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Optimizing optimizers, Tigo vs. SolarEdge August 1, 2013

Posted by Maury Markowitz in solar.
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I’ve touched on inverter concepts in the past, but I’ve only talked about “optimizers” in passing. I see lots of questions about these on the ‘net, so I think it’s time for a little intro to the two market leaders in this space. Those happen to be Tigo, the incumbent here in North America, and SolarEdge, a powerhouse in Europe. They both take different approaches to the task, each with its own ups and downs, and I’ll try to cover these in this article.

The basics of panels

Solar panels consist of a bunch of individual solar cells wired in series. Each of those cells behaves like two different electrical devices at the same time, a battery-like device that’s putting out DC power at 0.5 volts, and a very small of resistor that’s sucking some of that power back up. Each of these processes, production and resistance, is dependant on the amount of light.

In bright sunlight the battery is putting out much more than the resistor is sucking, so cells produce net output power. But in shading, or at night, the resistor side of things overwhelms the production – not entirely surprising.

When you wire things in series, they “add up” the voltages and resistances. So when you wire all of these cells together each of those cell’s 0.5 V gets added together. Modern panels normally have 60 cells, so they operate around 30V (actually closer to 35V) in normal conditions. But the resistances also add up too.

When you build a solar power system you normally have multiple panels wired together. The plugs on the panels are designed to be connected in series, so each time you add a panel the voltage goes up – 35V with a single panel, 70V with two, and so on. The cables and connectors are normally limited to either 600V or 1000V depending on where you are, so it’s very common to see strings of 12 to 20 panels. When you wire like this, it’s like you have a single bigger panel, with 720 cells instead of 60.

So here’s the problem… imagine you have a string of panels and the one on the left gets shaded by a tree. So that panel is putting out less power, but it’s also got increased resistance. And since every electron from every cell has to go through every panel on it’s way out to the world, that single panel is sucking up power from the entire array.

This is bad.

One solution would be to wire the panels in parallel, like modern Christmas tree lights, the ones that don’t go out when one bulb fails. In this case, if a panel was shaded, or broke, it wouldn’t have any effect on the string as a whole – the power from the other panels would “go around” that panel, not through it.

But this runs smack into a very practical problem. When you wire in series the voltage goes up and the amps stay the same, but when you wire in parallel the volts stay the same and the amps go up. Any wire is limited by the amperage, or current, it can carry. The 14 gauge wires in your home are limited to about 20 amps, so we put 15 amp breakers on them to be safe. A typical panel might put out 6 to 8 amps, so with that wire you could only put two panels together. If you want to collect the power from a string of panels in parallel, you need honking big cables. And copper is expensive these days.

So they don’t do that. We wire in series and live with the downsides.

The basics of inverters

Every “grid interactive” PV system looks the same when viewed from far enough – there’s panels, an inverter, and the grid. When I visualize these things I always imagine systems with panels on the left and the grid on the right, and the inverters sitting between them in the middle. Most of the heavy lifting is in the inverter. Lots of magic has to happen in the middle.

Basically, every inverter system has three parts. On one side (the left) are the panels that are connected to the inverter. In order to get the most power out of them,  the inverter has to use a system called “Maximum Power Point Tracking”, or MPPT. After that is a DC-to-DC converter that takes the output from the panels and turns it into a fixed output voltage, and finally there’s the DC-to-AC converter, the only part rightfully called an inverter, that takes the fixed DC output and turns it into AC power for the grid.

There’s no reason that all three parts have to be in a single box, although most systems do just that. Actually, there is a very strong argument for breaking them up.

Breaking up is easy to do

Recall that the output from a panel changes with the light. That’s the whole idea of the MPPT, to pull out the maximum amount of power for any given conditions. And since the lighting changes over time, we keep changing the MPPT settings as fast as we can, ideally hundreds of times a second.

But those conditions don’t just change over time, they also change from panel to panel. Imagine that one panel in a string is shaded, so its resistance is higher and its voltage is lower than the rest of the panels. The inverter at the end of the string sees the whole string as a single big panel, so it adjusts it’s MPPT to suit the system as a whole. But what that really means is that all of the panels are no longer on their MPPT point… the shaded panel is being asked to operate “too high” and the rest are being asked to operate “too low”.

So ideally, we’d want to put an MPPT on every panel – or every cell if that was possible.

This is the whole idea of the micro-inverter. By putting individual inverters on individual panels, you isolate each panel and get individual MPPT. And since you’re boosting the voltage from 30 to 240V, the wire losses aren’t such a problem, so it’s OK to use parallel wiring. Now if one panel fails, no big deal.

But think about it for a second… if the part we want on the panel is the MPPT, why put the entire inverter on the panel? Why not just the MPPT itself?

And that is the optimizer concept.

Tigo’s solution

Tigo MMJ

Tigo Energy of California is one of the big players in the optimizer market. They make a very small box that clamps to the frame of the panel and optimizes that panel. Actually their latest versions have two of these optimizers in a single box, which reduces the relative cost of the box – for a $95 item that’s a serious consideration. Using Tigo’s couldn’t be easier, you simply connect the panels to the Tigos, and then wire the panels together like any other array. The output from the strings is fed into any normal string inverter.

The Tigo uses a very clever concept known as “impedance matching“, which helps improve the collection of power from the array as a whole. This may sound counter-intuitive, but I’ll try my best here…

Imagine a string of panels with one shaded one in the middle. That panel not only produces less power, but also slows down the flow of current from all of the panels. So now the inverter at the end sees a certain voltage and current, and adjusts the MPPT trying to get those two values to their maximum. The problem is that with that extra resistance in the circuit, the point it will select will be wrong for every panel – too “high” for the shaded panel, and too “low” for the unshaded ones.

In many cases you can improve the total collection by turning that panel off entirely. Now the inverter’s MPPT sees only the good panels, and tunes them properly. But what happens if you have two shaded panels, or four? At some point you can’t just turn them off any more, the losses will be too great.

So what Tigos do is add more resistance to the circuit… that’s right, more. But they add this in parallel to the individual panels. When you do this, you end up with two paths for the power to take, one through the panel and one around it. Now you might think that you just want to add zero resistance in the around path, but that’s not right – at a minimum you want to add the resistance that the panel would have when it’s operating perfectly, small but not zero. But you can actually tune this even further, if you carefully control the resistance at every panel you can maximize the amount of power being taken from the array as a whole.

The amazing thing about all of this is that the Tigo does all of this with nothing more than some clever programmable inductors. There’s very little circuity in the panel-side boxes.

Since the system has to work at the string level, someone has to be in overall control. Tigo has another box, the MMU, which can see all of the optimizers and then send them instructions on what to do. They do this using a wireless connection, through a box known as the Gateway. Is this starting to sound a little more complex now? Another issue is that the wireless connection isn’t exactly very high performance, so the Tigo’s are only sent updates on what they should be doing every so often. This makes the system unable to deal with individual clouds or similar temporary effects.

Nor does Tigo help with the Christmas Light Effect – if one of the panels goes down the Tigo can cut it out of the string, but if one of the Tigos goes down… the string is down.

SolarEdge’s solution

SolarEdge P300

SolarEdge P300

SolarEdge is out of Israel, and while they’re not as well known as Tigo here in North America, they’ve been very successful in Europe.

Like the Tigo, the heart of the SolarEdge system is a small box that goes with the panel, but their box is slightly larger and heavier than the Tigo, so it clips to the mounting rails rather than the panel frame. Mechanically, that’s about the only difference.

Electrically, they couldn’t be more different.

The idea behind SolarEdge is to split the traditional inverter into two parts. One is the DC-to-AC stage which they put in a box that goes on your wall. The other two stages, the MPPT and DC-to-DC, they put on the roof. So it’s basically 2/3rds of a micro-inverter.

Now how does this improve things over a normal micro-inverter?  I touched on this in a previous article – basically the hard part of  inversion is the last stage, DC-to-AC, because it needs all sorts of energy storage in the form of capacitors. So if you leave that out of the rooftop portion of the system, you’re left with the MPPT and DC-to-DC, which is a lot smaller and more reliable. Meanwhile the “bad” part, the DC-to-AC with all its capacitors, goes on the wall, where it’s easy to service. This also provides a convenient place for all the associated bits and pieces, like monitoring and communications.

SolarEdge boxes are true MPPTs, so each panel is individually tuned to it’s MPPT point, all the time. The system then controls the output so that the string as a whole is always outputting 350Vdc. This simplifies the inverter at the end of the string, because it’s always doing 350Vdc to 240Vac (or 220/230 in Europe, of course).

Fight!

So which approach is better? In pure theoretical terms, the SolarEdge is going to get more power out of the system, it’s running MPPT on every panel rather than the array as a whole. But in practical terms, they get this by adding more circuitry on the roof, so the cost should be higher for the system as whole. They can offset this slightly because they don’t have a full string inverter at the end, their inverter is greatly simplified because it doesn’t have the MPPT and DC-to-DC stages. Tigo uses a normal, complete, inverter.

Tigo actually dismisses the SolarEdge approach in their white paper. If you look on page 5, the second section is saying that if you put a DC-to-DC on the roof, like SolarEdge, you’ll lose about 2 to 3% of the power. But read it carefully… they’re talking about an additional stage. SolarEdge removes the DC-to-DC in the inverter so there’s no “additional”.  From the numbers I’ve seen, SolarEdge trashes Tigo-based systems in overall production and efficiency terms.

And here’s the thing… in every scenario I’ve run, the SolarEdge system is also cheaper. Here, try going to this site and using the search bar a bit. Let’s say you’re building a 10 kW system with 40 panels and two 5 kW inverters.

Ok, two Power-One 5k’s are 2x$2222.33 = $4,444.66 = 44 cents a watt. Now add 20 Tigo dualies, 20x$92.40 = $1,848 and a Gateway/MMU kit at $335.89 = $336 + $1,848 + $4,445 = $ 6,819, or 68 cents a watt.

Ok, now compare that to SolarEdge, where I need two 5k inverters at 2×1,608.41 = $3216.82 and the optimizers at 40x$71.63= $2,865.20. $2,865 + $3,216 = $6,081, or 61 cents a watt.

I’ve tried all sorts of scenarios large and small, and SolarEdge always seems to come out on top, either because of the free monitoring, or the low cost of their simplified inverters. And I should point out, those are retail prices.

So it seems that Tigo remains an excellent solution where the inverter already exists, like adding on to an existing array or when you’re using central inverters. But for all of the small and medium sized systems that are going up fresh, if module-level optimization is a requirement, SolarEdge seems like the way to go.

Update: I feel I should also point out that in spite of all the advantages of optimizers versus microinverters, micros still have one enormous advantage. In any optimizer system the output eventually runs into an inverter, and that inverter needs to be sized to the array as a whole. That means you need to have all sorts of different inverters to hit different system sizes. In comparison, a single micro can be used on any system, from a single panel into the megawatts. This design-time simplification should not be underestimated. That said, the PV market is ridiculously price sensitive (to its detriment, IMHO), and it seems the lower costs of the optimizer approach will win many designs in spite of any theoretical disadvantage.

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Comments»

1. Mark - September 27, 2013

Great explanation and comparison that answered all remaining questions I had and agreed with all of the conclusions I had drawn before reading. Both are sharp products in their own right, and hopefully the competition between the two will only make them better.

2. Andy Blunt - October 30, 2013

brilliant explanation. Solar Edge kicks arse!

3. Jon - November 7, 2013

This is interesting… however, we’ve made a strategic shift towards Tigo for reasons not mentioned here. Primarily, what if Solar Edge goes under, what will you do when the inverter fails? Take all the panels off the roof and reinstall them? I thought they were addressing this by making an inverter agnostic implementation of their maximizers, but I’ve since been told that those plans have been scrapped. System owners and installers need to consider this aspect carefully. Are you willing to bet that Solar Edge will survive the booms and busts of solar over the next decade plus? That’s a big bet…

Also, monitoring is now free with Tigo…

Lastly, in terms of the system performance my guess is it’s negligible in terms of the overall system harvest… probably less than .5 percent. I’d still like to see some hard data on that…

I definitely think that DC maximizers are the way to go… just not willing to make a big bet that Solar Edge is going survive 10+ years.

Maury Markowitz - November 7, 2013

“what if Solar Edge goes under, what will you do when the inverter fails”

Which is the big concern that everyone has with Enphase too, of course. One might also consider what happens if Tigo goes under and your comms box dies – same problem. But it’s not the same… the comms box is probably running 5 Watts, not 5000.

So I hear ya, but I’m not totally convinced you can be *really* vendor neutral if there’s anything up on the roof. There’s going to be a critical path somewhere. I agree that the critical path is likely better on Tigo, but not entirely.

I’d much prefer to have a system that “fails on” in any case, and none of these systems work that way.

“Also, monitoring is now free with Tigo…”

Is that the case? I know they were offering this as a special for some time, but I was under the impression the offer was over. If it’s not, let me know and I’ll put an update in the article.

“Lastly, in terms of the system performance my guess is it’s negligible in terms of the overall system harvest”

Yeah, this is one where we have to wait and see. But in theory -theory- the SE system should definitely pull out more power. I know that in the side-by-side Tigo vs. Enphase systems here in Ontario the Enphase ones win every time, sometimes by a whole lot. SE should be even (slightly) better than enphase (they don’t have to buck/boost as much). Whether or not it’s more or less than 5%? That I can’t say.

Ariel Corona - March 23, 2014

If the inverter goes down and Power Edge does not exist, you can buy any inverter from another manufacturer and disable the MPPT system. The solarEdge optimizer will keep working. (

Maury Markowitz - March 23, 2014

I don’t think that is the case. PE systems normally go to 1V if they don’t have communications with the inverter. I know they were making changes so they could use their optimizers with other people’s inverters, but I believe that took the form of a separate comms/control box (but I’m not positive).

4. Jon - November 7, 2013

One of the key aspects of MTBF is part count… that’s where optimizers blow doors on micros. And I’m guessing that Tigo beats SE by some margin here as well, since the electronics are less complex. No one really knows how long this stuff is going to last… but I’m far more inclined to believe estimates of 100+ year MTBF estimates from a device that has 1/10 the part count (and less failure prone parts) than say micros. I supposed the same argument is true with Tigo vs SE… only to a much lesser degree.

One thing about SE… because the communication is integrated, the installation is easier than having to run a separate 485 cable to your array… to the unit that wirelessly talks to the panels. I’m told Tigo decided to go with wireless communication because it was more robust than “dirty power” line communication. Honestly, I find that hard to believe… but I see no reason why they would opt for wireless communication if it wasn’t.

Lastly, Tigo’s basic monitoring is now free for the residential arrays.

So bottom line (for my firm at least)… it’s a little more costly to go with Tigo / [pick your favorite inverter] but I think it’s the best balance of distributed electronic benefits, flexibility and potential longevity.

5. kelley - March 28, 2014

Would Tigo or SolarEdge optimizer’s be beneficial to an off grid setup?

Maury Markowitz - March 28, 2014

Oh, good question!

Well in theory the answer would be “yes”. I’d say especially so because many off-grid situations have partial blockages, and the optimizers really help there.

But the flip side is that off-grid systems also tend to be smaller. With smaller systems you can often arrange the panel layout to avoid problems, a typical cottage might have only six panels, and I’d bet you can find a spot *somewhere* for six panels all facing the same way.

This is especially true because off-grid systems also have short strings, often only three or four panels, so the grouping is much smaller. In comparison, an grid-tie system normally has 12 to 20 panels in a string, so its much harder to physically arrange them so none of them are ever shaded.

So I guess the answer is “it depends”. For a system with 12 or more panels I think I would seriously consider Tigos.

6. Ashish - April 10, 2014

Maury.. can SE distributed architecture be used for Utility scale (say 3MW) .. If so, how would the AC collection architecture look like ?

Maury Markowitz - April 10, 2014

Another great question.

I can’t say for sure what a large SE system would look like, as I’m unaware of any deployments at this scale. I can make a reasonable guess though, based on the fact that the optimizers require power line comms, and those generally support tens to hundreds of systems, not thousands. Another limitation is the inverters themselves, which are currently limited t 20kW in three-phase models, although it might be possible to use the new models with someone else’s inverter, although I am not really too familiar with this option.

Thus I would expect an SE-based system of this size to look somewhat like an Enphase system of the same scale; patches of about 80 panels fed to a single point, and then recombiners gathering up the AC for the grid connection. The main difference, it would seem, would be the number of junctions; a single Envoy can work with up to 250 M250s, but the 12-gauge (?) wiring in the inverter strings limits you to perhaps 20 per string. With SE you’ll likely be using 10-gauge running at 350V, which theoretically offers more optimizers per string, but we’re limited by the inverter capacity anyway. So perhaps a few less junction boxes in the array, but certainly not a major difference.

This does contrast strongly with the Tigo solution, which can be used with a central inverter system. In that case one might use something like the Santerno 250’s and connect 1000 panels to it. That doesn’t eliminate the complexity through, it simply moves it from the AC side to the DC side, gaining you a little in terms of wire use because you can bump up the voltage closer to 600V (or if you’re allowed, 1000). Tigo also relies on a comms system, and in this case the complexity is actually increased; their wireless system has to be deployed separately, close to the optimizers, and only controls about 50 of them. So lots of the repeaters have to be embedded into the array, and cabled back to the monitoring central point.

I suspect that in terms of complexity and design, the simplest system would be Enphase – you have one set of cables and monitoring boxes for every 250 panels. However, that comes at the cost of greatly increased amounts of wire. The Tigo is likely the most complex in terms of topology, as you have two entirely separate systems to install. But I would strongly suspect that it would also be the least costly in terms of wiring.

7. Ashish - April 10, 2014

great insights.. follow up question .. For SE distributed architecture for 3MW plant, power line communications will be between all panels on 2 strings (total about 100 panels) and the inverter that these 2 strings are connected to .. would the limitation that you mention really come into play ?

Maury Markowitz - April 10, 2014

No, not really, the inverters would be the limiting factor. This would only come into play with larger inverters like 100 kW and up, but as I mentioned, I’m not entirely familiar with SE’s ability to be used in these situations. I know it’s possible – that’s it.

8. Ashish - April 10, 2014

True.. can’t quite find much on the web about SolarEdge use of module level optimization based distributed architecture for utility scale.. Purely from thinking it out.. the challenge seemed to be on the MV side .. connecting 200+ inverters to step up transformer(s) to grid connection point..

Maury Markowitz - April 10, 2014

Agree completely. That said, Enphase did pull that off on their 2.4MW install here in Ontario.

9. Darius - June 2, 2014

Have tried Tigo optimizers. String with Tigo so called optimizers produced 5 -6 prc. less than string without Tigo optimizers all other things being equal. All evidence were presented to Tigo company. The answer was that my system is too good and in the future my panels will de-gradate and then optimizers will start to work. Even from theoretical point of view there is nothing can be gained with those optimizers.

10. Antony Tersol - June 6, 2014

Another benefit to the Tigo optimizer (at least when integrated in the SmartModule available from some PV module manufacturers, such as SilFab in Ontario), is that the Voc coming out of the optimizer is capped (at 6% over Vmp). This means the Voc will not increase at low temperatures, allowing more modules per string, decreasing wiring and BOS costs for larger arrays. An added benefit is higher overall string Vmp, and lower Imp for given array size, lowering resistive losses and allowing smaller wire size.

Maury Markowitz - June 6, 2014

Antony, this is interesting. This seems to imply that a “smart module” is more than just a module and a Tigo glued together. Do you have pointers to the tech side of this so I could take a look?

Richard - July 21, 2014

Hey Maury! Long time no speak! Glad to see you’re still kicking around…

From what I do know, the smart module is a bit more than a module and Tigo glued together. Since the Tigo functionality is embedded in the junction box, it can do voltage limiting because it knows exactly which panel it is paired with and for ESA purposes, they can provide a data sheet showing a specific maximum voltage. For example, a Trinasmart 60-cell module looks to be limit voltage at around 32.5V, which is temperature independent.

http://www.trinasmart.com/Events/pdf/Designing_Installing_Trinasmart_Systems.pdf
http://www.tigoenergy.com/sites/default/files/smart-curve_application_note_v1.3.4.pdf

Cheers,

Rich

Maury Markowitz - July 21, 2014

Ahh, that was the part I was missing. I was under the impression it was nothing more than a Tigo installed on the frame in the factory. Probably also a minor savings/improvement if they can eliminate the back feed diodes, but I’m not sure that they can.

11. Adam Schoonmaker - July 26, 2014

Thank you for this informative article. I have installed several SolarEdge systems, and have found them to be high quality. I generally prefer them over micros, because as mentioned, the solar market is price sensitive. They seem to be competing head to head in terms of performance with the Enphase systems we have installed and eliminate the problem of clipping during peak production. Those of us that are technically proficient understand that clipping is a relatively small consideration, but it is very hard to explain to a customer that they are getting a 275W module and a 250W inverter, and that it is somehow a good thing.

In terms of “bankability” on the future of the Solaredge after the solar bubble has burst, I believe Solaredge is as bankable as any other inverter manufacturer. I believe that if there is a solar bubble, that Enphase and Solaredge are two companies that will ride the wave well, since their products have set them apart from most others. I would argue that there will not be a bubble for the solar industry as a whole, but rather for companies that have grown too big and too fast. Solar is trendy right now, and the growth has been staggering. However, as power rates continue to increase and public demand for non-fossil-fuel power sources follows suit, the amount of households with solar will continue to increase. This means that demand may plateau, or even fall, but that does not mean that demand will disappear. I can only foresee one cause of death for the solar market: policy change. If the power companies start to gripe that they are being forced to administer power, but are unable to sell significant amounts of power to the general public, then they will want a way to cover their overhead and maintenance costs. We have seen the beginning of that slippery slope in Utah: Rocky Mountain Power will soon implement a net metering fee to “cover the fixed costs of providing electricity.” Obviously as more people adopt solar into their homes, the power companies’ variable revenue will decrease but their fixed costs will remain the same. If the fixed fees for net meter connection become too high, we could see the solar market landscape change very quickly. There are only a few solutions to this: 1. pony up an pay what the power companies’ demand, and 2. have the government seize the power companies and make their access public (like roads), or 3. make the fixed fees illegal. Those are the three options I can see, none of which are very palatable. The first would erode the strength of the solar market, and possibly destroy it. the second would erode the very foundation of our economy, possibly setting a very serious precedent. We could also find ourselves with a deteriorating infrastructure, subject to budget committees and political backlash. The third would cause the power companies to lose money, causing power rates to skyrocket, and probably creating deteriorating conditions for the power grid, eventual bankruptcies of the power companies and bailouts or government seizure anyways. We see the same problem with tax revenue, since if you are not buying the power you are certainly not paying the sales tax on it. This may ultimately becoming an issue subject to political abuse and could be blamed for low tax revenue.

Sorry to get all political on you. I recognize this is a page comparing the Tigo and Solaredge approach to optimizers. I started out writing about my experience with SE, and I will finish with this: the SolarEdge products I have installed have excellent initial quality, and according to theory, should outlast most solar modules “useful years.” My experience with SolarEdge as a company has been excellent and they have a very nice looking monitoring portal. I have all but stopped selling micros, and usually only do if the customer is specifically asking for them or hates the idea of having an inverter mounted on the wall somewhere. Solaredge recently boasted that they now have 4 million optimizers in place, meaning that they have moved some good revenue and are growing well.

Maury Markowitz - July 29, 2014

Wow Adam, great post. I’m glad to see my posts can invoke such a spirited response! And feel free to get political, everything is at some point, and if we don’t talk out the politics then the politicians will do it for us. That never ends well.

12. Rob - August 31, 2014

Thanks for all the informative info above guys… Maury it will be interesting to hear your (or anyone else’s) opinion on Tigo v Solar Edge, when considering adding a battery manager & storage to a grid tied system at a later date. My understanding is that this can be easily done when added to a Tigo/SMA type combo, but not so sure of the potential limitations or pitfalls when using Solar Edge.

cheers Rob

Maury Markowitz - August 31, 2014

Rob, this is a topic near and dear to my heart.

Certainly the Tigo kit should work perfectly with existing DC charge controllers (CC) like the Outback FX kit or similar. All these really do is fine-tune what’s already there on the DC side, so the CC shouldn’t even really know anything is going on.

But of course that only gets you so much, you’re not getting MPPT on every panel. SolarEdge gets you that, so it seems like a very interesting idea. Even more importantly, existing CC’s have very tight voltage inputs, typically 150 to 200Vdc, which means you have to make little tiny strings, driving up all your costs. SE has a standard 350Vdc chain, and great flexibility in stringing arrangements.

So… why doesn’t SolarEdge make a box exactly like their existing inverters, but with DC output? The MPPT boxes wouldn’t change one iota, and the output the inverter already has a DC-to-DC converter (almost all of them do). Certainly the current of that stage would have to increase as it exports to 48Vdc instead of whatever it uses internally (probably around 250 to 350V) and that requires changes to the workings, but that’s just details. Oddly, this box would always be less expensive, because if the output is under 50V, the cost to get it ULed is *dramatically* lower, if not zero.

The other “problem” with existing off-grid solutions is that they generally connect through the battery. Since that’s at low voltage, you have to convert everything down to get to the battery pack, and then back up on the way out again. And of course the current is going up, so now you need big #2 wiring for everything.

So what if your DC box had two sets of outputs, one for the battery at 48V and another at 350Vdc that exports any excess. Now you take that 350V output and plug it into any SE AC inverter, which, for all it knows, is being fed the end of a string from the roof. Now you can wire to the inverter with 10 gauge. Sure, you end up with two inverters, but SE kit is relatively inexpensive anyway, I strongly suspect overall costs would be extremely competitive.

The idea seems so good that either I’m missing something obvious, or there’s simply no market for it. I’ll leave it to you to decide which!

Harald EbelingHarald Ebeling - January 20, 2015

Just stumbled across your excellent blog, Maury.

I am puzzled that an SE setup for hybrid / off-grid solar does not seem to exist. No market? Not a huge one just yet perhaps, but as the rates charged by utility companies increase (including substantial monthly or one-time charges for roof-top solar installations), adding battery backup to reduce dependence on the grid and ultimately transiting to an off-grid system might become more attractive to grid-tied customers.

I see a fair chance of this process beginning soon in Hawaii. At utility rates of 35c/kWh and more, residential solar installations are as popular with customers in HI as they are unpopular with HECO. The latter just having been acquired by NextEra, a Florida utility company with a demonstrated hostility toward distributed generation, approval of grid-tied systems is likely to become harder and / or more expensive to obtain than at present. Perhaps SE and others view Hawaii as a niche market not worth exploiting, but one could just as well consider it a perfect testbed for the development of a simple SE-based hybrid system like the one you described.

Optimizers may not be needed for traditional (small) off-grid system, but they make a lot of sense for 6-10kW roof-top installations that are currently grid-tied but may soon become uneconomic unless that tie is severed or battery storage is added to reduce dependence on the grid at night and peak-demand times during the day.

Are you aware of activities toward the creation of optimizer / charge controller / inverter combos that allow the inclusion of battery banks?

13. Jake B - October 1, 2014

But even after all these claims and anecdotal stories from everyone, SolarEdge’s own third party testing shows that they only beat out an older string inverter by 2% a year. Even if the SE devices do not prematurely fail, they will eventually wear out.

Leaving the string inverter out of it, is 2% extra energy a year really worth the extra costs of getting up that roof and swapping out 30-40(?) optimizers? Or am I missing something?

Maury Markowitz - October 1, 2014

Given the electronics involved, I strongly suspect that SE kit will last as long as the panels. Enphase and SMA I can’t say.

14. Keith - December 10, 2014

This was a great article, thanks! I found this article after searching around for an unbiased comparison of microinverters, and power optimizers (didn’t know Tigo and Solaredge were so different). I think you may have the best explanation out there (very prestigious internet award!). If there have been relevant developments would it be possible to get an update on this article?

Looking forward to looking around the rest of the blog.

15. Rog - January 12, 2015

I will share my experience with Tigo. I have a 10kw fixed ground mount installation. After the initial installation, I would review the daily production online, and I noticed the inverters cutting in and out. I contacted the inverter manufacturer, who admitted they were having issues. They sent me a replacement inverter so I could swap mine out, one at a time, and send them in for repair. I would have had no idea how often it was happening without the monitoring.
Secondly, I currently have one panel with a bad diode. The interesting thing here is, the VOC is normal. The diode only shorts when under load. That would be the next thing to impossible to find without some type of per panel monitoring, as a quick check with a voltmeter would show normal. I really have no idea which is better, SE or Tigo, to me, the important thing is installing some type of monitoring.

Jake B - January 12, 2015

Hmm, I think you still would have noticed it by looking at your utility bill? Regardless, if you use good quality inverters and modules, for the most part, most people have nothing to worry about.

Rog - January 13, 2015

Not a chance you’d notice. It’s a 52 panel array. One panel’s production is down one third because of the diode. You’d never notice that difference. Even the output displayed by both inverters has never matched, so you’d never notice it there either. It’s just not that accurate. One inverter has always displayed slightly less output than the other. I actually swapped them side for side, the inverter which displayed slightly less still did when hooked to the other string. As far as the cutting out goes, there was no reference point to relate to. The system was new, so without the monitoring, (or standing in front of the inverters for hours) I wouldn’t have known any difference.
Now, say, in a few years, several diodes have shorted so you suspect a problem. Since the VOC reads normal, how would you determine which panels are defective? A simple check with a multimeter would read normal.
I really don’t know if the maximizers help the output at all, and really don’t care. I didn’t buy them for that. I bought them to monitor the system for defects throughout my 20 year contract. For what I’ve found in less than 4years, I am happy. I would strongly recommend some type of monitoring for anyone considering solar. I know mine will pay for itself through the life of the contract, as I’m sure more issues will arise.

Jake B - January 16, 2015

52 modules = 52 optimizers to find a module with one bad diode that was only losing 1/3 of its power? If you spent about $100 each for those optimizers ($5200), what is the payback time for that power loss?

Considering the fact that you will have to go up there during the next 20 years to swap out 52 optimizers (how much is your truck roll cost? $100? $200?) that adds another $10,400 to your O&M costs.

IV tracers are used to find those problem modules, not a meter. Get a good baseline and you have something to start with if your modules go bad. Then again, if you are using good modules you don’t have to worry about it very much.

Rog - January 16, 2015

The optimizers cost me 48 ea, not 100. As I said, my system is a fixed ground mount. I can walk up behind the panels, zero truck roll costs. How much is your IV tracer worth? They are not cheap. That would be a pain to check panels yearly, and hope you don’t miss something. I can glance at a screen and see what’s going on. I used to do it often, now it’s just a couple of times a month for a quick review. I agree one bad diode is not serious, but how many more will go? My bet is you’d miss a few bad diodes, and just chalk it up to degradation, while I will certainly catch them all. My panels are Canadian made, by Solgate in Woodbridge, out of cells made in Germany. I didn’t buy Chinese stuff for that reason you infer. To each his own. If you don’t mind not knowing what’s going on with your system, all the power to you. At 80 cents per kilowatt, I want to make sure my system is performing optimally for the life of the contract.

Jake B - January 18, 2015

Ah, FIT 1.0? That changes the ROI perspective a bit. In that case, it really didnt make sense because your payments would offset what little problems ypu did have. Sounds like you are the home owner and not an installer, but the curve tracers pay for themselves very quickly. All serious companies in the industry use them.

Not picking a fight but many professionals agree that MLE are great for problem roofs but cost you energy on areas with little to no issues. Whatever, people think the monitoring is good but most can get just as good yields and ROI without it. Feed in tariffs do change things a bit though.

Have penciled out youpr ROI? An $0 truck rolls are not possible. Your just fooling yourself.

16. Grant E - February 6, 2015

Just some comments on the TIGO data reporting and availability.

With the basic membership which is free that you get when you buy a TIGO system you get monthly reports, 1 minute data granularity (delayed by at least 1/2 hour) for the past 30 days for viewing, safety alerts, environmental impact and trending charts. They say that with the basic you get full history, but it must not be at the 1 minute granularity and you cannot download the data. With a premium membership which costs $20/Year or $342 for 20 years. You get all the basic benefits, but also with daily reports, delayed 1 minute data granularity for the full history for viewing only, performance alerts, device integration, and the ability to download.

I have a TIGO system and signed up for the Premium membership and was having some system problems and had hoped to be able to use the TIGO data to help with diagnostics however I have found that the data is not real time it is delayed by at least 1/2 hour, sometimes more. I have also tried down loading the data to help get better granularity in the diagnostic, but the fastest cut rate that they have in the downloaded data is 1 hour. I have gone back and forth with TIGO Tech Support and they recognize this is what is happening they have no solution or know when or if it will be corrected. It turns out that the MMU only pushes the 1 minute cut rate data every 10 minutes, but then it needs to get processed on the TIGO server and then uploaded to your TIGO account so this is why they say that there is such a long delay. I have asked if there is some way to access the data directly off of the MMU so I can more actively try to diagnose my issues.

Grant E - February 7, 2015

Correction to my previous post. In the charts section they have added an Advanced Tab which will let you download data as fast as a 1 second cut rate, but still on a delayed basis. There is no real time data availability on the web. You can look at the signal strength on the MMU, but there is no way to connect to the MMU so it is a matter of cycling through each PV panel by pushing the button on the MMU.

Rog - February 8, 2015

I’m curious. What kind of diagnostics are you doing that you can’t figure the issue out by reviewing the data logged for that particular day?

Grant E - February 12, 2015

With time I probably could, but my roof is very broken up and with the weather, snow and clouds, it may take a while. If I could get more real time data it might help accelerate the process. Issues have been present since installed and it is taking a long time to resolve.

17. NetComrade - March 24, 2015

The SolarEdge manual has a section called “For installation with non-solaredge” inverters. Which (to me) means MPTT will work with other inverters (monitoring data will likely be lost though)

Maury Markowitz - March 24, 2015

Yes they have had this capability since the versions released in 2014, or maybe late 2013

18. Ed - April 16, 2015

Hello Maury,
The off-grid part of this discussions caught my attention and I’m thinking I can get some advice here. I live completely off-grid on solar power(not just to save money, even though I like to, but because I live in Nigeria and the utility is not only epileptic and unreliable but far from where I built my property). I have no shedding problems and my current generation of 12kwh/day max is able to sustain my moderate household.
Now I’ll love to add air conditioning to my loads and I don’t want them to wear down my batteries so I’m thinking: What if I install micro inverters, change my Indian inverter to an AC coupled like Magnum then since my load bus is where the battery inverter output and the micros will meet, I’ll be able to run ACs somewhat directly off the panels at the same time charging the batteries using the bidirectional feature of the Magnum inverter? I know this is doable though I’m finding it difficult to find 50Hz version of Enphase micros. But my question is, will I really gain more efficiency this way or I should just increase my panels and continue with the traditional MPPT controller and central inverter?

Maury Markowitz - April 16, 2015

Great question Ed.

If you have clear sun and all your panels are the same and facing the same direction, go with the charge controllers. If you have different panels, blockage or orientation, adding Tigo is probably something you might want to look at. I recall running the numbers with Outback and concluding that AC coupling wasn’t the way to go – although I can’t recall exactly how I came to this conclusion. I think it was due to charging limits (typically the inverter can produce more than it can charge) but that might be limited to just the Outback gear. Definitely talk to Magnum and see what they say.

I think the ultimate solution to this is with SolarEdge. I would like to see them make a box that attaches to the side of the existing inverters and handles charging. They could couple on the AC or DC side, I think the DC would be slightly more efficient (or at least cheaper, no caps). Then you would have all the advantages of the micro solution, and can tailor the AC and DC sides to whatever you need. Need 3k max at night, but 5k during the day? Get a 5k AC and a 3k DC, and hook them up to your 12k of panels.

Ed - April 16, 2015

You just spoke my mind with your last paragraph which unfortunately is a wish for now. I wish that someone will talk to SolarEdge to come up with something flexible enough to be customized! I hear of using their system with other inverters but its not so clear to me.
Now, what if I get Enphase micros, dispense with the AC coupled inverter route and instead feed a battery charger/rectifier from the same AC bus where the micros and the battery inverter output plus house loads meet? This way the separate charger does the charging of the batteries irrespective of what kind of inverter I use, whether AC coupled or not.

19. Ed - April 16, 2015

You just spoke my mind with your last paragraph which unfortunately is a wish for now. I wish that someone will talk to SolarEdge to come up with something flexible enough to be customized! I hear of using their system with other inverters but its not so clear to me.
Now, what if I get Enphase micros, dispense with the AC coupled inverter route and instead feed a battery charger/rectifier from the same AC bus where the micros and the battery inverter output plus house loads meet? This way the separate charger does the charging of the batteries irrespective of what kind of inverter I use, whether AC coupled or not.

Maury Markowitz - August 31, 2015

Indeed. The only problem is that the inverter has to provide a load that’s within acceptable limits for the anti-islanding parts of the inverters to be happy. I know that it works with Outback, but I couldn’t say for anyone else, I simply haven’t looked.

20. nativewolf - August 31, 2015

Just wondering what you think of the new inverter meant to be coupled with a powerwall. There is a fascinating discussion on the tesla user blogs about a user that is putting together a massive off grid system with Tesla battery packs and 106 sunpower panels (435watt e20s). Lots of folks discussing the need for just the sort of inverter you were mentioning.

21. David Ahn - September 12, 2015

Is it true that optimizers are only important with shading? If I have a section of roof that is partially shaded and another part that is not, can I just put optimizers on the shaded string and a simple string inverter on the unshaded string?

Maury Markowitz - September 12, 2015

In the case of Tigo, yes. Tigo modifies the output of the panel but otherwise leaves it similar to what it would be without the Tigo.

David Ahn - September 18, 2015

Thanks, Maury! So to clarify, even without shading issues I should get SolarEdge for my panels?

Sam - June 23, 2016

Question:
if only one or two panels in a string may be shaded, do I need optimizers on all the panels are on just those one or two?

Maury Markowitz - June 24, 2016

With Tigo you only need them on those panels. This is one of the big selling points for the Tigo kit. On the downside, since you need the monitoring system, there is a large startup cost that only goes away once you have maybe two dozen optimizers or such. But if you have this problem, definitely look into the Tigo gear.

22. Richy Thompson - October 31, 2016

What will happen to TIGO optimizer MM-2ES-75?
I have SunPower and BenQ 96 cell modules with Vtyp 57V and Vopen up to 67.9V.
The specs of the new TS4-R simple stops at 48V at module (Vmax 52V).
Does somebody know a solution or NEW TIGO products for those ‘high voltage’ panels?

Any comments on the new Cloud connect advanced?
Based on description it is open to build ‘into’ inverters….
Does this means the TIGO module data is available directly on the RS485 bus? Open data specifications?

Maury Markowitz - October 31, 2016

More generally, what’s going to happen to non-60-cell designs in general? I cannot imagine anyone is going to attack this market. I know SolarEdge still lists their 72-cell versions, but to be honest, I’ve yet to see a single one in the wild. I think that the 72- and 96-cell market is so tightly focused at the utility-scale installer that the micro’s don’t have a lot of market to sell into.


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