The direction of current flow, produced by the solar cell, is allways from the positive to the negative terminal of the cell - It is called direct current (DC). DC is also well known from batteries. The power grid and, thus the wall sockets inside your house are operated using alternating current (AC). Due to the periodic changing direction of current flow, positive and negative polarity are periodically swaped, this type of current is called alternating. To prevent misunderstandings, the pins of the socket are called 'line' and 'neutral'. The periodic change of current flow offers some advantages for the operation of large power grids. That's why almost every power grid is operated using alternating current. Obviously, DC and AC grids cannot be connected directly. Therefore, the inverter provides the interface to the grid, needed to convert DC to AC.

The active part of an inverter is build up using semiconductor elements. These elements are used as simple switches. Using the semiconductor switches a current flow can be connected or dis­connected or the positive and negative poles of the PV module can be changed. By periodically switching the semiconductors, the DC current at the interter's input is converted into an AC current at the output. Through the switching pattern of the semicon­ductors, the inverter can control the energy flow from the PV system to the grid. Thus, the inverter controls the energy output of the PV-system and provides the interface to the power grid.

Depending on the environmental conditions, Solar cells have an operating point at which the maximum output power can be generated. Depending on solar radiation, temperature or shading conditions, this operating point changes constantly throughout the day. The task to optimize the operating point is taken by the maximum power point (MPP) tracker. It is constantly checking whether the input is operating on the maximum power point for the current conditions. In case of a detected deviation, the operating point will be adjusted. Therefore, the MPP tracker is often integrated into the PV inverter and is an important part of the system.

In addition to the differentiation into module, string and central topologies, inverters also can be classified with regard to their functionality in the system.
Conventional Inverters: Modul, string and central inverters are often used as a conventional inverters. This means that they are only convert­ing the DC power of the PV modules into AC power for the grid and control the power flow into the grid. Therefore, they are the most affordable type of inverters.
Hybrid Inverters: Just like a PV-system a battery storage also provides DC power, thus, an inverter is also needed to connect the storage system to the grid. So why not combining both inverters in one device? This is exactly what a hybrid inverter does, the PV-generator as well as the battery can be connected to the grid using only one device. Furthermore, the hybrid inverter is managing the energy exchange between PV-generator, battery and grid.
Off-grid Inverters: In the event of a blackout, conventional inverters get automatically dis­connected from the grid and turn off. For normal operation, these inverters always need a stable grid. Without the grid connection they are not able to convert DC current into AC current. Off-grid inverters behave differently, the integrated controller detects whether the system is connected to a running grid or not. In case of a grid breakdown, the controller disconnects the inverter within a few microseconds from the grid and a stable off-grid system is initialized. This ensures that your own power supply is maintained. When the public grid is back in operation, the controller synchronizes the off-grid system and reconnects the system to the public grid.