Seven advantages of AC-coupled systems

April 2019

Seven advantages of AC-coupled systems

Energy storage systems are offered as AC and DC systems. What’s the difference? And under what conditions is each system appropriate? Here we discuss the most important points and explain why we consider AC-coupled systems to be more future-ready.

Differences between AC- and DC-coupled systems

Whereas a PV installation on the roof generates direct current (DC), our domestic mains supply works with alternating current (AC). That’s why there’s always a PV inverter installed between them to make the conversion. Energy storage can then be installed on the direct current side (DC coupling) or on the alternating current side (AC coupling) of the PV inverter.

 

The most frequently cited argument for DC-coupled systems is that they eliminate one inverter and two conversions. This usually has no effect on the price because providers of integrated DC systems generally factor the inverter costs back in. In theory, however, this should at least reduce conversion losses.

 

Nevertheless, this assumption has never been proven in practice, because DC systems are also subject to conversion losses through the conversion of the PV string voltage and battery voltage by a DC/DC transformer. You also sacrifice some of the advantages of AC systems, which we’ll tell you about below.

Why AC-coupled systems are more advantageous

Whereas the maximum size of the PV installation in DC-coupled systems for directly charging batteries is limited, AC-coupled systems can be dimensioned independent of the size of the PV installation and PV inverter and can be installed anywhere in the house. You benefit from a simple and therefore cost-optimized installation of a system that you can flexibly expand at any time.

AC-coupled systems can draw electricity from the mains grid. This means that you can take advantage of the flexibilization of electricity prices – in other words, day and night tariffs – and that you’ll also be able to participate in electricity trading in the future. So you’ll not only earn money, you’ll also support the energy transition by decentralizing power generation.

Other kinds of power generators like cogeneration units and miniature wind turbines can easily be connected to AC-coupled systems, because these generators also supply alternating current. This is especially practical if you want to supply power to your home with a PV installation in summer and, for example, with hydrogen or natural gas via a fuel cell or cogeneration unit in winter. With AC-coupled systems, you have the option of combining multiple power generators, which gives you the flexibility to design your system sustainably and also to expand it at any time.

Whereas the output of DC-coupled systems is limited by the PV inverter, this isn’t the case with AC-coupled systems. During periods of high consumption, you can utilize the full PV installation’s capacity and the battery discharge capacity simultaneously, reducing the amount of conventional power you need to draw from the grid.

Because the dimensioning of the PV inverter integrated in DC-coupled systems is dependent on the PV installation, you can’t design it to optimally match your consumption profile. AC-coupled systems, on the other hand, are much more flexible, thanks to the separate inverter. Especially at night – when consumption is low – the system can be operated more efficiently at partial load, and you’ll benefit from maximum efficiency.

In the event of a malfunction, you benefit from the AC-coupled system’s independence from the PV installation. Retrofitting and troubleshooting can be performed more easily, faster, and more affordably because there’s no need to intervene in the PV system.

With DC-coupled systems, you’re extremely limited in your choice of combinable system manufacturers, because DC-coupled systems require that the PV installation and storage system be precisely coordinated. With AC-coupled systems, on the other hand, you benefit from open technology. Regardless of your PV system, you can choose your preferred battery manufacturer – and not just for retrofits. New installations are also manufacturer-independent, meaning that you can decide what modules will be installed on your roof and what batteries will go in your basement!

Released by Pawel Nicinski