Central Inverter vs Micro Inverters

[russ]

I just stumbled across this thread, I'm a first time poster. I currently have a 3.5 year old enphase system with (24) of the M190 inverters, with Sharp 224W panels. Those who follow enphase know that the M190's were sold with a 15 year warranty.

All excellent information! Thanks!

Russ

[ginahoy]

I just stumbled across this thread, I'm a first time poster. I currently have a 3.5 year old enphase system with (24) of the M190 inverters, with Sharp 224W panels. Those who follow enphase know that the M190's were sold with a 15 year warranty.

Several things may have been misreported in this thread... first, several people suggested the enphase modules have a low failure rate. Well, over the past year or so there have been an alarming number of M190 and M380 failures, as reported in the enphase community forum. So far I've had four microinverters replaced and I'm watching two others that are beginning to act up. Some customers have reported even higher failure rates, and even some failures among the replacement modules. Also, I'm seeing a significantly larger variance among module output (currently approaching 8% top-to-bottom) than when system was new (less than 2.5% top-to-bottom). The initial variance reflects manufacturing tolerances for panels + inverters, and is actually pretty good, but I can't imagine there being that much variance in panel degradation rate, especially given the age of my system.

It's impossible to know if this was a bad batch or more widespread among all M190/M380 modules. But if enphase doesn't come up with a reliable replacement module, I wonder what's going to happen after the warranty is up. So far, the newer M215 and M250 modules, which have a 25 year warranty, seem to be much more reliable. Only time will tell.

Here's a link to a thread on failures in the enphase community forum: http://bit.ly/1kIgMfl. Interestingly, this and another thread on M190 failures were deleted by enphase. This one was eventually restored after some folks screamed. But it was moved to the archive yesterday, which means no more comments and it will no longer appear in the topic list or search results.

Second, several here stated that the enphase warranty doesn't cover labor. I don't know if they changed their policy for the M215/M250 modules, but enphase paid my dealer a labor allowance for each inverter replacement. Replacements are quick on the roof, but the dealer isn't going to make money on the deal. Whether or not he loses money depends on logistical considerations. For example, my dealer has lots of customers in my area so he's able to coordinate warranty work with other work to save truck rolls.

Third, someone said enphase requires that modules must be returned to the factory and diagnosed before a replacement is shipped. I can't speak to what the policy may have been in the past but all of my replacements have been cross-shipped to the dealer. My first failure was a year ago and the most recent was last month. That said, they do seem to try to drag their feet. They've implemented a new policy whereby if a module fails soft (e.g., intermittent drop-outs), they initiate a 90-day observation period. Knowing that, when my last module failed, I just waited until it failed hard so I was able to avoid the what I think is a stall tactic. In any case, intermittent dropouts only cause minimal loss of kWh's.

One problem is that their auto-alert feature doesn't trigger a reportable event until a module's power output is 0 for 24 hours. Two of my failures reported 2 watts for a couple of weeks before I noticed. When my system was new, I checked the array pretty much every day, but that soon wore off. Since the failures began, I've been checking module output a couple of times a week. Someone started a thread asking why alert thresholds couldn't be tweaked to solve this problem. After several people chimed in, the admin posted a message advising that the event threshold can be modified in system settings and promptly closed the thread, marking it as "implemented"... He either didn't read, didn't understand, or didn't care that folks were reporting that it's the most aggressive alert option that doesn't trigger an event if module fails to 1 or 2 watts

And finally, enphase rolled out a new pared down customer portal earlier this year (MyEnlighten). The original, full featured Enlighten Manager is now intended for dealers and a one time fee of $500 is required for access. Fortunately existing customers and dealers were grandfathered. Interestingly, the new portal doesn't display the per-module graphs that make variances and failures obvious during playback. Some folks speculate that was one of the motivations for the new site design. They know that dealers don't have time to study output graphs for all of their systems. I'm sure the company hates anal-retentive end-users (like myself) that monitor their array regularly. We're the ones catching all the problems (and giving them all the headaches

Sorry to turn this thread into an enphase rant, but I just wanted to clear up a few misconceptions.

[kwilcox]

You also might want to check with your local Fire Dept. When the main disconnect is thrown, Micro-Inverter systems de-energize all shared power on the roof in about 2 seconds leaving only individual panels to deal with. DC systems that use central inverters can still have large string voltages present. Fire depts. are still working out how they will handle roof mounted arrays; however the 2014 NEC may shed some light on this:

690.12 PV Arrays on Buildings Response to Emergency
Shutdown. . For PV Systems installed on roofs of buildings,
photovoltaic source circuits shall be deenergized from
all sources within 10 seconds of when emergency shutdown
is initiated or when the PV power source disconnecting
means is opened. When the source circuits are deenergized,
the maximum voltage at the module and module conductors
shall be 80 volts.

[bcroe]

Enphase doesn't give him spares to swap in 1 trip. Enphase requires him to retrieve the failed microinverter and send it in for evaluation first. Then if Enphase agrees that it's a defective part, they'll send out a new part for replacement next. But that means 2 service trips for him, not 1, plus all the down time. So his customers are not happy and neither is he.

Th big annoyance to this customer is having the much extended time out of service. I have
in general avoided that by keeping a lot of spare parts, and trying to make swaps easier.

For my strings, a spare (near new) inverter I picked up is wired & ready; just requires the 2
DC input wires to be moved over, to replace either of the on line inverters. Bruce Roe

[russ]

I know Enphase's warranty doesn't include labor but more than a few installers whom I got quote from would include the labor to swap out the bad inverters within the 25-yr warranty.

Good luck finding one who will be in business in 24 years to honor that warranty - ain't likely. Five years is a maybe.

[Volusiano]

I know Enphase's warranty doesn't include labor but more than a few installers whom I got quote from would include the labor to swap out the bad inverters within the 25-yr warranty. I specifically asked this question about labor after the initial 10-yr period. They would also put it in writing for me. According to them, it's really easy to swap out the mirco-inverter. It's a 15-min job, I was told. I guess for most small systems, they can always install the inverters on the outside frame, so it's easy to access? So unless either Enphase or the installer bell up in the next 25 yrs, I really have nothing to worry about.

I think if you can convince the installer to include labor in THEIR warranty in writing to cover Enphase's lack of labor inclusion in their warranty, then that would help a lot for sure.

Access to the microinverter can be easy or not, depending on how your panels are laid out. My 44 panels are laid out together in 1 big slab of 4 rows of 11 columns altogether. So if the microinverters of the outside perimeter panels fail, it's not too hard to get at. But if the microinverters of the panels smack in the middle of the slab fail, it'd be harder to get to those because you have to remove some outside panels in order to get to the middle panel. So a 15 minute job can turn into an hour job easily if time is spent removing neighboring panels to get to the middle panel with the failed inverter, then put them back. This is also not counting the time to make the trip to the site, which can be up to an hour for a round trip service call.

And the worst part of it is that it wouldn't be a single trip or two in the 25 year period. If a microinverter can fail before the warranty is up, that means other microinverters can fail, too, and most likely at different times. So it's going to be SEVERAL trips over the life of the system, not just a trip or two only.

It may be a matter of time before the installers wise up and withdraw their own labor warranty offer. I know my installer has wised up. The owner of my installation company told me that he doesn't want to do microinverters here in AZ anymore because he's had to send people out on several service calls, and his labor cost is eating into his profit margin. He also said that Enphase doesn't give him spares to swap in 1 trip. Enphase requires him to retrieve the failed microinverter and send it in for evaluation first. Then if Enphase agrees that it's a defective part, they'll send out a new part for replacement next. But that means 2 service trips for him, not 1, plus all the down time for his customers. So his customers are not happy and neither is he.

Maybe the newer generation of Enphase microinverters are more reliable. But the first generation has a lot of short-term problems. And long-term reliability is still an unknown because this market is too new. It's probably going to be past 10 years before you'll see longer-term failures.

[inetdog]

Optimizers are basically small buck/boost converters.

Whose buck or boost ratio is set by a microprocessor based on MPPT algorithm on the panel side and communication with the associated string inverter on the output side.

[billvon]

I didn't know that optimizers can adjust the voltage to compensate for bad panels. This means they have DC-DC converter in them.

Optimizers are basically small buck/boost converters.

[thejq]

I don't see why a failed panel/optimizer can bring the whole string down if there's a fail-safe bypass. Other optimizers on the other panels in the string will simply adjust their dc outputs to put the whole string back to the fixed string voltage. That's the whole point of the design.

I didn't know that optimizers can adjust the voltage to compensate for bad panels. This means they have DC-DC converter in them. If they have good efficiency, I can see optimizers can be quite useful.

The idea of easy expandability is a novel idea but I need to question the practicality of doing this a little bit at a time due to the cost of resubmitting an expanded system for review and permit approval again. You don't want to just add 1 or 2 at a time and go through the whole cycle and cost of permit approval just for 1 or 2 more panels. Any expansion has to be significant enough to make the permit cycle cost effective and worthwhile. So if you're doing a significant expansion, it's just as easy to add more panels to a new string inverter. Actually in my system of 11 kw on a 7kw and a 4kw inverter, I can easily add 12 more 250w panels to up my system from 11 kw to 14 kw and still just use the 2 existing inverters I have, no new inverter needed.

Yeah, I'd agree with that. If permit is required, it's probably not worthwhile to add a small number of panels.

Misguided or not, unless a manufacturer includes labor warranty for any kind of electronics on the roof, I wouldn't touch it. It just costs too much to have to cover for labor on roof top services.

I know Enphase's warranty doesn't include labor but more than a few installers whom I got quote from would include the labor to swap out the bad inverters within the 25-yr warranty. I specifically asked this question about labor after the initial 10-yr period. They would also put it in writing for me. According to them, it's really easy to swap out the mirco-inverter. It's a 15-min job, I was told. I guess for most small systems, they can always install the inverters on the outside frame, so it's easy to access? So unless either Enphase or the installer bell up in the next 25 yrs, I really have nothing to worry about.

[bcroe]

Electrolytic failure rates depend on operating temperature and secondarily AC current (which causes temperature rise.) It doesn't depend on voltage. In general string inverters win here since string inverters operate cooler than microinverters do. Getting away from electrolytics will solve this problem.

Good luck getting away from electrolytic capacitors. I have built systems without them, but only
because the power was very low. Switchers are very hard on caps, because so much AC current
flows in and out of them each cycle. The Effective Series Resistance acts similarly to battery
resistance, heating them up. The heat degrades them from the inside, and they lose capacity
if not outright failing. Lose of capacity will eventually defeat the switcher.

More panels will get a switcher running sooner, because more power is initially available. Bruce Roe

[Volusiano]

My understanding is that with SolarEdge optimizer, if one panel/optimizer goes down, it can still bring down the whole string. It's because even if the optimizer can bypass the bad panel, the whole string needs certain voltage to operate. Say if the inverter turns on when the voltage reaches 500v, with one panel gone, it will reach that voltage later and shuts down earlier, or never reach it at all.

I don't see why a failed panel/optimizer can bring the whole string down if there's a fail-safe bypass. Other optimizers on the other panels in the string will simply adjust their dc outputs to put the whole string back to the fixed string voltage. That's the whole point of the design.

Most of the installers I talked to had very few problems with the Enphase micro-inverters and have installed tons of it.

Installing tons of microinverters is not an indication of high reliability. They're still relatively new with only a few years in the field so it hasn't been long enough to see field failures yet.

Also later on if I need more, with micro I can easily add one or more panels, but not that easy with string.

The idea of easy expandability is a novel idea but I need to question the practicality of doing this a little bit at a time due to the cost of resubmitting an expanded system for review and permit approval again. You don't want to just add 1 or 2 at a time and go through the whole cycle and cost of permit approval just for 1 or 2 more panels. Any expansion has to be significant enough to make the permit cycle cost effective and worthwhile. So if you're doing a significant expansion, it's just as easy to add more panels to a new string inverter. Actually in my system of 11 kw on a 7kw and a 4kw inverter, I can easily add 12 more 250w panels to up my system from 11 kw to 14 kw and still just use the 2 existing inverters I have, no new inverter needed.

The whole capacitor argument is also misguided...

Misguided or not, unless a manufacturer includes labor warranty for any kind of electronics on the roof, I wouldn't touch it. It just costs too much to have to cover for labor on roof top services.

[thejq]

Electrolytic failure rates depend on operating temperature and secondarily AC current (which causes temperature rise.) It doesn't depend on voltage. In general string inverters win here since string inverters operate cooler than microinverters do. Getting away from electrolytics will solve this problem.

Ok, I know I've been out of school for a while (ok a long while), but I still remember the basics of electromagnetism. You're right that heat is one of the cause of failure for the caps. But the amount of heat/unit time is power in watts, and power = V_rms*I_rms = V_rms^2/R. In another words, power is directly proportional to voltage squared. So if I increase the voltage by 100% or 3 dB, the power or heat quadruples or an increase of 6 dB. In fact, V_rms = I_rms * R, so voltage is directly related to current. If you say current is the problem, the same must be true for voltage. The ambient temperature maybe cooler for the sting inverter, but 600v vs 50v is a big differential for internally generated heat. I've heard about electrolytic capacitor failure as the main disadvantage presented by the sting inverter developers, but I haven't seen any solid proofs that it actually happened, granted the concept of microinverter is quite recent, so the long term reliability is yet to be seen.

[billvon]

The whole capacitor argument is also misguided, since for the optimizer there's one capacitor for each sting of 600v, but for micro, it's about 40-50v per capacitor. I was told the former works at much higher temperature and load vs the latter, so the failure rate is much lower for the micro.

Electrolytic failure rates depend on operating temperature and secondarily AC current (which causes temperature rise.) It doesn't depend on voltage. In general string inverters win here since string inverters operate cooler than microinverters do. Getting away from electrolytics will solve this problem.

[inetdog]

Your ideas about "reaching the turn on voltage earlier" seems to be based on the common misconception that the panel voltage increases as more light falls on it.
The voltages Vmp and Voc are almost independent of light level until you get down into the moonlight or lower light levels where the internal leakage resistance of the cells pulls the output voltage down.

You may find that you have to go from three panels in shade down to two panels in shade to get the inverter to turn on, but that is different scenario than just being down by one panel and not turning the inverter on.
A good GTI and panel configuration will not be running the string voltage that close to the minimum voltage spec of the GTI, among other things because V will decrease as the panels get hot.

[thejq]

I just started my research in the last few weeks and still haven't decided on the best configurations. But so far, I'm fairly certain that I want the micro-inverter. In San Diego, for the 4.5 KW system that I'm interested in, the installed price for SolarEdge and Enphase is the same. My understanding is that with SolarEdge optimizer, if one panel/optimizer goes down, it can still bring down the whole string. It's because even if the optimizer can bypass the bad panel, the whole string needs certain voltage to operate. Say if the inverter turns on when the voltage reaches 500v, with one panel gone, it will reach that voltage later and shuts down earlier, or never reach it at all. Most of the installers I talked to had very few problems with the Enphase micro-inverters and have installed tons of it. But not many had installed the optimizers. Also later on if I need more, with micro I can easily add one or more panels, but not that easy with string. The whole capacitor argument is also misguided, since for the optimizer there's one capacitor for each sting of 600v, but for micro, it's about 40-50v per capacitor. I was told the former works at much higher temperature and load vs the latter, so the failure rate is much lower for the micro. The industry as a whole seems to be convinced that micro is the future, now that SMA, APS, Tigo to name a few have all come up with micro-inverter products. LG just recently announced their AC modules where the inverter and panel are built together to produce AC. Unless I can get optimizer much cheaper, I think the balance tips heavily in favor of the micro-inverters.