Solar panels are a revolutionary source of power - arguably the most promising for the century. However, unless you are living in miserable conditions on the North and South pole each for six months, your panels will not generate power throughout the 24 hours of the day.
This is why solar power and energy storage have such a longstanding partnership - with batteries storing excess power during the day and providing it at night, which is only one of the many benefits.
For example, batteries can help save on power bills through peak shaving, i.e. not using grid power when it is costliest (e.g. peak hours). They can also offer resilience and protection against outages, and may also allow the user to participate in a revenue generation model in some parts of the world.
But this is not an article about the benefits of an energy storage system - it is meant to clearly differentiate between the two main types of batteries available for solar power systems today - DC coupled and AC coupled.
What is DC and AC Coupling?
For beginners, DC and AC stand for direct current and alternating current respectively - it is the way electricity travels. DC is the simple form wherein current steadily moves in one direction, and it was the common and only way to transmit power in the early stages of electrical infrastructure.
At one point, a brilliant scientist named Nicola Tesla came up with the equally brilliant idea of using current that flows alternatingly forward and backward (hence the name), and the world of power transmission was changed forever. How this technology greatly reduces losses and allows transportation over long distances safely is a topic for another time - for now, back to batteries.
Let us get one important thing out of the way - batteries are not, and cannot be AC equipment, the current that flows in and out of them is always DC. You’ll notice that in the title it says ‘AC-coupled’, not AC batteries. And now we’re ready to move forward.
Every solar system has a bunch of equipment - panels, controller, fuse box, inverter, etc. Some of these are AC devices and some DC, usually grouped together that way. DC-coupled batteries are batteries that are connected on the DC side of the system, meaning between the solar panels and the inverter.
On the other hand, AC-coupled batteries are connected after the inverter, meaning on the AC side of the system. This may not feel intuitive, but there are certain advantages of AC coupling which make this arrangement logical.
AC Coupled Batteries
The obvious question now is this - which of these is better? And like most other things, we’ll say the answer is a bit complex. It can be understood only by knowing the pros and cons of both the types.
Pros and Cons of DC-Coupled Batteries
To begin with, DC-coupled batteries result in higher efficiency. In any electrical circuit, wherever there is inversion of current from DC to AC or AC to DC, there is a loss in efficiency. An AC-coupled battery system means current that was converted from DC to AC (when it came from the panels to the inverter) once again gets converted into DC (inverter to battery). This results in losses.
A DC-coupled battery needs only one current inversion in total, and hence has higher efficiency.
The downside, however, is that installing this type of a system is fairly complex, especially in existing PV systems. This may drive up the cost of installation and even the time required.
Also, as you can see in the image above, DC coupling requires a charge controller for optimum and safe charging of a battery from solar panels. Thankfully, newer battery units often come with an integrated MPPT charge controller, simplifying things to some extent.
Pros and Cons of AC-Coupled Batteries
The most important advantage of AC-coupled batteries is their simplicity of installation. Whether it is new systems or existing ones, AC-coupled batteries can be installed swiftly. For anyone with even a little experience in installing batteries, the installation of AC-coupled batteries is an almost effortless task. And faster installation also means lower costs.
Secondly, being located on the AC-side, these batteries can also be charged through the grid. This might not be a huge USP, but can still turn out to be a decisive factor in some cases, such as when you are participating in an energy rate arbitrage program or a demand-response program.
The drawback of AC-coupling, as we saw earlier, is that the current that was converted from DC to AC in the inverter needs to be converted to DC again to be stored in the batteries, and then back into AC to use for our everyday appliances - which are all (or almost all) AC devices.
This leads to losses in efficiency. For example, if we suppose for a moment that the efficiency of all the equipment is uniform at 98%, inverting it thrice brings the total system efficiency down to about 94%.
However, the savings in installation costs often offset the lowered savings by the drop in efficiency, which is why a lot of installers recommend AC-coupled batteries.
An interesting way to combine the advantages of both while eliminating the drawbacks is to use a hybrid system that has both AC and DC-coupled batteries. Though a bit more complex, this can be done by using a hybrid inverter connected to the DC-coupled batteries while also having AC-coupled batteries on the other side. Obviously, this too increases complexity and cost.
Here’s a table that outlines all the pros and cons of dc-coupled and ac-coupled batteries
|Parameter||DC-Coupled Battery||AC-Coupled Battery|
|Simplicity of Installation||Lower||Higher|
Examples of DC-coupled and AC-coupled Batteries
The best example of a DC-coupled battery is the LG Chem RESU, a mighty 16 kWh unit that is compatible with most top inverters available in the market today. Other manufacturers like Panasonic (EverVolt) and Generac (PWRcell) also offer some good DC-coupled options. Interestingly, the first Tesla Powerwall was also a DC-coupled battery.
While DC-coupling was the common choice for a long time for remote, off-grid applications, the introduction of feature-rich AC-coupled batteries is causing the demand to shift towards AC-coupled batteries.
Which One Should You Choose?
Like we said before, none of the two types is a single better choice by all means, or else we wouldn’t have both options available today. What suits you best will depend on your requirements.
If you have a larger system, say 6 kW or higher, it is best to go with an AC-coupled system. Similarly, if you are introducing batteries to your existing solar PV system, AC-coupling is your obvious choice.
For smaller, slightly more remote systems that don’t rely much on the grid anyway, DC-coupling might make sense. The same stands true if you place efficiency above everything else.