Double glazed windows vs triple glazed. I've yet too see a ROI calculation on that.
This ROI argument can go on forever. The fact is a PV/battery storage will ( at some point ) pay for itself. The Bentley ( nor nissan micra ) probably won't, Neither will a 50" TV , my exclusive holiday abroad etc. etc. etc.
As I've said before ; most of you have got mortgages on your residential home. 25yrs later when it's paid off ; could have cost you easily over a £1million in interest payments alone. THAT IS DUMB!. But millions of people do it ( even with a repayment ) ; why?. No alternative?, maybe . ROI on that is well - pointless.
We all pick our own apples. Whether we eat them or not is a separate issue 💥