[bruce511]

The inverter keeps a log of electricity generated, used, battery used, imported and so on.

I put that in a spreadsheet which calculates money saved. (Which is reasonably complex because of pricing tiers although fortunately I don't have to deal with daily pricing changes.)

Then some math returns the return on investment per month and per year.

[throwaway2037]

Do you also subject the cost of upfront capital? Example: Imagine that you didn't have the upfront captial, and you needed to borrow it "from a bank" at X% for Y months.