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Efficiency and Relative Efficiency

Efficiency

The efficiency of a PV module is defined basically as the available electrical power, divided by the input power.

I.e. Efficiency = Pmpp / (Gincid * Area)

where:

  • GIncid = incident irradiance on the module [W/m²] |
  • Area is usually the module area, i.e. its full Width x Length [m²], including the frame.
  • We may also define an efficiency related to the sensitive area (i.e. the cells area) which is more representative of the PV technology basic efficiency. But this is only for very special uses.
  • When defining a nominal efficiency, Pmpp may be either the Pnom (nameplate) of the module, or the Pmpp at STC. The Pmpp at STC may be the Imp x Vmp, or the result of the model, which may be slightly higher.

Therefore we observe that there are several options for this definition !

Relative efficiency

For the study of the low-light behavior, we define the relative efficiency, with respect to the operation at 1000 W/m2, and the same temperature.

Relative efficiency = ( Efficiency (Gincid) / Efficiency (1000 W/m2) ) -1 = ( (Pmpp / GIncid) / (Pmpp [at 1000 W/m²] / 1000) ) -1

i.e. Rel Efficiency = ( (Pmpp / Pmpp [1000W/m²] * 1000 / GIncid ) ) - 1

For the Low-light studies according to measured data, we should use as reference the Pmpp values at 1000 W/m2, measured in the same conditions as the other points. We cannot use the Pmpp of the "father" module of course, as its values are usually the datasheet's values, which have nothing to do with the measured data of the specific module under test.