It is easy to get too hung up on efficiency though. Overall cost matters more. If a solar plus metal powder solution gives the same reliable power as a solar plus battery solution at lower cost then that will often be a superior solution, even if it requires more solar panels.

Efficiency matters when you run out of space or resources. But you got vast areas like Sahara where solar power production could be combined with metal production. Wasting space in Sahara is a non-issue if it means you get the overall cost of system down.