Comment by avhon1

The "System Losses" breakdown shows the various additional factors they are derating the system by. The figures seem reasonable enough, and give an additional loss of 14%.

I put in my own address, which is in the 4.0-4.5 kWh/day region, and set the DC system size to 1 kW, which corresponds to 6m2 of panels (courtesy of their rooftop calculator). The NERL website estimated that such a system would yield between 2.45 and 6.48 kWh/day, with an annual mean of 4.71 kWh/day.

That works out pretty close to what the map indicates for my region: 4.5 kWh/m2day * 0.2 conversion factor * 0.86 losses factor * 6 m2 of panels = 4.64 kWh/day