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Do Micro-Inverters Really Make a Difference? YES!
06/01/10
Do Micro-Inverters Really Make a Difference? YES!
In the residential solar marketplace there are two technology choices for inverters - conventional string inverters like those made by industry leader SMA - and micro-inverters - like those made by Enphase Energy. A solar PV system using a string inverter typically has only one inverter for the array and one or more series strings of PV panels are wired to the inverter. String inverters are large, typically heavy boxes that are mounted on the wall. For example, here is a picture of an SMA 4000US inverter mounted on the north wall of our customer’s garage:
(Interestingly, you see no conduit coming into or out of this inverter because we were able to bring all the conduit into the back of the disconnect through the garage wall, leaving a very clean installation.)
The string inverter requires all of the panels to be closely matched with preferably identical pitch and azimuth. The inverter handles a great deal of power which means that it gets hot. If exposed to summer sunlight, it gets even hotter. String inverters, like this one, typically have electric fans to provide additional cooling - but that creates another possible failure point for the inverter. String inverters come with ten year warranties.
The micro-inverter is a very different approach.
Micro-inverters, like these from Enphase, mount underneath the solar panels, so there is no large box to mount on a wall. Since there is one micro-inverter per solar panel there are no mismatches, portions of the array can have different orientations without difficulty, and most importantly, the problem of shading is greatly reduced. Those advantages come at a price - a micro-inverter system will typically cost 10-15% more than a string inverter system.
The question is - in the real world, is it worth that cost?
At Run on Sun we are starting to see some answers to that all-important question - and the answer is YES! We have been monitoring three of our Enphase installations and compiling our data to see how the actual energy produced tracks with the predictions that we made based on the CSI calculator and our Solar Pathfinder analyses at these sites. (In other words, the same prediction that the utilities make in calculating your rebate.) Here’s what we have seen so far:
For the three systems combined, we are seeing nearly a 17% improvement in energy yield overall - with a peak exceeding more than 50% improvement. While these results are still preliminary - after all, the oldest of these systems has only been in place for 8 months and none of them has yet gone through a summer season - we are very encouraged by our data so far.
Bottom line - if you need to maximize your system’s yield and/or are constrained at your site due to substantial shading issues, a micro-inverter system could well be the best choice for you. At Run on Sun, we can help you evaluate the trade-offs and whichever way you decide to go, provide you with the best possible solar PV system. Oh, and did I mention, all of our principals are NABCEP Certified Solar PV Installers?
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7 comments
This comparison is flawed. Predictions are based on average weather conditions, but the actual output may have been recorded at better-than-average conditions. 
Anton -You are certainly right that the actual output *could* be based on better-than-average insolation conditions. However, the results for June continue the trend that was reported in the post. Our June here in LA was very cloudy - the classic "June gloom" that every Angeleno has experienced. Nevertheless, our micro-inverter systems continued to outperform predictions, despite an at best "average" June, and quite possibly a below average month.
Please also note - we are acknowledging that we don't have a complete year's worth of data for any of these systems yet. Our results are admittedly preliminary, but clearly promising.
At the end of the day we have no dog in this hunt - our desire is to provide the best systems for our customers. If it turns out that micro-inverters cannot deliver on their promise, we will stop recommending them. But for now, they seem to be a great response to otherwise compromised sites.
Do you have 3 systems installed under the same conditions with a central inverter? What type of panel are you using? Are the voltage drops being acounted for? I am not against micro-inverters, just not so sure of the claims made. Though I love seeing individual panel production. I am seeing nearly identical numbers for systems using sma and fronius with Sunpower panels and proper heat management and array design. A dc combi switch with an AC contacter (sp?) On the roof also eliminates energized dc lines coming off the roof at a fraction of the cost and labor. Jesse - that is why I am comparing against the CSI model. It is assuming the exact same equipment, azimuth, tilt and shading issues. Yet we are seeing significantly better performance than what CSI predicts. We always account for voltage drop in sizing our conductors (which, as someone on the NABCEP list noted is a bigger concern w/micro-inverters than for DC returns which operate at higher voltages and lower currents). The panels in these three systems are all Sanyos - match up nicely against the Sunpower panels.
The difference as to safety is that the system is shutdown from the ground. And installing switchgear on a roof is expensive and largely useless (for residential systems) since never used. Way more labor to fabricate a vertical standard for mounting a disconnect on a pitched roof than it is to install micro-inverters.
Thank you for your quick responses. The back side of a chimney works great for the standard and my crew has ran all the lines and installed several of these switches in under 30 mins. a lot less time than mounting the micros to the railing. This is just an answer to the argument that micro-inverters provide easy way to kill all power coming down from the roof.When we compare our out puts from central inverters and arrays we are seeing the same production increases over CSI models. If we are seeing the same with central inverters as you are from micro-inverters where are the real world gains?
Or perhaps the fact that our two companies use modules that are far better than the rest, the CSI models are unable to accurately predict their output. Have you used the SAM models? Jesse - chimney mounting is all fine and good for houses with chimneys, but many of our customers do not have them! Pasadena has backed away from requiring roof-mounted disconnects because the cost was simply not justified. That said, the fire inspectors all are very happy to know that when the AC disconnect is pulled, there are no live wires on the roof.I certainly think that CSI understates performance for Sanyo panels since the high temperature performance is so much better than "typical." I do not think that accounts for all of the difference that we are seeing, however.
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