For an "investment" of £36,650, which includes the purchase price of the car, new, list price, I have a system that has eliminated both petrol and electricity bills completely, and is apparently leaving me over £300 in profit at the end of a year. (OK I'm not counting public charging in that, that's just my little luxury.) I'm not sure how to calculate a return on it all, but I would have had to buy a new car anyway.
If you leave the cost of the car out of it, it was £13,700. I reckon I'm saving over £3,000 a year in petrol and home electricity outgoings.