Why DC-Coupled Solar Plus Storage Is Finally Ready to Take Off

For years, DC-coupling was the way off-grid systems worked. Then AC-coupling took over. Now, interconnection bottlenecks and new tax incentives are bringing DC-coupling back – this time at commercial scale.

HeatSpring instructor Wes Kennedy walks through the economics and engineering: how DC-coupling cuts your grid impact in half, why Massachusetts is driving adoption with targeted incentives, and how the 30% ITC for energy storage equipment (including switchgear and transformers) is changing project economics.

He also explains how oversizing solar and storage behind the meter creates a dispatchable 24/7 renewable energy source – a breakthrough concept for grid reliability.

Ready to go deeper on solar plus storage design? Wes’s Comprehensive Solar Plus Storage course is part of HeatSpring PRO – 16 courses covering the technical and business skills solar professionals need in today’s market. PRO members also get access to exclusive discussions like this PRO Circle, where you can learn directly from instructors working on cutting-edge projects.

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Transcript below.

One specific thing that would segue into one of my favorite little pet technologies that I want to make sure we cover today is solar and storage co-located and then connected to each other on the DC side. So DC-coupling is what we call that. And that’s continued to be kind of this “maybe it’s going to take off as the next big thing in our industry” for a while. And I think that finally, circumstances are aligning to where it could actually happen now.

So DC-coupling used to be always the way that we did stuff back in the old days. Solar panel went through a charge controller, charged a battery, and then the inverter took energy out of the battery to give you AC. And then AC-coupling came in where you have whatever the resource is – solar panel, wind turbine, battery – inverts, and then everything talks to each other on the AC side and is controlled with the control layer.

Now as we’re starting to see kind of congestion, either real or imagined, at the point where we’re trying to connect these projects to the grid – so the interconnection queue – when you AC-couple, you have to account for the potential full output of both of those AC sources, right? So you have a one megawatt battery, one megawatt PV. AC-couple means it’s a two megawatt impact on a grid.

If you DC-couple, then you just have the one megawatt impact at the grid and you have energy moving back and forth solar to storage on the customer side. The holy grail of this idea that I’ve had an opportunity to get a couple of projects that kind of start to realize this promise is you put so much solar and so much storage co-located behind the meter that you can drive the meter at that rated output 24/7. So you become in effect like a dispatchable energy source.

And you do that by saying, okay, if I can only export one megawatt, you say, okay, one megawatt times 24 hours – that’s 24 megawatt hours. So how much solar would I need to make 24 megawatt hours of juice, and how much battery will I need to store 24 megawatt hours? And then I just backfeed that point of interconnection for 24 hours.

This is really like where we can get the most resiliency with these little pockets of dispatchable renewable energy. So that’s where I see a lot of this stuff heading.

And then specifically, the state of Massachusetts has a program that is really focused on this idea. So they really restrict your AC input or your AC point of interconnection for your PV projects, and they incentivize DC-coupling. So there’s a little bit of a cluster of projects happening in Massachusetts that are in this mid-market scale, so to speak, that are doing solar and storage DC-coupled.

What continues to be a barrier is balance of systems equipment to make all this work and work as easy and trouble-free as AC-coupled systems. So what the industry needs to do is come up with more power electronics solutions that do the DC to DC conversion to make the solar array and the battery talk to each other, and real unsexy stuff like combiner boxes. So big 4,000 amp, 1,500 volt DC-rated switchgear that can physically couple all those cables from all your solar panels and all your batteries.

So right now all that stuff is custom and custom equals cost. So we’re starting to see this happen and every year more and more of these kinds of projects go in, and the regulatory restriction is what’s driving it.

And then the other piece that’s now exciting is with this ITC change that we talked about at the beginning. There’s still an energy storage 30% tax credit, and if you build out a big DC-coupled system, more and more of that equipment can be considered energy storage. So the switchgear, the transformers, those combiner boxes, and of course the energy storage itself – all of that, guess what, that’s a 30% tax credit.

And then the solar is basically just an adjunct to charge that big battery. So maybe we’ll finally start to see DC-coupled co-located solar and storage take off.