Your solar energy isn't free. It comes at the cost of depreciation, and an invested money opportunity cost of not deploying/investing the money elsewhere. It comes with future benefits of lower operating costs for your home and car and potential tax benefits. Your fuel costs should be adjusted by the amount of the cost of your fuel/battery replacement. Take that cost and apportion it backwards to get the true cost of the fuel expended thus far. A ICE or hybrid car would have the same range as its original range at your miles but be without the cost so you have to account for it. Just as an ICE car has to account for $400 in oil changes for about 100k miles covered.
