Because the process to make magnesium requires so much heat, the system stores a lot of heat, and has places where heat "leaks" out of it. If you took some of this waste heat (such as the heat you need to take away to cool down freshly made magnesium) you can run good old steam turbine generators to make electricity on demand. In fact, you might even build a magnesium refinery in decommissioned fossil fuel power plants, where that steam equipment already is!

So the upshot is that renewable energy and magnesium refining probably would complement each other well.

Refined magnesium would allow sunlight captured in one season to be used somewhere else months later.

Temporary fluctuations in production could be evened out by efficiently using the waste heat from the refinery process to generate power.

Momentary spikes in demand are where batteries work the best, and we're well on our way on deploying those.