When calculating utility expenses over the full 25 years, the system takes into account both how much you expect utility rates to rise and lowered solar production over time due to degradation. Thus, a homeowner will be purchasing more power from their utility over time (due to lowered solar production due to degradation) and will be purchasing that power at a higher rate (due to the utility escalation rate).
Please note that your organization can control the utility escalation rate and we're happy to update the default rate for solar degradation at your request.