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  • Enphase M250 Clipping

    I have a new solar system, and love it. It's working great but I have noticed clipping of my production during most sunny days now (see attached) My system is ground mounted and consists of;
    (20) Canadian Solar CS6P-265P panels, (20) Enphase M250 micro inverters, 180 degree azimuth, 43 degree angle. I never see the panels produce more than 252W, which is limited by the M250 inverter. My question is, why would you buy a higher producing panel (285-300+Watts) if the inverters clip production at 250W?
    Attached Files

  • #2
    You are getting the most from your investment in inverters. If you wanted to get the most from your PANELS, you might have
    used a larger number of 250W panels and more of that size inverter. Some say your inverters are at best efficiency; from the
    curves I consider that a much smaller effect. You have the advantage of collecting some 20% more energy under clouds, for
    the size of your inverter investment.

    My curves look a lot like that, but the maximum inverter plant is restricted, and I can run up to 50% performance under clouds.
    Bruce Roe

    Comment


    • #3
      What's your zip?

      Run PVWatts with 10 % system losses, choose the hourly output option and look for a clear day output near an actual clear day you have data for and compare output for a SWAG..

      If you are at ~ 40 or so degrees N. latitude, on a cool but sunny and dry day this time of year, your panels may well be producing slightly more than 265 W/panel and you may well see some clipping for an hour or 2 on either side of solar noon. Most days and most times of the year, your expected output will be less than 250 W/panel.

      Comment


      • #4
        I have the M250's with 260W panels and sometimes see clipping like you have - but not often. The day after it snowed - cold - with a cloudless sky - every inverter was maxed for a couple of hours (sweet!). The inverter is speced for max output of 250W but I have seen 254W or so on a few on those really cold clear days. Most of the time with normal conditions - you will not clip and output will drop off over time. Heck today - its cold and cloudy but for a brief moment right at noon the sky parted and bang - system clipped for about 5 mins until the hole covered back up.

        One of the difference between string type inverters like bcroe has is he can add panels and just make sure the strings meet the input requirements for his inverters. But with micros - every panel added has to be calc'ed into your entire system as full output. For example right now I'm guessing you have a 25 amp breaker on your service panel for your system. Just adding one more micro/panel and you would then need a 30amp breaker. You could then add 3 more but on your 25th (total) you would have to move to a 40 amp breaker. And whatever wire used would have to be adjusted, etc. And a new application with the PoCo...

        Enjoy your clipping with your new system while it lasts. Given its a ground mount, you can also take the time once a week and wash it down to get the dust/leaves/bird droppings off it easily if you feel its worth it.

        And since you are using Enphase, its super easy to setup a pvoutput.org account and have it pull your data automatically. It has better historical viewing than Enphase has but it only has the totals - no per panel data. Looks like this: http://pvoutput.org/list.jsp?id=49477 with lots of options to go back and look at any day you want.

        Comment


        • #5
          Hello and thanks everyone for their feedback on my PV system. I'm new to this PV system, so the clipping of my M250 micro inverters was new to me. I was thinking of it a bit differently, in that when clipping occurs, its wasted energy produced by my panels that I am unable to harvest. I wish the inverters wouldn't clip at all and just let the panels over produce beyond the 265W rating they have. If for instance hypothetically, if I had 285W panels with the M250, it would still clip at 250W and the extra money I paid for the 285W vs 265W panels would be a waist of money. My panels clip on every sunny day, and it's wasted energy that I can't harvest. Regardless, I'm pleased with my system and I appreciate everyone's feedback.
          /Dann

          Comment


          • #6
            Originally posted by dannw1431 View Post
            I was thinking of it a bit differently, in that when clipping occurs, its wasted energy produced by my panels that I am unable to harvest.
            That is pretty much how it works.
            OutBack FP1 w/ CS6P-250P http://bit.ly/1Sg5VNH

            Comment


            • #7
              My number one complaint on microinverter installations. I see quotes that folks put up on occasion with 330 watt panels with 250 watt inverters. I also feel that running any electronic at 100% is somehow going to reduce its long term lifespan.

              Comment


              • #8
                There are a lot of complaints on the forum here too, often ironic because a system with a small amount of clipping can often be more cost effective than one without.

                In the OP's case, let's say the *average* clipping throughout the year was 10 W for 2 hours per day (it is probably much less than that). That is 20 Wh per day of energy not produced, or $0.004 / day if that energy is worth $0.20 / kWh. Over the course of the year, that is about $1.50 worth of electricity. If it costs $15 to increase from the M250 to the S280, it would appear that cost is recovered in 10 years, except that you'd also have to consider that each year, the panel will be degrading slightly. By year 10, that 265 W panel might be more like a 248 W panel, with no clipping at all.

                In cases with larger panels, modeling tools like PVWatts can help estimate where the breakeven point is between using a lower rated, less expensive, vs one that reduces clipping but costs more. These kinds of trades are throughout the PV design... for example, in a 265 W / M250 pairing, the transmission loss through the wires might be more than the clipping loss. Yeah, you could spend more for heavier wires that lose less, but the value of the energy you recover from it not worth the additional cost. Because this source of loss doesn't show up in a pretty flat-top chart, no one pays attention or gets upset by it, but the decision making is the same.

                The idea that an S280 will last longer than an M250 because it is being loaded less than it rating is far from proven. Power isn't truly the enemy, usually temperature is, in the form of absolute temperature, gradients inside the device, or the rate of change of temperature through the daily cycle. An M250 running 100% in march (when it is cooler) might be happier than one running 80% in August (when it is hotter).
                CS6P-260P/SE3000 - http://tiny.cc/ed5ozx

                Comment


                • #9
                  Originally posted by sensij View Post
                  There are a lot of complaints on the forum here too, often ironic because a system with a small amount of clipping can often be more cost effective than one without.

                  In the OP's case, let's say the *average* clipping throughout the year was 10 W for 2 hours per day (it is probably much less than that). That is 20 Wh per day of energy not produced, or $0.004 / day if that energy is worth $0.20 / kWh. Over the course of the year, that is about $1.50 worth of electricity. If it costs $15 to increase from the M250 to the S280, it would appear that cost is recovered in 10 years, except that you'd also have to consider that each year, the panel will be degrading slightly. By year 10, that 265 W panel might be more like a 248 W panel, with no clipping at all.

                  In cases with larger panels, modeling tools like PVWatts can help estimate where the breakeven point is between using a lower rated, less expensive, vs one that reduces clipping but costs more. These kinds of trades are throughout the PV design... for example, in a 265 W / M250 pairing, the transmission loss through the wires might be more than the clipping loss. Yeah, you could spend more for heavier wires that lose less, but the value of the energy you recover from it not worth the additional cost. Because this source of loss doesn't show up in a pretty flat-top chart, no one pays attention or gets upset by it, but the decision making is the same.

                  The idea that an S280 will last longer than an M250 because it is being loaded less than it rating is far from proven. Power isn't truly the enemy, usually temperature is, in the form of absolute temperature, gradients inside the device, or the rate of change of temperature through the daily cycle. An M250 running 100% in march (when it is cooler) might be happier than one running 80% in August (when it is hotter).
                  Aside from and noting the separate issue of being generally more negative about micros and generally more favorable toward string inverters, I'd probably agree with most of that, and I'm not one to oversize stuff, but, I'd think that some overdesign, with that overdesign considering more than just new output for example, and depending on the application, is perhaps a good starting point. With that in mind, a 280 Watt micro on a 330 Watt panel somehow doesn't seem to be in the same locus as a design philosophy that started with a slight overdesign.

                  Comment


                  • #10
                    Originally posted by J.P.M. View Post

                    Aside from and noting the separate issue of being generally more negative about micros and generally more favorable toward string inverters, I'd probably agree with most of that, and I'm not one to oversize stuff, but, I'd think that some overdesign, with that overdesign considering more than just new output for example, and depending on the application, is perhaps a good starting point. With that in mind, a 280 Watt micro on a 330 Watt panel somehow doesn't seem to be in the same locus as a design philosophy that started with a slight overdesign.
                    330 / 280 = 1.18. Inverters of all types are routinely designed at that ratio, most manufacturers recommend up to 1.25 and some even more than that. Even for an optimally oriented array, the loss at 1.25 may not be worth the cost of increasing the size of the inverter. I don't know the extent to which the life of the inverter is compromised by increasing the array's DC power rating, but as long as the array is within the manufacturer's guidelines, and the known or modeled loss from clipping can be financially justified by the reduced equipment cost, paying more to increase the size of the inverter is something like insurance against a failure mode that is known to exist, but with an unknown influence on any individual system.
                    CS6P-260P/SE3000 - http://tiny.cc/ed5ozx

                    Comment


                    • #11
                      Originally posted by sensij View Post

                      330 / 280 = 1.18. Inverters of all types are routinely designed at that ratio, most manufacturers recommend up to 1.25 and some even more than that. Even for an optimally oriented array, the loss at 1.25 may not be worth the cost of increasing the size of the inverter. I don't know the extent to which the life of the inverter is compromised by increasing the array's DC power rating, but as long as the array is within the manufacturer's guidelines, and the known or modeled loss from clipping can be financially justified by the reduced equipment cost, paying more to increase the size of the inverter is something like insurance against a failure mode that is known to exist, but with an unknown influence on any individual system.
                      You're preaching to the choir, but, depending on particulars, I thought most string inverters can handle more than their nameplate rating, but not micros, at least not to the same degree. Aside from the cost considerations you write of , the logic of which I mostly agree with, if a 280 W micro clips at 280 W, how does that make a ratio of > 1.0 justifiable in an engineering sense ?

                      Comment


                      • #12
                        Originally posted by J.P.M. View Post

                        You're preaching to the choir, but, depending on particulars, I thought most string inverters can handle more than their nameplate rating, but not micros, at least not to the same degree. Aside from the cost considerations you write of , the logic of which I mostly agree with, if a 280 W micro clips at 280 W, how does that make a ratio of > 1.0 justifiable in an engineering sense ?
                        generally string inverters can handle more but also you have the string dynamic. In a string, usually not all of the systems are performing at max at the same time so for the same bit of oversizing you get much less clipping. With micros obviously, there is a 1 to 1 so you get clipping someplace.
                        OutBack FP1 w/ CS6P-250P http://bit.ly/1Sg5VNH

                        Comment


                        • #13
                          Originally posted by J.P.M. View Post

                          You're preaching to the choir, but, depending on particulars, I thought most string inverters can handle more than their nameplate rating, but not micros, at least not to the same degree. Aside from the cost considerations you write of , the logic of which I mostly agree with, if a 280 W micro clips at 280 W, how does that make a ratio of > 1.0 justifiable in an engineering sense ?
                          I know you understand this, but for the sake of others following along...

                          The PV panel's rating is not the same as the inverter's rating, in the sense that the panel does not have electronics and logic that limit its output capability, while an inverter does. What the panel is capable of producing is primarily a function of its construction, the environmental conditions, and the irradiance it sees. The output distribution throughout the year is a curve with an upside tail that as an engineer, I would want to consider. That tail doesn't end at STC power... under real world conditions, powers in excess of that could be generated, especially when snow albedo or cloud edge reflections are involved, or cool temps in high elevations with clear skies. In other words, STC (25 deg C cell temp, 1000 W/m irradiance of AM 1.5 spectrum) is just a point on a continuum.

                          If I'm specifying an inverter without regard to cost, I'd be trying to estimate the maximum power the array could ever produce under all conditions that the site might realistically see, and size the inverter at least at that size. That is probably somewhat larger than the array's STC power. Complicating the calculation is that an inverter's efficiency is a function of the operating voltage of the array. An LG 320 W panel has a Vmp at STC of 33.6 V, while the 280 W panel is 31.9 V. The slight boost in efficiency that you'd get from operating at a higher voltage might justify a very small amount of clipping, looking just at the engineering side, and not at the price.

                          Once price comes into play, I'm looking at the overall curve of power distribution and trying to find the most cost-effective point to design at, from the commercially available options. There isn't anything particularly special about the array's "rated power"... it would operate at that power for some number of hours per year, probably fewer than the hours it operates at the PTC rated power, and more than the number of hours it operates at 1.05x the STC rated power.

                          Without data on how loading of the inverter affects it life, rightly or wrongly, I don't put much weight on that as long as I'm within the manufacturers guidelines. With industrial VFD's, there tends to be better documentation around the expected loading... the same inverter might be rated for 2 HP, 1.5 HP, or just 1 HP, depending on its duty requirements. The differences in duty between a PV inverter designed at 1.25X STC and one at 1.0X STC are so small relative to the differences in the VFD world that truly affect the inverter's rating, that I have a hard time making the leap to it being a significant factor in the PV inverter's expected life. I'm not making any claim of being "right" in this judgement, and fully support that others might conclude differently.
                          CS6P-260P/SE3000 - http://tiny.cc/ed5ozx

                          Comment


                          • #14
                            Originally posted by sensij View Post

                            I know you understand this, but for the sake of others following along...

                            The PV panel's rating is not the same as the inverter's rating, in the sense that the panel does not have electronics and logic that limit its output capability, while an inverter does. What the panel is capable of producing is primarily a function of its construction, the environmental conditions, and the irradiance it sees. The output distribution throughout the year is a curve with an upside tail that as an engineer, I would want to consider. That tail doesn't end at STC power... under real world conditions, powers in excess of that could be generated, especially when snow albedo or cloud edge reflections are involved, or cool temps in high elevations with clear skies. In other words, STC (25 deg C cell temp, 1000 W/m irradiance of AM 1.5 spectrum) is just a point on a continuum.

                            If I'm specifying an inverter without regard to cost, I'd be trying to estimate the maximum power the array could ever produce under all conditions that the site might realistically see, and size the inverter at least at that size. That is probably somewhat larger than the array's STC power. Complicating the calculation is that an inverter's efficiency is a function of the operating voltage of the array. An LG 320 W panel has a Vmp at STC of 33.6 V, while the 280 W panel is 31.9 V. The slight boost in efficiency that you'd get from operating at a higher voltage might justify a very small amount of clipping, looking just at the engineering side, and not at the price.

                            Once price comes into play, I'm looking at the overall curve of power distribution and trying to find the most cost-effective point to design at, from the commercially available options. There isn't anything particularly special about the array's "rated power"... it would operate at that power for some number of hours per year, probably fewer than the hours it operates at the PTC rated power, and more than the number of hours it operates at 1.05x the STC rated power.

                            Without data on how loading of the inverter affects it life, rightly or wrongly, I don't put much weight on that as long as I'm within the manufacturers guidelines. With industrial VFD's, there tends to be better documentation around the expected loading... the same inverter might be rated for 2 HP, 1.5 HP, or just 1 HP, depending on its duty requirements. The differences in duty between a PV inverter designed at 1.25X STC and one at 1.0X STC are so small relative to the differences in the VFD world that truly affect the inverter's rating, that I have a hard time making the leap to it being a significant factor in the PV inverter's expected life. I'm not making any claim of being "right" in this judgement, and fully support that others might conclude differently.
                            Understood and generally agreed, and perhaps an example of how engineering has some of the aspects of an art as much as a science.

                            Comment


                            • #15
                              Thanks again for the insight into inverters. I realize now it's not so simple to determine what is best. But is there a moral dilemma with the companies selling these systems, in that they never mention the micro-inverters limit the production of your panels? You may think your buying a higher power panel and you'll produce more power, but the micro-inverter limits what the panels can produce. Its like buying a car with a five speed transmission, and never getting it out of 3rd gear.

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