Some practicians - and most of PV module's catalogs - usually specify the NOCT coefficient ("Nominal Operating Cell Temperature"), which is the temperature attained by the PV modules with free air circulation all-around, under standard conditions defined as:
Gincid = 800 W/m², Tamb=20°C, Wind velocity = 1 m/s, Open Circuit
The exact NOCT conditions are defined in the norm IEC/TS 61836.
Equivalence with the U-factor
The NOCT factor is related to our loss factor U by the thermal balance:
U · (Tcell - Tamb) = Alpha · Ginc · (1 - Effic) where U = Uc + Uv · WindVel.
which gives with the NOCT conditions:
(Uc + Uv · 1m/s) · (NOCT - 20°C) = Alpha · 800 W/m² · (1 - 0)
The definition concerns open-circuit modules (i.e. modules exposed at sun, but not really in use), therefore the Effic value = 0.
However in working conditions, the electrical energy is drawn from the module, and this thermal balance is affected by efficiency value, of the order of 15%. PVsyst also proposes this operating NOCT, issued from our U-value definition, to the choice of the user.
Validity of the specified NOCT
To our understanding, the NOCT value for a module is only (slightly) dependent on the cover (glass) and back kind of the module (plastic, glass), which are same materials for almost any module (concerning heat convection). It could be slightly different according to IR losses, but the IR emissivity is about the same for any materiel used in the PV collector.
As a confirmation of this, a team of the NREL measured side-by-side, at sun during several days, 3 modules with respectively 42.4, 49.7 and 52.3 NOCT manufacturer's values, and these 3 modules showed exactly the same temperature within 0.2°C !
On the other hand, the NOCT doesn't include any information about the mounting mode (free ventilated, integrated and insulated, etc). It is always given for a "nude" module.
Therefore the NOCT is useless for the module temperature evaluation in any conditions during the simulation, and this is the reason why we did not include it in the PVsyst parameters.