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Efficiency and Relative Efficiency

Efficiency

The efficiency of a PV module is defined 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 can also define an efficiency based on the active area (i.e., the cells area), which is more representative of the PV technology's inherent efficiency. However, this is used only for specialized applications.
  • When defining nominal efficiency, Pmpp can be either the module's Pnom (nameplate) or the Pmpp at STC. The Pmpp at STC can be Imp × 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 studying low-light behavior, we define relative efficiency with respect to 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 low-light studies based on measured data, we should use the Pmpp values at 1000 W/m² measured under the same conditions as the other points as the reference. We cannot use the Pmpp of the parent module, as its values are typically datasheet values that bear no relation to the measured data of the specific module being tested.