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Micro-inverters: Are They Worth the Cost?


  08:10:00 am, by Jim Jenal - Founder & CEO   , 400 words  
Categories: All About Solar Power, Solar Economics, Residential Solar

Micro-inverters: Are They Worth the Cost?

Last June we reported some preliminary performance data for three of our installed systems that use Enphase Micro-Inverters to try and answer the question, Do Micro-Inverters Really Make a Difference?  That continues to be a popular post so we wanted to update our results to show you a full year’s worth of data - and the answer is still YES!

We started with the output data from the CSI calculator to estimate what the monthly yield would be for each of these three systems. After all, that is the estimate that the utility is relying upon to determine your rebate so we consider it a reasonable baseline estimate. However, the CSI calculator does not take into consideration any performance improvement factor for micro-inverters.

Year's worth of Enphase micro-inverter performance data for three Run on Sun solar power systems

We then took the daily output for each system - easily available thanks to the Enphase Enlighten website - and compared monthly totals to the CSI predictions.  The results are summarized in the table above.  All together, the three systems experienced a combined yield of 15.6% above the CSI predictions.  The individual systems ranged from an annual increase of 5.1% to a whopping 28.5%.  All three systems had individual months where the yield improvement exceeded that overall yield, whereas two systems experienced months where yield was below predictions - mostly likely due to weather impacts.

We have also included the averaged shading percentage from the CSI/Solar Pathfinder data and it is interesting to note that the highest yield improvement did not correspond to the system with the greatest degree of shading.  (However, the system with the least shading did have the smallest yield increase which would be expected.)  Why would that be if shading is what drives the performance improvements?  We believe that the difference is in the type of shading.  System 2, even though it has the highest degree of shading, has long periods of time when no portion of the array is shaded at all.  Thus its performance during those periods is comparable to what would be seen with a string inverter, and it therefore experiences a more modest - but still robust - yield improvement.

The bottom line here is that for a relatively modest increase in price over that of a string-inverter system, these three systems show substantial, real-world performance improvements thanks to using micro-inverters.  When we do a solar evaluation at your home, we will be able to let you know which type of system makes the most sense for your site.



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Comment from: bill rossi  
bill rossi
5 stars
Hi Jim, Thanks for the post. I’m curious as to your thoughts why system 1 performed so dramatically better that estimated at 28.5%. Any ideas? Bill
06/29/11 @ 09:26
Comment from: Jim Jenal - Founder & CEO
Hi Bill - It turns out that the layout for that system is highly irregular in shape. As a result, the 4-corner shading analysis - which assumes a more conventional arrangement - may be skewed to suggest more shading than the system actually sees, particularly one corner which is heavily shaded. So the system probably performs much better than that corner evaluation predicts. It is also possible that we have some other factors at work here - we are using Sanyo panels and the majority of them may well be performing better than their nameplate rating would predict. Bottom line - the client is *very* happy! Thanks for reading and taking the time to comment. Jim
06/29/11 @ 09:42
Comment from: John  
Jim, Thanks for the analysis. We use both Enphase and string inverters on our installs and I too have been very curious on the performance advantages of the microinverters. While I wholeheartedly agree that micros should improve performance where there are shading issues, basing a study in this area on the estimated performance from a Solar Pathfinder analysis leaves a lot of room for error. With only one exception, every system we have installed (micro or string inverter) has out-performed the Pathfinder estimate. Certainly, better than normal weather may be the reason. Perhaps a better guage of their performance improvement might be a side-by-side comparison of a partially shaded system using micros vs. the same system using a string inverter. This would be a costly and time consuming endeavor, and I haven’t attempted it yet. However, one test that I currently have running is a side-by-side comparison of identical systems that have minimal early am and late pm shading. Both systems use (22) 220W modules. One uses the Enphase M190s, while the other uses an SMA SB5000. Enphase claims that their inverter will outperform a string inverter by about 5% even without shading. I can say without hesitation that this has NOT been the case in my test. The SMA has consistently out-performed the Enphase in energy ouput. I have verified this with actual current/voltage measurements (sorry, don’t have the numbers here at the office). The only expeption was early or late in the day when the system was outputting about 200W or less; in this case, the Enphase did better. A long conversation with Ephase about my results didn’t get very far. Nonetheless, I still use them for installations where I think they are appropriate and will be installing thier new M215 in the next week or so. And yes, I love the Enlighten web monitoring! -John
07/01/11 @ 07:02
Comment from: Jim Jenal - Founder & CEO
Hi John, thanks for taking the time to comment. The estimates that we used actually came from the California Solar Initiative’s EPBB rebate calculator which relies on our Pathfinder results for shading and then uses PVWatts on the back-end for its calculations. My point was that since this is what every rebate calculation in California is relying upon, we thought that it made for a fair benchmark. I agree with you that a real-world, side-by-side comparison would be the best - but no, I haven’t had an opportunity to run that experiment either! (Although I seem to recall that Enphase has. Don’t know whether SMA has done so - if they have, I haven’t heard about it.) In any event, Enphase claims a greater energy yield, not higher instantaneous power (which is what your current/voltage measurements must yield). Have you had a chance to compare energy production day over day, and month over month? That might provide more informative results. Still, I would love to see your numbers - please post them if you have a chance. Best regards… Jim
07/01/11 @ 07:21
Comment from: Gary  
Anyone have a pbetter approach when using pathfinder to predict the output of an enphase installation, than taking a reading at each inveter location. It seems the standard approach of using the 4 corners of the array would not do this predict the correct output.
08/08/11 @ 04:39
Comment from: Jim Jenal - Founder & CEO
Gary - I have not seen anyone offering a tool (including Enphase) that they claim will give a better performance match. No one is going to do a pathfinder at each inverter location because to make that work, you would really need to calculate the rebate amount at each location with its own set of shade values. (Hence, a 24 panel install becomes a 24-sub-array install for the purpose of the rebate.) What is needed is a better modeling approach that accounts for the actual array configuration and the overall shading.
08/08/11 @ 06:48
Comment from: Mike T.
Mike T.
2 stars
How about providing test data for a side-by-side comparison of a micro-inverter and non-micro inverter system (e.g. install the two systems side-by-side)? This is the only way to get any meaningful evaluation data. You opinion in this article is based on calculated output data, which may be masking the fact that array output is always initially higher during the burn-in period, and site conditions typically vary widely from CSI generated data.
01/10/12 @ 09:00
Comment from: Jim Jenal - Founder & CEO
Hi Mike - If you have a site where we could do such a side-by-side installation, we would be happy to install two systems for you! Joking aside, I recall seeing data from Enphase early on where they in fact did just that - a micro-inverter system and a string inverter system on the same roof. Not surprisingly, the micro-inverter system outperformed the string inverter system. I don’t think, however, that the only meaningful evaluation would come from such a comparison. The site conditions are included in the CSI rebate calculator using location (zip code), azimuth, pitch, shading, panels used, height above roof and inverters used. As I noted in the article, the output from that calculator is the basis for rebate payments throughout the state (and possibly elsewhere) so it is at least a reasonable baseline. You are correct that solar panels often display higher output power during the initial burn-in period, but that period is at most a few months, not an entire year. Each of the systems that were monitored for this analysis had been operating for months *before* the 12-month segment of data that we reported. So I don’t think the burn-in period contributed to these results.
01/11/12 @ 08:47

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Jim Jenal is the Founder & CEO of Run on Sun, Pasadena's premier installer and integrator of top-of-the-line solar power installations.
Run on Sun also offers solar consulting services, working with consumers, utilities, and municipalities to help them make solar power affordable and reliable.

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