What do you think about including a Diesel Price Index and a Consumer Price Index in the Net Present Value calculation?

HOMER assumes that all prices inflate at the same rate over the project lifetime.  That allows us to factor inflation out of the analysis by using the real interest rate, and reporting all costs in constant dollars.

To model the effect of price changes over time, you could simply do a sensitivity analysis on any price variable.  Another option is to export the cash flow sequence, draw it into a spreadsheet, and modify each year's grid power costs to reflect escalation over time.  (You can see the cash flow sequence by clicking the Details button on the Cash Flow tab of the Simulation Results window.)