Much of the energy generated by a photovoltaic system, is produced during the day when there is little or no use at home. This energy is sold back to the grid.
When electricity consumption is at its highest in the morning or in the evening, the energy generated by a photovoltaic system is either not sufficient or no longer available to power appliances like washing machines, ovens, televisions or needed lighting during the evening.
When it happens you are forced to buy energy from your power provider which generally costs you more than the refund for the photovoltaic energy you feed back into the grid.
Why? Because most photovoltaic systems lack a fundamental feature that allows you to store the energy you produce during the day for use when you actually need it.