Marginal cost is never zero though. That would imply truly free unlimited energy. There is a cost to build solar, wind, storage, etc. that needs to be amortized. We also want to incentivize folks to not waste energy, so a reasonably strong link between usage and price is helpful.

It’s going to be interesting to see how this all plays out for gas infrastructure as folks electrify and cut their gas service. Once the spiral starts, fixed costs will grow for the remaining customers and push more people to cancel their service.

But the size of the array, and therefore the cost of the array, are intimately tied to the production of said array. So there can’t be flat rate unless consumption never surpasses production, which is of course will when you have zero marginal cost.