There would not appear to be any place to enter assumption about the future cost of electricity from the grid - it appears the economic analysis assumes the price will remain constant for the next 25 years.  Is that what HOMER assumes - no escalation?  It seems to me that future escalation of the cost of energy from the grid would be a significant factor in the economics of a PV or other renewable system.

You are right that HOMER assumes a constant grid power price over the project lifetime.  The same is true of fuel prices and other prices.

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.)