Central Inverter vs Micro Inverters

[BGuy838]

Hi all,

I am close to signing a contract to have 18 Canadian Solar CS6P-250P panels installed on my home. The current proposal is for a SolarEdge Central inverter with individual DC Optimizers. The company has offered an alternative of Enphase Micro Inverters and the Enlighten system for $500 more.

From my research on the differences, I am having a hard time deciding on whether or not the additional cost is warranted for the Micro set-up. The panels will have very little shading issues, (which is one of the only big factors I have read to go with the Micro). Also, the Enphase Micros have the 25 year warranty vs the 12 for the SolarEdge Central.

If anyone can add any input on the two set-ups, I would be greatly appreciative. I am hoping to sign the proposal soon (as in by the end of the weekend).

Thanks,
Brian

[J.P.M.]

Hi all,

I am close to signing a contract to have 18 Canadian Solar CS6P-250P panels installed on my home. The current proposal is for a SolarEdge Central inverter with individual DC Optimizers. The company has offered an alternative of Enphase Micro Inverters and the Enlighten system for $500 more.

From my research on the differences, I am having a hard time deciding on whether or not the additional cost is warranted for the Micro set-up. The panels will have very little shading issues, (which is one of the only big factors I have read to go with the Micro). Also, the Enphase Micros have the 25 year warranty vs the 12 for the SolarEdge Central.

If anyone can add any input on the two set-ups, I would be greatly appreciative. I am hoping to sign the proposal soon (as in by the end of the weekend).

Thanks,
Brian

You'll pay $500 more for what in all likelihood is very similar performance with more potential failure points that may very well be hard to get at. For those reasons, plus few shading issues, I'd skip the micros. Also, check the labor/shipping part of the micro warranty. Finally, even though I'm usually against extended warranties as a complete rip off, that may be a consideration for part of the $500 you won't spend on the micros and put instead toward an extended warranty on the string inverter if burning out a central inverter in, say, 10-12 yrs. is a concern.

Regards,

[silversaver]

Depends on size of central inverters, you can always get the extended warranty from $500 to 1000 to extended to 20 yrs coverage. I have my inverter exchange recently and it is really simple ground level job with few wirings within an hour. Most of time were spend on testing the voltage of arrays and making sure the arrays were in working condition.

[Volusiano]

You should also ask if the 25 years warranty on the Enphase microinverters include labor or not. My understanding is that it does not.

On SolarEdge, I think the DC optimizers on the roof have a 25 years warranty both parts and labor. The 12 year is probably on their central inverters only.

If you don't have any shading issue at all and your panels are all pointing in the same direction/slope, you probably don't even need the SolarEdge local DC optimizers either. Just traditional central inverters on the ground will do. It'll save you some money and also reduce the risk of failure on the DC optimizers that will be put on the roof.

If I have shading issue, I would pick the SolarEdge solution over Enphase any day. The reason SolarEdge chooses to use central inverter is to reduce the risk of multiple points of failure for inverter circuitries up on the roof for each panel. Each time an Enphase microinverter fails, you'll have to go up on the roof to retrieve it, then back up on the roof again to replace it (after Enphase validates the failure and sends you a replacement). And they'll more than likely fail at different times, forcing you to go up on the roof for retrieval and replacement more often than you care to do. And if labor is not covered under warranty, you're left holding the bag.

So now maybe you see why SolarEdge prefers to put a central inverter on the ground, and why Enphase doesn't include labor in their warranty.

[Naptown]

You should also ask if the 25 years warranty on the Enphase microinverters include labor or not. My understanding is that it does not.

On SolarEdge, I think the DC optimizers on the roof have a 25 years warranty both parts and labor. The 12 year is probably on their central inverters only.

If you don't have any shading issue at all and your panels are all pointing in the same direction/slope, you probably don't even need the SolarEdge local DC optimizers either. Just traditional central inverters on the ground will do. It'll save you some money and also reduce the risk of failure on the DC optimizers that will be put on the roof.

If I have shading issue, I would pick the SolarEdge solution over Enphase any day. The reason SolarEdge chooses to use central inverter is to reduce the risk of multiple points of failure for inverter circuitries up on the roof for each panel. Each time an Enphase microinverter fails, you'll have to go up on the roof to retrieve it, then back up on the roof again to replace it (after Enphase validates the failure and sends you a replacement). And they'll more than likely fail at different times, forcing you to go up on the roof for retrieval and replacement more often than you care to do. And if labor is not covered under warranty, you're left holding the bag.

So now maybe you see why SolarEdge prefers to put a central inverter on the ground, and why Enphase doesn't include labor in their warranty.

very well said

[billvon]

The reason SolarEdge chooses to use central inverter is to reduce the risk of multiple points of failure for inverter circuitries up on the roof for each panel. Each time an Enphase microinverter fails, you'll have to go up on the roof to retrieve it, then back up on the roof again to replace it (after Enphase validates the failure and sends you a replacement). And they'll more than likely fail at different times, forcing you to go up on the roof for retrieval and replacement more often than you care to do. And if labor is not covered under warranty, you're left holding the bag.

How is that different than SolarEdge? Optimizers fail too.

[Volusiano]

How is that different than SolarEdge? Optimizers fail too.

Yes, as far as the local optimizers go, both SolarEdge optimizer and Enphase optimizers are on the roof with the panels, so both have the same potential for failure. So that's why if you don't have a shading issue at all, you don't need a local optimizer either from SolarEdge or Enphase.

But if you have shading issue and use SolarEdge optimizers, you'll only have to go up on the roof for service if the optimizer fail, but at least both parts and labor are in the warranty.

On the other hand, if you use Enphase microinverters, the optimizer and microinverter are packaged together, so you'll have to go on the roof for service if either component fails -> 2 components in 1 package -> higher chance of failure for the whole package altogether, and labor is not included.

[billvon]

But if you have shading issue and use SolarEdge optimizers, you'll only have to go up on the roof for service if the optimizer fail, but at least both parts and labor are in the warranty. On the other hand, if you use Enphase microinverters, the optimizer and microinverter are packaged together, so you'll have to go on the roof for service if either component fails.

But again same issue; indeed, optimizers present a bigger problem there.
One microinverter fails? You lose 250 watts of generation until you get up on the roof and replace it.
One optimizer fails? You lose either 250 watts or the entire string (say, 1560 watts) until you go up on the roof and replace it.

Overall you are better off with a single failed microinverter.

-> 2 components in 1 package -> higher chance of failure for the whole package altogether

Not sure that's true. The number of active components isn't radically different. Both are basically an H-bridge (4 MOSFET's) driven by a controller.

and labor is not included.

OK that part makes sense to me.

[silversaver]

How is that different than SolarEdge? Optimizers fail too.

yes. I'm not sure if the failure rates are lower comparing with Enphase's build in micro inverters. Even with partial shading issues, I'll still pick the central inverter with dual MPP tracker like SMA TL-US-22 series. Seriously, if you have lots shadow, micros wont help too.



This is the best method for shading:

[Volusiano]

But again same issue; indeed, optimizers present a bigger problem there.
One microinverter fails? You lose 250 watts of generation until you get up on the roof and replace it.
One optimizer fails? You lose either 250 watts or the entire string (say, 1560 watts) until you go up on the roof and replace it.

Overall you are better off with a single failed microinverter.

Not true. When the Enphase microinverter fails, you lose 250W of power (or whatever the panel is). When the SolarEdge optimizer fails, you don't lose 250W of power, nor does the whole string go down. The panel with the failed optimizer still produces power just fine. It just won't be managed by an optimizer anymore. Eventually you do want to and should replace it, but it doesn't cause a critical down situation.

Not sure that's true. The number of active components isn't radically different. Both are basically an H-bridge (4 MOSFET's) driven by a controller.

Both microinverter or central inverter need electrolytic capacitors to manage large input capacitance, but electrolytic capacitors contain fluid that can dry out over time. This is the main reason why inverters don't last longer than 10-15 years. But SolarEdge optimizer doesn't need to use electrolytic capacitors. It can use ceramic capacitors which can last much longer. On top of that, much of SolarEdge's optimizer is integrated into an ASIC, eliminating the need to use discrete components -> better reliability. So the likelihood of the optimizer failing is much lower than of the microinverter failing.

This allows SolarEdge to offer 25 year part & labor warranty on their optimizer, but still realistically just 12 year warranty on their central inverters.

This would also explain why Enphase doesn't want to cover labor in their warranty, because they know their microinverters won't last the 25 year, so they only cover the part, which is cheap, but not the labor, which will be the bulk of the service cost.

[SunEagle]

yes. I'm not sure if the failure rates are lower comparing with Enphase's build in micro inverters. Even with partial shading issues, I'll still pick the central inverter with dual MPP tracker like SMA TL-US-22 series. Seriously, if you have lots shadow, micros wont help too.



This is the best method for shading:

And what did your neighbor have to say when you cut down his tree?

[billvon]

Not true. When the Enphase microinverter fails, you lose 250W of power (or whatever the panel is). When the SolarEdge optimizer fails, you don't lose 250W of power, nor does the whole string go down.

Why do you assume that? There are four FETs in the optimizer, operating in buck-boost configuration. Those are the power devices. If they fail, the device goes open circuit (if they fail open) or the panel is shorted (if they fail closed.) In both cases, in a multi-string configuration, you lose the string. In a single string case you lose only one panel.

Both microinverter or central inverter need electrolytic capacitors to manage large input capacitance, but electrolytic capacitors contain fluid that can dry out over time. This is the main reason why inverters don't last longer than 10-15 years. But SolarEdge optimizer doesn't need to use electrolytic capacitors. It can use ceramic capacitors which can last much longer.

Agreed there. Over time electrolytics degrade and EMI / harmonic content goes up.

On top of that, much of SolarEdge's optimizer is integrated into an ASIC, eliminating the need to use discrete components -> better reliability.

True of microinverters as well. But the failure mode in power electronic is almost never the controller - it is power devices, or electrolytic capacitors, or magnetics failing due to shock.

[Volusiano]

Why do you assume that? There are four FETs in the optimizer, operating in buck-boost configuration. Those are the power devices. If they fail, the device goes open circuit (if they fail open) or the panel is shorted (if they fail closed.) In both cases, in a multi-string configuration, you lose the string. In a single string case you lose only one panel.

I remember reading about it from this source but of course it's not an official SolarEdge source so we can't believe everything they say. But it seems plausible that SolarEdge would do something in their design such that a failure would cause the optimizer to be bypassed altogether instead of being in the critical path of the panel's operation. That's why I chose to believe it.

But if the device fails closed and the panel is shorted, why would you lose the whole string in that case? Wouldn't you just have 1 less panel in the string and the other optimizers in the other panels would make adjustment on their DC output to compensate for the loss in voltage of the failed optimizer?

If the device fails open then I can see losing the whole string. But if I were SolarEdge, if there's a fail open situation, I would try to put something to either put it in bypass mode or in fail closed mode as to not disrupt the whole string. It just seems like it'd be something they would be very remissed to not have considered it up front and have some kind of fail-safe design built-in to minimize system disruption.

And I agree that microinverters can be integrated into ASIC as much as optimizers for better reliability. So the reliability differentiation is not in the ASIC controller, but in the power devices like the electrolytic capacitors.

I heard that Enphase claims that they've figured out how to get rid of electrolytic capacitors in their design to eliminate this weak point. That's fine and dandy, but what really matters is that they should update their warranty now to include labor if that's the case. They still haven't.

[billvon]

I remember reading about it from this source but of course it's not an official SolarEdge source so we can't believe everything they say. But it seems plausible that SolarEdge would do something in their design such that a failure would cause the optimizer to be bypassed altogether instead of being in the critical path of the panel's operation.

It does seem plausible, and it would probably increase sales if they claimed that whether that feature worked or not. But I have never seen a failed optimizer and I have only seen one failed microinverter (and it was a very old one, and it had been abused.)

On the other hand, the parts that TI sells to implement optimizers (those ASICs you mentioned) has a recommended design associated with it, and it does not "fail safe." Perhaps there is additional circuitry inside the optimizers that does that; I would be curious to see if there is.

But if the device fails closed and the panel is shorted, why would you lose the whole string in that case? Wouldn't you just have 1 less panel in the string and the other optimizers in the other panels would make adjustment on their DC output to compensate for the loss in voltage of the failed optimizer?

Good question. If they have a far enough power-point adjust range I could see them being able to compensate for the loss of one panel.

[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.