This indicator is particularly important as the photovoltaic modules typically lose a 0.45% productivity for each centigrade degree of temperature of the cells above 25 degrees of temperature. In a summer day, in the presence of high solar radiation, the cell temperature can arrive for example at 60 centigrade degrees, ie 35 degrees over the nominal 25, with a loss of productivity (PR) of the order of 15%.
The PR net loss due to temperature recovers these losses, in fact representing what the PR would be if the temperature of the cells had been stable at 25 degrees. So it is not affected by temperature changes during the day or depending to the seasons, resulting THE ONLY TRUE STABLE INDICATOR OF THE PLANT PERFORMANCE.

Lost solar hours = solar energy loss, calculated and weighed on different inclined planes of different sections of a photovoltaic plant that are not producing the expected energy output for a malfunction.
Eg if 100% of a plant is shut down for an hour in which the solar irradiation was 0.6 kWh/sqm, it will be lost 0.6 solar hours; if it was stopped only 50% of the plant, it would have lost 0.3 solar hours.

The lost solar hours trace the malfunctions of the system (plant availability).
With this information, the PV Asset Manager calculates the PR net of lost solar hours in a given period of time, that is, the PR that would be obtained if there were no lost solar hours (100% availability).
It is then immediately possible calculating the economic losses resulting from such malfunctions, that the PV Asset Manager shows in several reports and graphs.

1 standard solar hour = 1 kWh/sqm solar energy incident perpendicular to the plane of the photovoltaic modules. Ex. One hour irradiation at 1 kW/sqm or 2 hours at 0.5 kW/sqm, eventually weighed over the different plant sections.

The PV plant performance PR (Performance Ratio) is normally defined as follows:

PR (%) = actual energy yield / solar energy input through PV panels


  • Actual energy yield is expressed in kWh / kWp, which is the energy measured by the output meter divided by the PV plant rated output
  • Solar energy is expressed in kWh/m2

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