Micro Inverters vs. String Inverters

[Cameron Morrison]

Hello. I am new to this forum, but I am glad I found it. We are looking to have a microfit solar install done on our barn. The barn roof faces about 237 degrees or west-southwest. There are no trees or any shadows near it. It is about 130' to feed back to the grid. I have a few questions, and I am looking for comments from other members:
1) Micro inverter setup or string inverter? I have been told micro is better, but the labour to replace a down inverter is not covered in writing. They come with a 25 yr. warranty, APS is the model, but the company (from Toronto) I have been given a quote from won't put labour coverage in writing. The string system seems fine for a barn with no shade / shadows, and is about 6K cheaper up front. I don't care too much about detailed monitoring...For a string system (from a smaller local company), the inverter would be a Fronius inverter.
2) If we run a large enough cable back to the grid, I don't think there would be much line loss. I am not an electrician, but I don't think 130' is too long of a run with proper cable size?

Thank you very much for any comments - Cam.

[Rob Beckers]

Hi Cameron,

My business used to install solar systems (including lots of MicroFITs) for many years. We stopped installing as of last year, to focus on helping people design systems and supply panels/inverters etc. Here's my take.

I'm not crazy about micro-inverters. The beauty of solar PV is that panels have 25-year warranties, they're expected to live for 30+ years, and there's no maintenance. It's really extremely rare for a panel to fail.

Now you add complex electronics to every panel (or every second panel). While the warranties on micro-inverters may be 25 years, I can with certainty guarantee you that of the 20 or 40 micros, at least one, more likely a few will fail before the 20 years of the MicroFIT contract are up. Yes, you'll get a new micro-inverter, but you'll be paying someone to pull panels to get to the broken one, replace it, and put those panels back. Those micro-inverters are in an incredibly hostile environment, subject to almost daily temperature extremes. Electronics have a hard time with that!

History bears this out too: The most popular micros are made by Enphase. Well, all, and I do mean ALL of the M190 either have failed or are failing. The ones that still work will fail over the next few years. Some were still under warranty and the owners got new ones (to be installed at their cost), some were not. Enphase didn't yet have 25-year warranty at the time those M190s were sold. In fairness, their M215 and M250 seem to be holding up much better, but still. On a roof of panels some of those will fail too for the average user.

It's not that string inverters won't fail. The way I see it (so far) is that most will likely live for 20 years (or more), but some won't. I've seen string inverters fail as well, all of those were still fairly new. That is typical for electronics; stuff fails quickly, or if it doesn't it's likely to live for a long time. It is much easier to replace a string inverter though!

It's a myth, welcomed and propagated by micro-inverter manufacturers, that micro-inverters deal better with shading. Only under very special conditions would that be partially true. In reality string inverters handle shading just as well or as poorly as micro-inverters, as long as the person that did the string layout knew what they were doing. There is a longer story to this, and I'll be happy to explain in person or over the phone, it's a bit too lengthy to write all down.

What sells micros is their monitoring. It is what makes people willing to shell out a few grant more for a micro-inverter system vs. string inverter. Some installer really like micros because in many ways they are just much easier to install.

Now, there are situations where using micros makes perfect sense: If you have a roof consisting of many different planes, with different orientations, with a few panels here, a few there. That is a case that micro-inverters are a great solution!

Regarding Fronius for the string inverter; they are a great brand! We're a distributor for them, and it's not because of that. They have been proven very reliable. The company has been around since 1945, family owned. Their support is superb if you need it.

Listen, you can build solar PV systems with micros or string inverters, and either one will work really well. Just be aware of what can happen down the road, or why you would choose micros.

Regarding 130' of wiring: That's nothing. I can run the line-loss numbers vs. wire gauge if you like.

My 4.5 inflated cents...

-RoB-

[Cameron Morrison]

Hi Rob,

Thanks SO much for this information you've provided and fast reply. This is almost exactly what the local solar company told me. I was uncomfortable with a lot of the information the company from Toronto was giving me (and they were very pushy), and I'd much prefer to stay local. The inverter they've quoted me on can be monitored via WiFi. We applied for the microfit 1 week ago with the ieso, so we hope to get a spot. I have now decided to go string inverter. Cam

[Ralph Day]

Hi Rob and Cameron
On one point I beg to differ Rob. While sitting on a block of wood with fingers and toes crossed...I haven't had one M190 inverter fail. Everyone else I know with M190's has had some replaced, but not problems here.

Perhaps it's because they're on open racking, not a roof, so there's loads of ventilation. Another factor might be they're programmed (by Enphase via the Envoy and internet) to the highest voltage grid profile ie: to the inverter's max line voltage.

At any rate, no failures in almost 7 years. Not the lifetime of the contract or warranty, but not bad. A replacement doesn't take very long on ground mount racking, but I wouldn't want to have to rent/hire a cherry picker to get at a middle pannel on a roof install.

How do you discover you have a panel malfunction with a string inverter? Enphase you can see it immediately on your computer/tablet/phone depending on how you're set up. Just curious.

Ralph

[Cameron Morrison]

The system would be going on our barn, and it would be about 50' in the air. Roof would get very hot. I understand that with the string system, if we had a bad panel, an amp meter could be used to located which of the three strings were bad, and then you would need to check each of the 12 panels up on the roof. I also understand it is very unlikely for a panel to fail.

[Rob Beckers]

Ralph, I remembered you had Enphase micros, but didn't know which type. Good to see yours are still working, and I hope they will continue to do so. The odds are unfortunately not good: In talking to many installers, and dealing with end-users who needed replacements, I see them failing on a massive scale. This isn't a few here and there, this is whole FIT installations with hundreds of them. It's not limited to any particular production run either. You're right that heat has almost certainly a lot to do with it. Hopefully yours will keep going!

Regarding panel issues & string inverters: You are correct Ralph that it is of course much more difficult to spot. Luckily it is really, really rare for a panel to fail. I've seen some spectacular failures, with junction boxes burned out (leaving the top of the panel discolored too, easy to spot), and even one where they forgot to caulk the frame, so it fell apart while lifting it out of the box (not a clue how they got it in there in one piece!). Then there were a few, very few, bypass diode failures. Overall though it's truly rare to have a panel fail.

What I use as a measure for my own rooftop, with string inverters, is how much the system produces on a very, very good day. The maximum I've seen made from my 6.5kW is 43 - 44 kWh per day. Happens a few times a year. It did so during the first year, and I've still seen it happen this year (now over 6 years in). As long as I see 38 - 42 kWh on a nice sunny day coming out during the rest of summer I know it's working.

If you want to be more diligent, you can keep an eye on the string Voltage and currents (as reported by the inverter) around noon on a sunny day. Those will change if there's a broken diode (string Voltage will drop), or other issue.

In case you have enough MPPT inputs on string inverters to match the number of strings, so it's just one string per MPPT input, it's easy to track current and Voltage of each string. The inverter will report it, and most online monitoring will do so too. You don't even have to get a meter out. When strings are combined it's a bit harder, though today it's at most two strings per input (for example for a single 10kW Fronius Primo inverter with 35 panels).

-RoB-

[Dayton Johnson]

Glad to hear you prefer the string inverters Rob...we were undecided too when we built our ground mount system. 7 years in we have not had a panel failure. We did have to replace both Fronius inverters as one had fizzled completely and the 2nd was on it's way out too. Company replaced at no cost but it took a bit of time for shipping, installer to show up the usual bumps in the road. Luckily it was not peak season but we still lost a fare bit of income. I believe (could be wrong but) Fronius will only do this once!
After a # of years of daily walks to the meter we have installed a transmitter to send info to the computer.

[Mabel Smith]

String inverters are any day better according to me. Even though both have the same life, it is easier to replace a string inverter as compared to a micro inverter...