Hey,
With my very limited experience of the software, I found there are a couple of way you could address out of scope things like that, but both require doing external calculations.
Either on the Cashflow page click on "Send to Excel with equation" and modify the exported spreadsheet.
Otherwise you could do the calculation externally and plug the results in the "Operation and Maintenance Costs" section, using scheduled values.
I "faked" having a variable surcharge on self-consumption using both the spreadsheet method and a scheduled Operation and Maintenance variable cost by generation when looking at a potential project in Germany. (I'm digressing but in Germany apparently they will start charging you about 2 or 3 cents for using your own electricity!)
The only "problem" was that you cannot do a parametric analysis on that (by providing several schedules for example). I switched to a fixed value instead of schedule to do parametric analysis. It was introducing a tiny bit of an error in my case (on the first 2 years only) but not enough to prevent parametric analysis from doing it's job, so once I was roughly satisfied with my simulation, I switched back to a scheduled value to get proper financial metrics.
Pros of doing it in Excel: you see exactly what's going on, it's a little bit cleaner and apples and bananas aren't lumped in the Operation and Maintenance costs.
Pros of doing the calc and plugging the results back in SAM: everything stays in SAM, so if you make changes it's less of a pain. But the biggest pro is that you can still do parametric/sensitivity/statistical analysis and get proper results.
I hope this helps.
Julien
