On grid System means your solar system is tied to your local utility’s GRID. This is what most residential homes will use because you are covered if your solar system under or over-produces in regard to your varying energy needs.
Off Grid System you are not connected in any way to your grid’s power system or utility company. This is appealing because you are 100% self-sustaining your energy use. However, there are disadvantages because off-grid systems back-up battery which can be expensive.