Which is why they need a network of the grids like the local ones in existence today. If you have 10 lines feeding into a mega city from different directions the reserve for a single failure can be lowered to 10%. You can also build pumped water hydro stations, which I don't see how batteries could ever match in capacity or efficiency. Batteries are good for smoothing out spikes but how can they deal with a failure that could take some time to resolve?