Written by Maria Beck
When purchasing an energy storage device, various aspects must be taken into account that influence the performance and efficiency of the device. We have already explained the influence of cell chemistry on this topic. This article will highlight the difference between the AC and DC-Systems. Which system is the most suitable has to be decided by the buyer on the basis of his individual needs and requirements. Each system has its advantages and disadvantages, which is why we will describe them in more detail.
The difference between the AC- and DC-System is how the energy storage system is coupled to the energy sources of a photovoltaic system. DC-coupled energy storage systems share the same inverter with the PV-system. Therefore no additional inverter is necessary. At first glance, the system efficiency seems to be higher in such systems, since fewer conversion stages are needed than in an AC-system. The conversion steps represent the number of transformations of direct current into alternating current. However, it is not possible to make a general statement about this, since besides the number of conversion steps, other factors also influence the efficiency. For example, especially with a lower base load of 100 – 200 watts, as is the case at night, the efficiency is lower. This is because the PV-inverter must cover the maximum output of the PV system, which is usually greater than that of the energy storage unit. For example, an inverter of 5-8 kW is installed in an 8 kW system, which leads to a higher energy loss during conversion with lower base loads. In addition, the total output of the system is limited by the PV-inverter, which has to operate the PV- system and the energy storage unit.
An AC-coupled energy storage system has its own inverter. The energy storage system is therefore operated independently of the PV-system. This constellation requires two more conversion steps. However, with a well-matched ecosystem of PV-system, energy storage and domestic consumers, this has no significant influence on the system efficiency. With a maximum output of 2.5 kW at low base loads, the VARTA pulse is much more efficient than a DC-system with an 8 kW inverter. On the basis of using two inverters, it is also possible to use the power of both inverters, so that in our example a maximum output of 10,5 kW is possible. Also the flexibility speaks for an AC-System. Because the energy storage system can be installed independently from the PV-inverter, it is not necessary to ensure compatibility with the system. This means that it is not only possible to use any PV-system, but also other energy sources such as a combined heat and power plant or an electricity-producing heating system. Even a combination of PV-systems and combined heat and power plant would be conceivable. An increased energy demand, e.g. because an electric car has been purchased, does not pose a problem either, as an additional AC-coupled energy storage system can be installed quickly and easily, so that the increased energy requirement can also be covered by solar power*. A further advantage is the higher reliability. In case of a problem both systems do not fail at the same time, which also facilitates troubleshooting.
VARTA’s energy storage systems are AC-systems. Since we offer a complete system that already includes both the battery inverter and the DC/DC converter, our system is optimally matched to each other. The system efficiency does not differ significantly from DC systems as follows. In return our customers gain flexibility and reliability.
Incidentally, neither of the two systems can do without an inverter. This is because the PV-system always provides direct current, whereas most household appliances are operated with alternating current. The conversion into direct current for energy storage system takes place, since energy can only be stored in this form. However, there are hardly any losses in efficiency due to an optimally coordinated system overall.
More detailed information to the differences between AC- and DC-systems can be found in the BVES and the BSW Solar efficiency guide (only in german).
If you have any further questions, please do not hesitate to contact our service team.
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*With VARTA energy storage systems, only the VARTA Link is required for this purpose, which takes over the control and communication of the energy storage system.