Most energy modeling software incorporates the full load energy rate in terms of COP, EER, kW/ton and so on. To incorporate part load efficiency, manufacturers often list an IPLV. However, in lieu of entering an IPLV, building energy simulation software uses unloading curves - which are far more accurate than a simple IPLV. The full extent of unloading curves requires some education. The typical question is: where do you get them? They are usually obtained from product literature but are being made available for direct import from some manufacturers. If you cannot obtain them, it's usually best to use the defaults. Questions on this complex concept?
This is indeed a very important concept, in some cases it is difficult to obtain the partial load coefficients in order to define the unload curves, but in other cases it is easy to ask for the manufacturer for this data, so in my opinion one should always try to get this data in order to accurately model the proposed system.
Regards,
Santiago