Below mentioned Video will help you in selection of Solar roof top project :

OffGrid System : Where there is scarcity of electricity and grid electricity generally not available. Mostly Offgrid system are of small capacity from 100watt to 20 KW.

On Grid System : Where there is plenty of electricity is available. Per Kw cost of on grid system is cheaper as compare to Off grid system. Disadvantage of Ongrid system is that it will not work in absence of reference grid electricity. More details you can see in above you tube link