Li-Ion has dropped in price to the point where it is cheaper than lead-acid. To come up with a realistic lifetime cost you have to factor in: - Cycle count (l

Li-Ion has dropped in price to the point where it is cheaper than lead-acid. To come up with a realistic lifetime cost you have to factor in: - Cycle count (lead-acid ~500, li-ion 1000-3000) - Usable capacity (lead-acid 50%, li-ion 80%) - purchase cost (lead-acid $n, li-ion > $n) I'm about to over-generalize, but to get good lifetime out of your batteries, you only discharge lead-acid to 50% of its rated capacity, whereas li-ion has good cycle life working between 10-90% state of charge. You have to buy more lead-acid than you do li-ion to get the same usable storage. Then, your lead-acid batteries will degrade significantly after 500 charge-discharge cycles. The li-ion batteries will tolerate 2-6 times more cycles before significant capacity loss. Environmental friendliness is sort of a wash. Lead-acid is more toxic, but recovery and reuse is like 99%. Lithium is less toxic, but recycling is difficult.