The Top 3 by E3

Choosing the Right Inverter in 2025: Central vs. String

• E3 Consulting • Season 3 • Episode 3

Share episode

Send us a text

Ever wondered if central or string inverters are the way to go for your solar project? Join host Daniel Tarico, E3 director of renewables and Mike Kosmersky, E3 Consulting's head of field engineering services, as they discuss the impact of project size and location on inverter choice, sharing tales from our own experiences, including a one-megawatt project in rural India. They  reflect on the importance of serviceability, reliability, and the changing landscape of inverter capacities, challenging the once-clear definitions of "central" and "string." Whether you're deep into the world of renewables or just curious about solar tech, this episode shines a light on the crucial decisions shaping solar energy today.

Ginger Elbaum:

Welcome to the Top 3 by E3 Consulting, a monthly podcast about the intersection between engineering, energy and project finance. E3 is a firm of technical advisors who provide independent and owner's engineering services to capital providers, project developers and other stakeholders. One of our goals is to ensure that projects are bankable, meaning that they are both technologically and commercially feasible projects, allowing them to obtain funding for development. This gives us a front row seat to the so-called brave new energy world. In addition, we've chosen a field that allows us to be lifelong learners, and we enjoy sharing what we have learned and educating our clients. In each episode, we'll explore the top three takeaways about a particular topic, examples being hydrogen, wind power, energy storage or simply market trends. We hope you can join us and we would love to hear your questions and suggestions for upcoming episodes. Write to us at E3CO, at E3COcom.

Daniel Tarico:

All right. Hello, this is Dan Tarico. I'm the Director of Renewables our field engineering service. Mike's an electrical engineer with decades of experience in solar and expertise with solar inverters.

Daniel Tarico:

Now I've been working in the solar field since 2007, and a lot has changed over the years. The equipment has improved by leaps and bounds, and it seems that design standards change every year, so it can be really hard to stay on top of what's going on. One question, that's , that's outstanding, actually, since I started in solar a long time ago, is this: Are central inverters or string inverters better or better to use now? put this in perspective, when I started, a string inverter was a few kilowatts, and a central inverter was maybe 250 kilowatts, often a lot less. These days, string inverters are, on the small side, still a few So, in some regards, the terminology has changed with the technology, but the question still keeps coming up: what's the better choice for a given project?, but up to 250 kilowatts on the largest, and central inverters, which were once 250 kilowatts at most, are now four megawatts and larger.

Michael Kocsmiersky:

Yeah, it still is a question in terms of which is better a central inverter or a string inverter, and it's an open question. I've seen plenty of both in my 20 plus years. As you would expect, there is no one size fits all. The correct answer depends on the many details of the project.

Daniel Tarico:

Okay, well, I guess before we get into that, if you don't mind, I'll share a war story here from some years ago that might help illuminate this issue. One of the early projects I worked on was a one megawatt project and when we were designing that we debated for a long time should we use a central inverter or string inverter? Now, what's unusual about this project is it was located in rural India and we knew that it would be very difficult, if not almost impossible, to get a service technician to the site to maintain the inverter. Of course we had faith that these inverters would be reliable, but I was worried about the service aspect.

Daniel Tarico:

I think at the time string inverters were maybe 10 or 15 kilowatts the largest. They weren't necessarily very reliable, but if a string inverter does go bad you can replace it with a new one. Even in rural India you can probably carry it out to where it needs to be replaced and find someone who's capable of swapping it out. The customer ended up going with a central inverter, which was kind of expensive, and they put that inside a building to protect it and the building itself was kind of expensive. But looking back at that project, I wonder if that was a good choice.

Michael Kocsmiersky:

What I've seen in my experience is that I like to see up to 50 or 60 string inverters before going to a central inverter. Okay, and with string inverters in the 250 kW range nowadays, you're going to get to about 10 to 15 megawatts of AC system size before jumping into a central inverter. Okay. So for utility-scale type projects, you're going to see central inverters because they're going to be up towards 100 megawatts plus.

Daniel Tarico:

Or more. Yeah, these days it seems like 100 is the smallest utility project.

Michael Kocsmiersky:

Yeah, you'll have over 100 3.5 to 4 megawatt inverters on a large project which are technically central inverters, but at this scale they almost become string inverters. Yeah, that makes sense.

Daniel Tarico:

Yeah, that's a good way to look at it. You know, I hadn't really thought of it. It's quite like that. But you know, one one thing I'll say about E3, one of our important roles is it is independent engineers as owners engineers. So we want to guide developers so that when they're designing and building solar facilities they'll perform and that they will last and they'll be maintainable. And that means the architecture has to be maintainable, which is kind of what we're alluding to here with this inverter quantities and inverter types. We kind of need to look at everything in the power plant construction right and we provide the diligence around that design so that those developers can design projects and have confidence they're going to function properly and the bankers who are funding them will fund them and they'll get built. With that in mind, can you give some other examples of factors that might favor one architecture over the other?

Michael Kocsmiersky:

Yeah, the thing I like about the string inverters is that, for starters, they're easy to replace. You can get two technicians out there in four hours. They can replace a string inverter For a central inverter. When those go down, you generally need to use a factory-trained technician, right, and those guys are so booked. Oftentimes what I'm seeing nowadays is they're three months out. Wow is they're three months out? And they just cannot supply enough technicians for the amount of service that's needed. And the central inverters are generally require specialized, specialized knowledge. They want factory trained technicians for it. So if you've got the 10 megawatt system, let's say, where you have the option of 50 string inverters or two central inverters, if one of those string inverters goes down, okay, you're out 2% of your system. You can schedule that repair. The string inverter is readily available. You can get that really quick and schedule that over the next week or two. Get the technicians to the site. Central inverter goes down 50% of your site's down, and you're waiting three months to get it replaced or repaired.

Daniel Tarico:

Rather, yeah that's going to kill you. But basically with a string inverter a competent solar electrician can go and just swap it out and they don't require necessarily extensive factory training or certifications.

Michael Kocsmiersky:

Yeah, that's generally the case For the string inverters. There's not a need to do advanced-level troubleshooting to fix it. They're swappable. Put a new one one in there, send it back to the factory. It's probably still under warranty. They can do the troubleshooting on their factory bench. Go a lot quicker than your technician trying to get a hold of tech support, being on on hold for two hours and then trying to do stuff that they're not familiar with or finding that, oh yeah, it needs a replacement board for this or this component's blown. If a component's blown, you're going to send that inverter back anyway.

Daniel Tarico:

Yeah, yeah, yeah, okay. Well, that is helpful. You know, I can certainly see how that long delay in factory service is a really big risk for central inverters. So certainly if there's only a few on site, that's going to take down a major portion of the project. I've got another question I'd like to ask you about.

Daniel Tarico:

I spoke with an inverter sales representative a while back and he mentioned kind of a new approach to this, which is a centralized system layout with decentralized inverters as a concept. I also read a white paper on this subject. It seems to be promising and essentially what it is is. The inverter manufacturers will take string inverters and skid mount them, similar to what they do with a central inverter now, and add the medium voltage transformer. So you basically have a power conversion system that you can drop in, factory pre-assembled and wired, and so it's very similar to a central inverter type configuration, but it has some of the advantages of the string inverters, where you have a bank of DC disconnects or not, but you have these bank of string inverters mounted on a skid ready to go. What are your thoughts?

Michael Kocsmiersky:

on that. I think it's a great idea because you've You've lessened the installation cost by putting them prefabbed on a skid and you've made a central service location for the multiple string inverters. Makes it easy for the technicians to do regular O&M on it or any type of corrective maintenance. You can put a cover over it to block out direct sunlight. Makes the string inverters run at a lower temperature, which the temperature is what kills these devices, kills the lifespan on them. And having those string inverters centrally located, you now have your higher voltage DC wiring running the longer runs to the inverters, versus having the inverters out in the field and running something like 480 over to an AC combiner panel and those longer runs being at the lower voltage. So you're upping your voltage on the longer distance wire runs, which means smaller gauge wire, saving you some money on the wiring.

Daniel Tarico:

Okay, so you definitely get some benefits on that. Yeah, I certainly like the idea of putting a cover over them so you can service those inverters in bad weather, standing in the rain and snow, something you probably appreciate out there in the Northeast. Yes, that's definitely helpful. So basically, what I'm seeing here and this is just wrapping up this brief discussion is that a couple things you might keep in mind is that the string inverters are the better way to go, I'd say, on smaller projects, up to maybe 10 or 15 megawatts, because they're more readily serviceable and you're kind of eliminating that risk of extended downtime. I think that's what I heard you say yes, that's exactly right.

Daniel Tarico:

And then for the large utility projects these days they seem to start at 150 megawatts. They're gosh approaching 1,000 megawatts in some places. Then, really, the central inverters are probably just as serviceable. You'll have full-time staff on site, and if you lose one, you're only losing a small percentage of the total production, and so it's not so hard for you.

Michael Kocsmiersky:

Yeah, similar to the idea of the string inverter. If you lose one, it's not a critical red flag emergency because you've got 50 on site. If you've got 100 central inverters on site, you lose one. Yeah, you still want to get it back up and running, but it's a small percentage of the overall. The other thing I'd like to add too is that on a utility scale site you should have some of your O&M staff that's maintaining that site factory trained on that. That makes sense, so that you're going to shorten that repair time. Yeah, because then you can get component level boards or equipment direct from the factory and have your technician servicing that rather than waiting for the factory guy. So get your staff factory trained on the equipment that you're using.

Daniel Tarico:

So a good upfront investment probably in the project, I would say. And then, last of all, I guess that centralized layout, decentralized inverter concept seems to capture some of the advantages of both the central inverter architecture and kind of the technical advantages, both the central inverter architecture and the technical advantages of the string inverters.

Michael Kocsmiersky:

And that's what I've seen the trend towards centrally locating the string inverters. And now the inverter manufacturers are running with that and saying, well, rather than do all that installation in the field, why don't we pre-install a group of central inverters together? A group of central inverters together, string inverters? Yeah, so you have a group of string inverters together and put that on a rack and ship it right to the site. That seems to incorporate the best of both worlds.

Michael Kocsmiersky:

Now, if you're operating a bunch of these string inverters well, maybe you have more than one central location, maybe you have two for that size site or as you need There'll be a certain amount of inverters that you want to put together, string inverters you want to put together before. Okay, we want another centralized pad because the home run lengths are going to be just so long, right? Okay, it makes sense to have another centralized location. Run lengths are going to be just so long, right? Okay, it makes sense to have another centralized location. That's going to be kind of dictated by what your step-up transformer can handle. Again, every project is a little bit custom.

Daniel Tarico:

Okay, no, it seems like again. Well, that's good to know. That's all we wanted to talk about today. Mike, thanks for stopping by to say hello. We'll catch up to you next time on the Top 3 for Me 3.