Battery cost, weight, volume, efficiency, longevity, and charge cycle times are all key factors for *portable* batteries for use in vehicles (and cell phones, tablets, etc).

But if you just need to store energy at a fixed site, flywheel systems might do the trick (already in use in some applications). With magnetic (vice mechanical bearings) they are extremely efficient for a time period in the range of day/night cycles, essentially maintenance free, and the expensive parts last forever. Storage capacity is limited only by the size of the wheel and tensile tensile strength of the metal used. And of course you can use as many as needed in parallel. Construction cost (comparable to other large grid equipment) is almost irrelevant, since they literally last forever, you could amortize the cost over a very long period (assuming you design the entire system right in the first place). Such devices could also be dispersed down the grid if that made sense, although you might not want want a very high RPM unit in a residential neighborhood, just in case.

You could also do reverse hydro for overnight (or longer) grid power accumulation. Pump water up into a reservoir or tank at night, extract the power with normal hydro gear as needed. I suspect flywheels would be better in most applications (where you don't have a good place for water storage).

It is utterly astounding to me that all of the Good Idea Fairies STILL cannot grasp this basic concept from Electrical Engineering 101. Transmission losses pile up over the miles, and 300 miles is about the practical limit. Slight possibility that could be extended with superconducting lines, but those would be orders of magnitude more expensive that current long-distance lines, and would probably require utterly impractical amounts of energy to cool the lines. Plus maintenance of a complex system stretching for hundreds of miles?

Solar is a nice supplement where it makes sense but isn't going to be the solution, any more than wind.