I wonder how HOMER takes into account the efficiency of the PV panels. Could you tell me more about that?
PV panels are rated according to how much power they produce under standard test conditions (incident radiation of 1 kW/m2 and a particular temperature, which I forget). HOMER always deals with the PV array in terms of rated kW, not in m2. So it does not need to know the efficiency. That's not to say the efficiency doesn't matter, but it is already accounted for in the power rating of the panel.
By the way, HOMER assumes that the output of the PV array is linearly proportional to the incident radiation, so if the radiation is 0.75 kW/m2, the array will produce 75% of its rated output.