AC Coupling - Methods

Schematic of AC coupling
AC coupling configurations for on-grid and off-grid
Schematic of AC coupling

Even though they usually include an inverter, most battery-based PV systems are “DC-coupled”—the PV array sends DC electricity to the system through a charge controller to the battery bank. That DC power is then drawn from the batteries for the loads, including through a battery-based inverter for AC loads.

AC-coupled PV systems are gaining wider acceptance and support through several battery-based inverter manufacturers. In AC-coupled systems, the DC power from the array is first converted to AC by a batteryless inverter, to be used by the AC loads through an AC load panel. Any unused energy is used by a separate battery-based inverter that either converts the AC to DC to charge the batteries, or, if it is a grid-tied system, it can also pass through to additional AC loads and/or the grid.

A batteryless inverter would normally not turn on without the utility grid present, but many battery-based inverters create a quality sine wave that’s good enough for the batteryless inverter to synchronize with. In grid-tied systems, when the grid goes out, the battery-based inverter isolates both the load subpanel and the batteryless inverter from the utility grid via an internal transfer switch allowing the batteryless inverter to remain on without being connected to the grid.

For residential systems, a primary advantage of AC coupling over the traditional system design is that you can add battery backup to an existing batteryless grid-tied PV system without changing the existing system’s wiring. An AC-coupled system can also be more efficient than a typical battery backup system because the batteryless inverter is doing the majority of the power conversion. Efficiency is generally in the 96% to 98% range compared to 90% to 95% for a typical battery-based inverter.

Disadvantages are a more complicated system to design and program, and more expense, since you’ll need two inverters (or more) instead of a single inverter and a charge controller. In off-grid use, AC-coupled systems are not self-restarting if the battery-based inverters happen to shut down because of low battery voltage.  If this happens, the batteryless inverter does not sense AC voltage, and thus does not turn on to send array energy to the batteries. A DC-coupled system can self-restart even if the inverter shuts down from low battery voltage, because the charge controller can still charge the batteries.

Comments (6)

ke6cvh's picture

I found the grid zero function from Outback on the fxr's to be useless in my case as there is very little power with it. I have 4 series/parallel for 14kw continuous. Instead to get what I needed was to use Meanwell 2kw 24v power supplies (1/2 rack unit wide and 1 rack unit tall) that also can be purchased in other battery voltages. The Meanwell is upstream from the MBT from the grid. It is on a knife switch that goes from grid to off to generator. With this I "sip" from the grid only when the float voltage drops below the higher float voltage of the CC's (midnite solar classic 150's). Passive power consumption is 15w to 17w with 14kw coming off the inverters when the sun is providing all. They now make a 3kw version and they can be paralleled. I am now investigating the best solution for the ac coupling to work with it. I use the 14kw on a non-critical and have 5kw on the critical power panel but a common battery bank fed by all panels. I hope to use the ac coupling with the system from an adjacent building negating the need for excessively long dc power cabling and another separate battery bank.

bob tarzwell's picture

the 200 amp transfer relay has two sections and a mechanical interconnect so one relay can not pull in if the other is already engaged,its out of a Allan Bradly generator transfer switch , i bought it on ebay for 175.00 one side of both relays is load , and each relay is connected to grid and the other to solar, i have a 80 amp current sensor on load which triggers the relay to go back to grid and a timer so it does not switch back and forth to much then a 60 amp current sensor that says ok load is now low enough go back to solar,my problem is this system is on a resort so i have no control over how much electricity they use , and the outbacks do not have a very large switch so this way i protect the inverters from the poor quality power and provide a bigger transfer to grid if needed in over load .

Geoffrey Kaila_3's picture

I'm interested in AC coupled systems. I would like to know how to size the components just like we size PVs,charge controllers, inverters and batteries in DC coupled system. Please help.
Geoffrey kaila

AtlSwiss's picture

Interesting, wonder how would this would be designed to use my two EV's as the battery sink. Have 2x 1 KW inverters to pull out from each car (a Leaf and a iMiev) and they can be charged via 110-120v using their standard J1772 chargers. planning in getting 4kw solar panel system. How should I segregate the house loads to make sense of it all? Fodder for a new article?

bob tarzwell's picture

I have 35 kw off grid ac coupled system with 120 batteries in the Bahamas using 4 outback radian 8 kw inverters , the design was, when the batteries go low voltage the inverters switch to the grid for power. One problem to watch is in many countries such as the Bahamas the incoming power your connecting the inverters to is very poor, with huge spikes, changing freq, and voltage that varys all over, after blowing up a few inverters , I disconnected the ac power from the outbacks, who don't seem to be able to handle poor power and put on a large 200 amp transfer switch, driven off a flex 80 relay output set up for low battery voltage and a few over current sensors. The switch over bump from the transfer relay is bigger then the internal switching from the inverters, but the inverters do no see the poor quality power from the grid.

dominic wild's picture

Hi Bob,

May I ask what kind the 200A transfer switch is? DIN rail or panel mounted, automatic or manual, cost, model? Any schematic of that set up?

So many questions, so little time!

Dominic Wild
Perth, Western Australia

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