Watt-peak
The nominal peak power is the power rating given by the manufacurer of the module or system.
Power
Ppk= A*effnom
In other words, if Ppk is the nominal peak power and A the area of the module(s), we have Ppk= A*effnom (1)
The actual power depends on the irradiance G and the real module efficiency eff which is a function of irradiance and module temperature Tm (and sometimes more things which we will forget about here). So you have the actual power:
P = G/1000 * A*eff(G,Tm) = G/1000*A*effnom * effrel(G,Tm) (2)
where we have written the actual efficiency as the product of the nominal efficiency effnom and the relative efficiency effrel(G,Tm).
Combining Eq. 1 and 2 you get: P = G/1000 * Ppk * effrel(G,Tm) (3)
Therefore, if you know the relative efficiency and the peak power, you don't need to know the nominal efficiency or the area.
BUT: if you want to know either the nominal efficiency OR the area, you will need to know the other of the two parameters. If you know the nominal efficiency and the peak power, you can calculate the area using Eq. 1: A = Ppk/effnom (4)
So we don't actually have a nominal efficiency in the program. That would be useful only to tell the user how much area his/her system would use.
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