The solar insolation (in German: solare Einstrahlung) is the total radiated power of the sun (total solar radiation) impinging on a horizontally oriented surface and summed up over the day. The unit of solar insolation is power per area times time, corresponding to energy per area and is given in kilowatt hours per square meter (kWh/m²). On the X axis the time ME(S)Z means Central European (Summer) Time CE(S)T.
The measured values of the solar insolation are interesting i.a. for operators of photovoltaic (PV) systems, since from the solar insolation the input and thus the efficiency of the PV system can be estimated. However, it should be noted that the specified measured values were recorded on a horizontally aligned surface. Ideally, PV systems that are tilted to the south will achieve higher levels of solar insolation.