I have been attempting to use HOMER and I could not find the way I could input generator efficiency data nor is there a built-in data base of generators to choose from as is the case with wind turbines for example.  However, the sample files have examples of microturbines and fuel cells with efficiencies that differ from the default one.

In HOMER, rather than entering the generator's efficiency, you enter its fuel curve, which specifies fuel consumption versus power output.  For most constant-speed internal combustion generators, that fuel curve is very close to a straight line with a nonzero intercept (the no-load fuel consumption).

So you can specify it with just two numbers.  The corresponding efficiency curve (efficiency versus power output) is therefore nonlinear and harder to specify.  Note that the two numbers HOMER asks for are the slope and the intercept coefficient, which is the intercept divided by the size of the generator.  That allows HOMER to apply the two numbers to a range of generator sizes.

The default fuel curve inputs are appropriate for diesel generators.  Yes, the fuel curve inputs for fuel cells and microturbines in the sample files differ from the default values.  That's because fuel cells and microturbines do not have the same fuel curves as diesel generators.