Can you tell me how HOMER calculates the efficiency of the diesel gensets?

The Fuel tab of the Generator Inputs window lets you enter the fuel curve parameters, which define the graph of fuel consumption versus electric power output.  The fuel curve calculator helps you calculate those parameters when you have the kind of fuel consumption data typical of diesel spec sheets.  HOMER defaults the fuel curve parameters to reasonable values, but it doesn't assume anything.  You can enter any fuel curve you want.

Please look up 'fuel curve' in the index of the Help system for more information.