The streelights is a set together with the battery. At times the batteries are in build. In case of a rooftop solar system I can decide the type of battery I will use. Li-ion batteries by Tesla is very good one. In sizing of the battery, we have a term called days of autonomy. For example, depending on the customer , I can use 3 days autonomy which means if there is a continuous rain for 3 days there would be supply of energy. 3 days of auntonomy has more battery capacity as compared to 1 day of autonomy. As I said earlier in my post, even if the sky is cloudy still there would be light rays . In case of industries since the demand of energy is high, diesel generator can be added to prevent inconveniences. Solar energy does not pollute the atmosphere. It's environmental friendly unlike fossils fuels (diesel) which pollutes the atmosphere