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Micro Inverters (My Turn)- the boring technical stuff

A few weeks ago, Dan wrote a post about micro-inverters for beginners, comparing them to the inner-workings of a Peruvian diamond mine.   This article is more technical and boring, because that’s who I am, Mr. Technical and Boring.  (Remember the Mr. Men books? OK, never mind)…
Mr. Solar
I believe micro inverters will be a major new part of the residential solar energy market (the other being photovoltaic and hot water integrated into one panel, but that’s a post for another time).  There are definitely competitors getting in the microinverter game, but for now, Enphase microinverters are the only game in town. Much of this article is based on their product.

I’m going to go over all the advantages and disadvantages of micro-inverters as I see them.   The big picture that needs to be understood is that, with microinverters, we’re talking about one inverter PER SOLAR PANEL all linked in parallel, whereas a traditional string inverter configuration links several panels together in SERIES.  This allows microinverters to maximize the energy harvest of each panel.

First, let’s go over what max power is.  Well, Power (with electricity) is voltage * current, or P=V*I.  Solar panels change their voltage output based on a number of factors, the most dominant of which is how hot they are (up to 50%! that’s why some neighborhoods can get more out of Solar in San Francisco than in some places in Arizona!).  The panel current also changes based on how much light is hitting them (irradiance).  The maximum power of a panel is just the highest number you can get by multiplying it’s voltage and current.

Take a look at this graph courtesy of the NABCEP study guide for PV Installers:

Solar Energy Voltage Current IV Curve

You can see that light hitting panels changes their max current, and heat changes their max voltage.  The Max Power Point moves accordingly (Imp and Vmp).

With a string inverter configuration, the inverter is getting several of these panels linked in series.  In series, their voltages of the panels add together but the current does not.  It’s the inverter’s job to try and track the maximum power point, but it can only “see” a string 0f panels, it doesn’t know which panels are doing what.

The big benefits of micro-inverters come from the fact that they can maximize this power point for EACH PANEL. So that if one panel is say dusty, faulty, aligned differently, a different type of panel, etc., it does not drag down the whole string. Additionally, conventional string inverters are limited in the configurations they can accept, and disregarding those limitations can sometimes result in zero system output at times.  micro-inverters, on the other hand, are free of most of those limitations.

Due to this individualized “panel harvest,” Enphase claims you will see anywhere from a 5%-25% gain in power output when compared to a string inverter configuration.  The low end of that range would assume a well engineered string configuration with panels on the same orientation and tilt, without shading.  You would need some heavy shading or panel orientation differences to achieve the upper end.

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ADVANTAGES

Modularity – Scalability

Let’s say you want to add an electric car in two years, but you don’t want to oversize your system now because you won’t get a check from the utility for excess production (or even a thank you.) Due to the nature of microinverters, you can almost always just slap on another panel/micro-inverter.

  • Ideally you’d want to get it sized right the first time to avoid having to increase the racking system and number of roof penetrations, but this is still more scalable than a string inverter.
  • With string inverters you have to adhere to a regimented string design, and an upgrade would require a number of things from a different inverter to a large wing of panels to fit the string configurations. For example, a 3000 watt string inverter may be limited to five specific configurations: 2-strings of 7-panels, 2-strings of 8-panels, or 1-string of 9-panels. That means you are limited to 14, 16, or 9 panels. Not a lot of scalability there.
  • Up to 16 micro-inverters can be linked in parallel per AC branch. That does not mean you can only have 16 max panels, you can have literally any number of them from one to whatever your electrical service will accommodate. It is just good because you can fit more per branch than most string inverters which may cap out at 11 or 12 per string. This isn’t a giant advantage but it’s nice for the installers wiring small systems.
  • No clunky box downstairs. These inverters go on the back of each panel, on the roof, so there’s no need for a big string inverter downstairs by your meter. That eliminates concerns about string inverter placement and clearance issues, conduit run eyesores, keeping the inverter out of the sun, etc. Also no need for DC switching points.

Shading

This is the big one. If you shade enough panels in a row, you can bring the voltage of the entire string low enough so that the inverter just stops, creating zero power output. However, if you have say, 5 panels shaded on an 8-panel array, those 3 panels just keep kicking with micro inverters, each one creating maximum power.

Furthermore, if dust, grime, or bird poop are getting on panels disproportionately, they can drag down the power of the whole string, much like stepping only a small portion of a water hose. Remember: the string inverter maximizes power output for the whole string, not each panel, so if some panels are suboptimal, so is the whole string.

Differences in Panels

Enphase conducted a beta test by creating massive solar installs in a checkerboard pattern. Meaning, every other panel on the roof was connected to a micro-inverter or a conventional string inverter. Over time, panels got more and more mismatched because of dust and de-rating differences. As a result, the power output of the micro inverters increased comparatively to the string configured system.

  • Production methods for panels have gotten better and will get even better, but still, panel to panel mismatch is exploited best by microinverters, maximizing each panel’s max power point
  • If you live in a home where certain parts of the roof are sprayed with road dust, the impact could be particularly significant
  • This would be uncommon, but if you have a bunch of different panels lying around, or you have a system installed but your buddy gave you a bunch of different type of panels that fell off a truck, you can mix and match them with micro-inverters since each are independent to their own panel. As string inverters require that all panels on a string be the same orientation, they must also be the same type.

No Single Point of Failure

This is really up for debate. Some would argue that no single point of failure is better because if one inverter goes out you still have power production while you’re waiting to get fixed up. Others would argue that you have simply multiplied the possibility of failure dozens of times. Currently, I believe what I hear about their design practices and think these things, although not proven, will last considerably longer than string inverters. I hope I don’t have to eat my hat.

Burst Mode

In the early morning and early evening there may not be enough current to drive a string inverter. It will “flip on” when it gets enough light. Micro-inverters (at least Enphase models) have a “burst mode” which stores up energy until they can release it in these low light conditions. This allows for SOME power output in times when you may have none with a string inverter. I don’t have numbers on how much, but I don’t think it’s a ton or they’d be making a bigger deal out of it.

Cheaper Labor Costs

Electricians do not need to be there all day wiring, they simply perform the connection at the main panel. Installers are doing most of the connections up on the roof. The installation is also quicker with less conduit bending and what not, so some money can be saved here.

Monitoring

Another huge one. Because it’s panel by panel, monitoring system sophistication is really powerful. Not only does the software know your system as a whole is underperforming, but which panels are the culprits. The monitoring system is smart enough to know that power depression on all panels is probably a cloud, whereas if one panel is blinking on and off you’ve got an issue. You currently can’t get this type of information with string inverter monitoring. There also is some other cool whizbang stuff like getting text messages if a panel is out. Your installer gets this info too, so as soon as you know there is a problem, so does the installer. The “EMU” is smart as hell and just plugs into the wall, grabbing RF signals that the Enphase modules throw out. No need to connect it directly to the system!

How much is the monitoring price? I think they have different programs but from what I understood, they recommend salespeople offer 1 year free and $10/month for the whole system after that (with lump packages that make it slightly cheaper than $10/month).

Orientation

Let’s say you have a small roof with a west and south face. You can fit 10 panels on the south face and 5 panels on the west face. String inverters require all panels on a string to be the same orientation and tilt, so a string of 5 and a string of 10 is probably not a possible configuration, and splitting them up 8 and 7 won’t work because of this requirement. Even if you were allowed to perform these two string configurations, different string orientations need to be carefully designed and can lead to disastrous situations if not carefully looked at, something which even seasoned solar salespeople may not be able to do correctly. This allows for roof area maximization.

DISADVANTAGES

Field testing data is relatively early

By relatively I mean that they have thousands of these things operating in the field, it’s just that they’re supposed to last for decades so it’s early until we’ve reached decades. This is a big issue for most people considering micro-inverter systems. As you may know I have a big beef with this sort of thing. “We are waiting for new technology! We are waiting for new technology!” Then you bring new technology and everyone is like “It’s untested! We don’t want it!” So here are some reasons why it’s not a big deal:

  • Their MTBF is calculated for 119 years. They’ve got some people on their design team that would design stuff to be put on power poles in the middle of the desert. Believe me, they understand very clearly that if these have a high fail rate, they’re screwed as a company, so they put a whole ton of effort into nailing that beforehand.
  • There are lots of micro-inverters in the field and according to Enphase, none have ever failed. There are more being installed every day.
  • They have no moving parts and soft switching, unlike some fan cooled string inverters. That should aid in lengthening their lifespan.
  • In California, inverters are all warranted for 10 years anyway, and the lifespan of a string inverter isn’t much longer than that. I could see being concerned in say, year 5, if tons of them started failing all at once and the company goes belly up. This is a legit concern, but I think it’s worth the small gamble based on what I’ve seen, plus they just got $15M in more in funding.

Exposed to the elements

So yah, they’re up there on the roof. They are shielded by the panels, but they are outside none the less. (However, I’ve already gone over how these are designed to be rugged as hell because the company’s future depends on it)

Limited in some applications

  • No off grid situations, not overseas yet (50Hz), and haven’t been tested or designed for other forms of renewable energy other than solar
  • They DO have a 3-phase version for commercial applications but that is outside the scope of this article

Are they cheaper or more expensive?

The answer is that it depends. I didn’t put this as and advantage or a disadvantage because it’s complicated. Currently Enphase is $200 MSRP last I checked. Let’s call a 3000 Watt string inverter $1600 for simplicity’s sake. So once you go over 8 panels you’re looking are more money on the product side, but you’re weighing that against some other things like, well, the advantages listed above.

The extra power output is worth money, and so is the decreased labor costs, so you have to do that calculation. On a tiny system, micro-inverters are a slam dunk in my mind.

What’s Next, who’s entering this market?

There are other micro inverters that will be manufactured, as well as some string inverter technology that attempts DC based max power point tracking like National Semiconductor’s Solar Magic technology, which as far as I’ve heard, isn’t in the field yet.  There’s something called “solar bridge” but there’s virtually nothing on the internet about it and I haven’t seen it in the field yet.

So that’s probably more than you ever wanted to know. My take is that for residential, you can’t beat these things, they rock. In some large, perfectly unshaded, same orientation, residential applications you could still make a case for string inverters I think.

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45 thoughts on “Micro Inverters (My Turn)- the boring technical stuff

  1. Joe Seraphin says:

    As a professional solar installer and enphase system owner since 2015, I can tell you right now that you should not use bigger than a 275 watt module with the M250. If used 285W modules on my system and I have had two out of thirty microinverters burn out already with in the first 6 months.

  2. Anonymous says:

    Have just signed a contract for SunPower panels to be installed. I was told that micro inverters don’t work on the 325 W panels so we have to string inverter. Is that true? And if so would I be better off having the lower wattage panels with the micro inverter over the 325 W with a string inverte?

    1. Ben Zientara says:

      Well, there are certainly a lot more factors than just panel wattage to consider in deciding whether the installation would benefit from micro-inverters. But just in general, 325W panels are pretty beefy, and using them means you’re probably not going to have an installation on the smaller side. If you’re looking at a 6-kW or higher installation, and they’re all going on an un-shaded roof at the same orientation and tilt, I think the string inverter and 325W panels will work great for you.

  3. Anonymous says:

    There is now a very strong contender from SolarEdge. It gives virtually all the benefits of micro inverters AND string inverters.

  4. Anonymous says:

    Can I put 20 enphase 215 on a 2 ple 30amp breaker with appropriate sized feeder cable?

  5. Anonymous says:

    There is a third option, an emerging new class of inverter called Micro-Parallel inverters. They combine the best of string and micro inverters, along with a slew of new features designed to reduce inverter installation, cabling and activation costs by nearly 75%. TRC manufactures a micro-parallel inverter called SmartPhase™ that fits this class inverter. http://trci.net/products/energy-products

  6. David Larson says:

    Actually, Enphase is limited to 60 cell panels thus making not usable for high efficient panels like Sunpower.

    1. Dave Llorens says:

      Hey David!

      A micoinverter shouldn’t be limited to a number of cells – they wouldn’t know how many cells are in the panel – but could certainly be limited to a max imput power. However I think Enphase’s more recent M215s should be able to handle most just about anything. The reason they used to not work with SunPower in the past was because their panels were positively grounded – but I think they have since fixed that problem. (I’m not 100% up on this so I could definitely be wrong about last part).

  7. prabhu says:

    Gentlemen,
    The bggesr disadvantge is price and complexity…Better option is Panel maximiser with say 400V DC constant power used in a All Parallel DC array wih maximiser embedded intelligent modules we buy in India fom Mitramax.

  8. david variyan says:

    Is the Enphase inverter 50hz available in Australia. If so at what cost and details of agent/company

  9. Curtis says:

    I just installed a 42 panel ground-mounted system with Enphase 190s. The panels are 235W – and, on the few sunny days we have had this winter… max panel output is 199.x watts – clipped just as “A Solar Installer” mentioned. The most irritating thing though, is having to pay ENPHASE for my data! I’ve seen forums of computer geeks talking about hacking the LINUX based ENPHASE EMU, but it seems to me that it might be easier to crack the communications protocol, pole the inverters, and ditch “Big Brother’s” EMU. Anyone of a similar mind-set?

  10. Bruno says:

    For info,
    Few other companies of Micro Inverters:
    SPARQ
    Direct Grid Technologies
    Enecsys
    Solarbridge Pantheon
    Apparent (Active and Reactive Power)
    Microchip
    ….

  11. gearjam says:

    Can you use microinventers on a pole system? I didn’t find any information on this type of application. Great information and article – thanks.

  12. Justin says:

    I don’t think that when microinventers fail, replacement will result higher expensive, that fact is that both centralised inverters and microinverter have warranty just like any other electronic product, microinverter now has develop systems that would give users far better flexibility of monitoring each panel’s performance from data server. It actually reduces labor cost in the long-term.

    In fact, Involar in China has been pioneering in research,develop, and manufacture microinverters since it started in 2008, and is first company that has been shipping its product to European market in small volumes.

  13. a solar installer says:

    Two disadvantages that were not pointed out:
    1. Any inverter, whether string or micro, will fail eventually. When a string inverter fails, it is a simple matter to swap it out: less than an hour of labor. When microinverters fail, however, the installer has to get up on the roof and potentially remove and replace every module to replace every microinverter, incurring a much higher expense.

    2. Enphase really only has two offerings at this point, the 190 W and 210 W. The 210 is only good for Sanyo and SunPower, because of different voltages. All other modules are stuck with the 190. As module wattages get higher, many are at 230 W, this creates a poor match between module and inverter. In locations with high irradiance and cold, clear winter days, a 190 W microinverter will clip throughout the middle of the day when paired with a 230 W module. Running the inverters maxed out will also likely shorten their lifespan.

  14. Burt says:

    If enPhase is saying that none have failed in the field, they are not telling the truth. Locally we have already seen failures in systems that are only a couple years old.

  15. George Sverlow says:

    Senior Building Inspector.
    I found your articles very informative and down to earth commentary. Countinue the good work. Maybe in the futher something on calculating microinverter strings sizes. This maybe to technical for some, but interseting to others.

  16. Horea says:

    Thanks for the article! I am interested in more details about the subject as I’m doing my bachelor work on Solar MicroInverter technology and connection to a Smart Grid. Any idea where I could find some more info (except IEEE and their $840 annual fee :P so I can read full articles).

    Also I forgot to mention that I would love to make a few measurements on one of those beauties :D but I can’t seem to find a sponsor or someone that could borrow me one.

  17. Paul Bostwick says:

    I’m not sure if this is the best spot to float this idea but here is a ‘go”.

    the part when you wrote
    “This would be uncommon, but if you have a bunch of different panels lying around, or you have a system installed but your buddy gave you a bunch of different type of panels that fell off a truck, you can mix and match them with micro-inverters since each are independent to their own panel.”

    I thought YES someone is thinking like me. Here is what I hope to offer an installer as a plan. Come. Install the biggest rack we can fit up there. Up down side to side. max my racks! Then with whatever $ i have left, put in panels with micro inverters.

    Then in the following year or two as the installer is ordering panels for other customers… anytime he sees a chance to bump the number up to get a better discount or meet a monthly quota he does so for me and installs them as a fill-in job for crews who have spare fractions of a day. Or a training gig… I give up timelyness and give on flexibility and in return I get lower cost and higher output.

    yes my roof would look like a sale rack at the PV store… But only one neighbor can see it. And the only risk there is she’ll want one too…

    Is there some gotcha I am missing on this? I don’t think it makes it free just that it could be another slice at the price. and could be good for the installer.

  18. Christof says:

    Agree, nice piece. I’m inclined to think micro-inverters are the way to go — but it’s too late for us (I think), we signed on to a system from REC Solar (to be installed in June 2010) with a Sunny Boy central inverter. For me, the big thing is the durability. It’s disturbing to me that I know that we’ll be shelling out $3,000 or $4,000 grand for a replacement central inverter in year 12 or so. What’s also disturbing is that many solar proponents (and companies) don’t calculate this major cost into the long-term cost projections of a solar system. That, to me, seems fundamentally misleading, and, when I get a chance to, I’m going to do a piece on this issue for my website, SolarChargedDriving.Com. Thanks again guys for putting up good stuff!
    .-= Christof´s last blog ..‘EV Nut’ sticks it to Big Oil, fills up with sun =-.

  19. Drifty says:

    I am new to microinverters, and would like to learn more about the way in which they work. I am an electrical engineering student and would of course like to construct one. Any links to sources of that type of information would be greatly appreciated.

  20. Silver says:

    Good article David. We are moving into grid-tie systems from the off-grid becuase the grid has been extended to reached the off-grid areas. The micro inverter provides a way of utilizing PV energy into the grid using existing small PV system. Will the Enphase work under our 230VAC 60H system?

  21. Dane Ericksen says:

    So how “clean” is the AC generated by each micro power inverter? That is, is the output a good representation of a 60 Hz sine wave, or are we talking about a stepped inverter with lots of harmonics? What about RFI in the AM broadcast band?

  22. Greennose says:

    Great article. The big reliability issue in micro-inverters is aluminium electrolytic capacitors – they’re chemical reactors in a can. That’s why the warranty on inverters is so much lower than the warranty for modules. At couple of companies are shooting to solve the problem buy the end of this year.

  23. Jouvet says:

    Solar Magic has been out for a month now. Any comparisons? Also what about SolarEdge?

  24. Randy says:

    I echo Kevin’s concern. Going directly to the EMU appears to only give you summary data – not at the individual inverter level (per screen shots at the Enphase web site). The money Enphase wants to charge for the monitoring of individual module data cuts into the ROI of the PV system over its entire life span. Given the extra cost of these inverters I feel the data access should be free.

  25. KNSato says:

    We are looking at microinverters but have one concern. Batteries are not so great now, but are probably going to become more and more efficient and economical in the future. What if eventually we want to attach a battery so we can have power during a blackout or be completely off-grid? Isn’t that not possible with a microinverter system as it is now?

  26. Kevin says:

    I received a reply from Enphase on my question and found that I was incorrect in my reading of the information from the website. The EMU can be monitored by directly browsing to it. So you do not have to rely on the Enlighten website monitoring, however the Enlighten monitoring can setup alerts and provides additional features. Also the Enlighten Monitoring is free for the first 90 days and is recommended for initial setup to help identify any install issues early on.

  27. Kevin says:

    I am very interested in the Micro Inverters however I have a reservaion on getting a system that can only be monitored through the companies website. I would much prefer a system where I can monior it locally without relying on an outbound internet connection especially one that requires an ongoing subscription fee to monitor items you have installed on your house.

  28. Doug says:

    No if new homes could be built with the requirement of an AC disconnect as standard, then the industry could sell these units with a standard appliance plug and we could just pop each in the wall.

  29. patrick, the inverters are designed for 72 cell panels, and can handle basically any of them. I think you can find this info at enphase’s site.

    You can have 16 enphase inverter on the same AC branch circuit.

  30. Patrick says:

    What is the capacity? Your article implies that it can only handle 100 watts. Is that the case?

  31. Marco, I have heard the same, but I am always wary of “time to market” projections. I hope to be able to do a solid comparison of these two technologies at some point. Micro-Inverters vs. DC MPPT tracking AT the inverter.

  32. Marco says:

    I have heard about DC/DC converters perfoming single-module MPPT tracking being developed by various manufacturer (ST Micro, national Semiconductor). Expected to be on the market in six months. They promise to improve the global performance of a PV park by 3 to 5%, and this only for avoiding the mismatch losses due to uneven module tolerances (+/- 3 or 5%)

  33. There are a number of changes about to arrive in the next few months. I believe you will also start to see some transformer-less inverters.
    This is an industry coming out of it’s extended infancy; and it’s about time.

  34. Pretty sure that as an individual on your own property you can only get the tax credit once, but I’m not a tax professional so please consult your CPA.

    Micro Inverters make solar more modular and scalable, but talk to your installer because they may not want to roll a truck for one more panel and do more roof penetrations. The key is that you don’t have to redesign the whole string configuration you can just add more without having to do any math.

  35. Nathaniel says:

    Ok, so it is now reasonable to do a small solar installation and then say next year add another panel or three and maybe the year after add even some more correct? With micro-inverters system expansion becomes easier? So my question is does that mean I could do a relatively small solar installation, get my tax deduction for the year and then next year add some more panels and get yet another tax deduction? Wouldn’t that turn a $15000 installation with a $2000 credit into two $7500, or even for extra labor $8000 installations with $4000 worth of credit? Presumably this scales up even more in places like California where there are more incentives than the rest of us dopes have managed to convince our politicians to enact.

  36. Jaubert says:

    Hi everyone, nice article.
    I just wish to give some complementary information about “the market”. In Europa, Netherlands has begun to explore AC inverters some years ago (before 2000 I guess)through a company named NKF. You can see here a brief description of their product http://www.photon-magazine.com/products/products_02-08_nkf.htm (I don’t know if they still manufactures it).
    Looks like some scientific experimentation have been made about the reliability of the products also (http://cat.inist.fr/?aModele=afficheN&cpsidt=14036987), so I’m a bit suspicious because not so much information have spread out about this technology…

  37. Jan says:

    Hey David, Excellent article, but I’m understanding can’t use these babies yet for off the grid??? Am moving towards a system for my compound here in sunny St. Thomas, V.I., but the local utility’s infrastructure and reliability is so scary, I’m terrified to do any kind of tie-in system!! Suggestions?? I had not heard about micro-inverters, so seriously THANKS! Suggestions

  38. Steve,

    The idea is that these last much longer than conventional inverters. They won’t be able to “prove” it till they’ve been around 20 years, but what they say makes sense.

    And yes, if they do start to fail their distribution will be annoying, but if one fails the system stays online.

  39. Craig I’m sure that’s in the cards for the in the near future, or at least that’s my guess

  40. Craig says:

    Nope, that was exactly what i wanted to know, except for one thing, When can i get one at 50Hz?

  41. Steve says:

    Great article. One concern: what is the impact on system reliability with a “micro-inverter” on every panel? Are not inverters typically the first component to fail? Is the failure/service rate increased many times with the introduction of micro-inverters?

  42. Dad says:

    Nice article, David, highly technical and a little boring to be sure, but educational.

  43. Kathleen says:

    This is really good stuff – I even understand it, and it’s amusing too.

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