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The inverters or power converters don’t operate always at their maximum efficiency, but according to an efficiency profile as function of the Power.

The  "European Efficiency"  is an averaged operating efficiency over a yearly power distribution corresponding to middle-Europe climate. This was proposed by the Joint Research Center (JRC/Ispra), based on the Ispra climate (Italy), and is now referenced on almost any inverter datasheet.  

The value of this weighted efficiency is obtained by assigning a percentage of time the inverter resides in a given operating range.

If  we denote by "Eff50%"  the efficiency at 50% of nominal power, the weighted average is defined as:

Euro Efficiency  =   0.03 x Eff5%  +  0.06 x Eff10%  +  0.13 x Eff20%  +  0.1 x Eff30%  +  0.48 x Eff50%  +  0.2 x Eff100%.

Now for climates of higher insolations like US south-west regions,  the California Energy Commission (CEC) has proposed another weighting, which is now specified for some inverters used in the US.

CEC Efficiency   =   0.04 x Eff10%  +  0.05 x Eff20%  +  0.12 x Eff30%  +  0.21 x Eff50%  +  0.53 x Eff75%. + 0.05 x Eff100%.

as defined in  the   Sandia_Guideline_2005.pdf  document, p17.

 

See also the construction of automatic efficiency profiles.