When a solar PV system is generating more electricity than the home is using the 'excess' energy is usually exported to the grid. Home owners normally receive an extra 3p/kWh for this exported energy, in addition to the payments they receive for generating renewable energy.
But since a unit (kWh) of electricity costs home owners about 13p to buy from the grid it makes sense to use as much of the energy a solar PV system generates as possible. Obviously there is no point in turning on lights, or other devices which do not need to be on, but if the 'excess' energy can be diverted into things which can store the energy, or make use of it to avoid buying electricity in the evening or at night, there are big savings to be made. Controlling a PV system in this way makes great sense and can reduce the time it takes for the system to 'pay back' its' installation cost.