What a great question!
In a nutshell, with your solar system installed: sun will shine on the panels generating one type of electricity (DC), this electricity is then fed into an inverter to convert it to the energy you use at home (240V 50Hz AC) - but this is not always used entirely. So any surplus electricity is then fed back into the main grid, to be used elsewhere.
Therefore once it has been fed back into the grid, it is unable to be saved for a later date. However if ...