On-Grid: An on-grid system is interconnected with your existing electrical system. The solar energy generated is simultaneously consumed, and if the solar panels are not generating sufficient energy to cater to your consumption, the differential is automatically sourced from the grid. It is also possible to connect the system with batteries, home inverters, diesel generators, etc.
Off Grid: An off-grid system is an independent, battery-backed Solar PV system which is not connected with grid. During daytime, solar energy generated runs your appliances. Any excess energy is stored in the batteries, which can be utilized at night. Off-grid systems are used for standalone applications like traffic & streetlights, billboards, agricultural pumps, telecom towers, rural applications, etc.