It is a usual query that people ask before installing a solar rooftop, and this question is completely justified. The answer to this question is hidden in the working mechanism of rooftop solar that can be installed in three ways.
The rooftop On-grid system is connected to the main grid supply. When your system is unable to produce the required electricity, it withdraws from the main grid. But when it produces excess electricity, you supply it to the grid and earn money.
Rooftop Off-grid system is set independent of main grid supply, so you can use grid only if there is not enough supply by rooftop.
The rooftop Hybrid system is a combination of both systems.
These rooftop system arrangements allow minimum to no use of electricity from the grid, hence saving your bills.