Pretty recently, maybe in the last year or so?. It's pretty crazy to see it - CATL have started selling LiFePO4 cells as low as US$63/kWh while lead acid tends to cost more than US$100 per kWh! Obviously a finished battery + retail margin is going to cost more than those figures but we're coming to the point where it will just be cheaper than lead acid on an amp-hour to amp-hour basis.

Of course, on a Total Cost of Ownership (TCO) basis, it was already cheaper quite a while ago (for the last 5 years I'd guess). It was a combination of LiFePO4 batteries having such longer cycle lives than any lead-acid chemistry, and the greater depth of discharge meaning you could use a lot more of the energy. So if you build a new lead-acid based system, you'd need almost double the amp-hours of batteries to supply the same amount of energy with a safe depth of discharge, and you'd still have to replace them much sooner. So you'd not only have to factor buying a lot more batteries with lead acid but also replacing all the lead acid batteries at least once while a LiFePO4 system would continue to operate.