I find it difficult to get the generator fuel curves correct and have to fudge to get the right consumption figures

You mentioned that you have had trouble with the generator fuel curve, so I wanted to let you know that the new version of HOMER has a "fuel curve calculator" window that helps you calculate the fuel curve slope and intercept inputs.  Now you can enter real fuel consumption data (such as L/hr versus kW output) and let HOMER figure out the corresponding fuel curve inputs.