The basic feature of Grid-connected systems is the fact that there is a direct conjunction with the grid of electrical power transport.
The relationship between the grid-connected system and the national grid of electrical power transport is a two-way relationship. This means that the system can absorb energy but also feed the grid with energy.
Particularly, we distinguish three types of grid-connected solar systems:

1. The installation aims to feed energy into the grid. In these situations the aim is the maximum production of electricity and its sale to a supplier (consumer). These are the solar stations, the solar parks etc. The wattage in this case can be ranged from some KW to many MW. In Greece the most common investment at these levels is 100KW (because it combines the advantages of the high subsidy of KWh with the easier licensing procedure of the photovoltaic station.
2. The installation uses the grid as an alternative source of electrical power, in the case that the production of the local solar station cannot supply the electrical needs of the installation for some hours during the day, or generally. In the above cases, the installation can absorb energy from the grid to cover its energy needs. This can happen conversely: when there is an energy surplus produced by the system, bigger than the one consumed, it will be fed into the grid. Such a kind of system must have two metric systems. One of them will count the incoming energy and the other what goes out. These systems are also called grid interactive.

A power station of an infrastructure does not produce enough energy for its local loads and is using the grid to complete its necessary energy needs. These systems are called grid back up. In fact, in this case the design of the system is made in a way which ensures that the amount of produced energy will be absorbed by the fed installation loads.