There are three kind of solar roof top systems:-
SOLAR OFF-GRID SYSTEMS- These systems allow you to store your solar power in batteries for use when the power grid goes down or if you are not on the grid. You can install batteries as per your need. You can not export the surplus energy to the grid. This system is mainly used in rural areas & areas where there are huge power cuts.
SOLAR HYBRID SYSTEM- This system is a combination of both on-grid and off-grid system. This system provides power to offset the grid power whenever the sun is shining and will even send excess power to the grid for credit for later use.