In principle, the energy in an AC storage system is also stored in a direct voltage (DC), as this is not possible in any other way. However, a conversion takes place here from the AC network (alternating voltage network), where the coffee machine, for example, is also connected. An inverter, which is separate from the generation system, smooths the alternating voltage during storage and stores the energy in the direct voltage battery. If the energy is needed, the inverter converts the direct voltage back into the alternating voltage of the house network.

The advantages of this system compared to the DC variant are that it can be used in existing systems and the more flexible location, since the batteries do not have to be located directly next to the inverter of the photovoltaic system, which often leads to space problems. Furthermore, other energy producers such as combined heat and power plants (CHP), wind power plants and small biogas plants can also use the same storage unit, as these are connected to the identical AC power grid. A future-oriented advantage lies in smart metering, where electricity can be purchased cheaply from the public grid at certain times of the day or week.

A disadvantage of the AC system is a low level of efficiency due to the double conversion, since the direct voltage of the photovoltaic system is first converted into alternating voltage and then converted back into direct voltage for storage. Another disadvantage is the higher acquisition costs due to the separate inverter, which is not required for the DC storage system. Polycrystalline PV modules can be recognized by their mostly bluish color and crystalline structure. The use of polycrystalline solar cells or solar modules is currently one of the most frequently used PV modules in PV module production worldwide.

For the production of polycrystalline silicon cells, raw silicon is heated very strongly, cooled in a controlled manner in a casting mold and then cut into silicon wafers, 0.2 to 0.3 mm thick.

The crystal structure is inhomogeneous due to the irregular arrangement of the many crystals and is easily recognizable due to the pattern. Further chemical processes and the application of the contact tracks complete the manufacturing process of the solar cell.

The efficiency of poly- or multicrystalline modules is lower than that of monocrystalline modules. However, compared to monocrystalline modules, the yield prospects of polycrystalline modules per installed kWp are not lower.

When it comes to storage media, lead gel & lithium are currently the most important technologies.

Lead-gel batteries have a lower purchase price than a lithium system and can be discharged with high currents. However, the service life is shorter and only 50% of the capacity can be used in normal operation, which in turn brings about 30% reserve for the exceptional case of emergency power operation.

Lithium batteries usually do not have such a high discharge current capability, which can be necessary in emergency power operation. However, a load of 95-100% is possible with constant performance, which is more problematic with lead-gel systems. You are also more flexible when choosing a location with a lithium battery.

Lead-acid systems are not advisable in residential buildings, as they are more maintenance-intensive and also have a higher degree of degassing.

In addition, electrically powered vehicles and hydrogen systems can also serve as energy storage devices in the future.