Batteries ain’t cheap when you need them to output GIGAWATTS of power for 12 hours. “Keeping the lights on” is figurative, in reality you need to keep enormous industrial capacity running. Li-Fe batteries have gotten cheaper, but not cheap enough that it makes any kind of sense for that application (even in developed countries we’re just beginning to see battery storage, and the goal is smooth momentary changes in demand, not to power the entire grid at night. That’s still a faraway dream).