During a zombie apocalypse, not many people are left. The demand for electricity decreases significantly. Whatever automated systems are in place before the zombie apocalypse started will keep going for a few months until they eventually fail. We survivors will be too busy looking for food to try to repair them. At least that's what I see in the movies. Because your supply of gasoline will be depleted quickly, your left with wind, solar, and hydroelectric. Hydroelectric would run all the time, but would take a lot of work to set up. Solar is good during the day. Wind works well day or night. Solar is the most efficient power source. You hang up the solar cells and they work all day.
