This is just too much mathematics for me and based on your initial calculations, gas powered mowers are still more economical in the long run. It is scary to think about the price of the batteries after 5 years...and after 5 years that price at the present time will double in cost after 5 years. IMO, you are better off with gas powered since you are very good in maintaining your equipment. Battery powered mowers are for homeowners who don't like to change oil on their equipment. FWIW, I am still using my JD tractor that I bought in 2001 and is still running strong. Maintaining it is not so expensive either.