This is a great question. Basically, the losses can be very accurate _if_ you have accurate data, especially when provided from datasheets or the manufacturers directly (see some available device thermal loss models here: https://www.plexim.com/download/thermal_models
). The effects of output capacitance can be built into the data, but essentially, PLECS linearly interpolates/extrapolates from the two nearest datapoints for voltage, current, and junction temperature, with the ability to add other dependencies such as gate resistance. Of course, you can also use the tool to provide a rough estimate if you do not have highly granular data, or data for the very specific operating point you may need to study. But generally the goal when the manufacturers benchmark their thermal loss models for PLECS is to be within a few percentage points of accuracy, which is anyway the best you can expect from a model, idealized or physical in nature. I hope this helps!