How much can solar panels reduce the electricity bill?

50-100% is the rightway to describe this question,depending mainly on electricity consumption,solar system size,sunlight and state's net meetering rules.

In India,if a 1kW solar panel gives 150 units in a month it depends on net metering rules/DISCOM's billing.

India's PM Surya Ghar scheme also provides residential rooftop-solar subsidies,with support of up to 78000 INR under this scheme.