If solar were free, but we still needed to pay for battery storage, how would it then compare in cost to fuel-based alternatives (fossil fuel, nuclear etc)?
Might be of interest to you: the think tank Rethinkx is forecasting wind and solar + lithium ion batteries will be cheaper than continuing to run already existing coal and gas power plants by 2030. They believe this will cause the capital invested in other types of power plants to become "stranded".