Enphase's AC Battery and interface. Image: Enphase.
As solar becomes more mainstream, energy policies change, and electricity prices continue to rise, there is no shortage of vendors looking to offer batteries and energy storage for the residential market. Those systems come in two varieties – AC- and DC-coupled – each with their own characteristics. We’ll try to explain some of those differences here and explain why we think AC-coupling is the topology of choice.
Oversized vs. right-sized
An AC-coupled storage system interfaces with a solar PV system in the AC load panel, while DC-coupled systems connect directly to the solar array. A DC-coupled system allows both the PV and battery to be served by a single inverter, but this also means that there is a single point of failure that can take down not only the solar, but also the storage system.
For retrofitting storage to existing solar arrays, adding a DC-coupled battery means either replacing the existing inverter or adding a second inverter, and for consumers that may want to add more solar in the future or get solar now and add storage in the future, it means the added expense of either oversizing the inverter today or integrating a second inverter with the update. AC-coupled energy storage, on the other hand, can be retrofitted to existing solar installations without needing additional expenses for oversized inverters or additional load panels. Another advantage with AC-coupled systems is that they are capable of charging off both solar and the grid, while DC-coupled systems can only charge off solar. This gives AC-coupled storage more flexibility since they can theoretically charge with solar during the day for evening consumption and charge again at night when electricity rates are lower for morning consumption.
For retrofitting storage to existing solar arrays, adding a DC-coupled battery means either replacing the existing inverter or adding a second inverter. Image: Rexel.
Conversions and efficiency
There is some debate about the differences in efficiency of AC- and DC-coupled systems. The key is to look at the entire system, and not the battery alone. Some DC battery vendors claim that round-trip efficiency for an AC-coupled system is lower, as there is an extra conversion from AC to DC when the battery is being charged, which then goes back from DC to AC as the battery is discharged. What Enphase has found with its own AC Battery is that this not always the case. Based on various factors such as chemistry and cell type, an AC-coupled battery can actually retain more energy through the extra conversions than some of the DC storage systems currently on the market. Thanks to the high energy efficiency and lower voltage of the integrated Enphase microinverter embedded in every Enphase AC Battery, an Enphase AC-coupled system ultimately has about the same efficiency as a typical DC-coupled battery system.
Why AC wins
AC-coupled systems offer simplicity, flexibility, reliability, and safety—and the best ones offer equivalent efficiency to DC-coupled systems. Enphase feels that an all-AC distributed architecture makes it easier to add to new and existing PV systems, with no need for an expensive, oversized ‘storage ready’ inverter that can accommodate a DC-coupled system. We recommend that installers take a closer look at AC-coupled systems.