case it is an aggregation over 1 hour). Irradiance refers rather to instantaneous values.

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