The stand alone photovoltaic systemsare used mainly in isolated areas where it is not possible to connect to the electricity network. With this type of system, energy storage is guaranteed by batteries, with the possibility of having a direct current voltage of 12, 24, 48 V and, thanks to inverters, an alternate current of 110, 220, 400 V (Fig. 1).
The system must be overdimensioned in order to be able to supply energy even in bad weather or during winter months.
Only certain special systems can supply energy directly during the night too.

The grid connected photovoltaic systems (Fig. 2) are attached to the electricity network and act as producers and/or consumers of electric energy: when the domestic (or industrial) system overproduces energy (i.e. when it manages to cover its own needs), the excess is passed on to the electricity company. It is introduced into the network and counted with a "debit" meter by the electricity company.

If the system does not manage to cover its electric energy needs, for example during the night, the energy for the consumers is taken from the electricity network with a "credit" meter.

The Law of 28 July 2005 (Criteria for offering incentives for the production of electric energy by means of a "photovoltaic convention") states that for small systems of less than 20 kW_peak the electric energy sent to the network is paid for, over a 20-year period, at 0.445 Euros/kWh (while that taken from the network is paid for at approximately 0.15 Euros/kWh).