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Using 2 panels on one micro inverter (numbers not adding up)

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  • Will792
    replied
    Originally posted by Ampster View Post
    I use some IQ7+ and specifically chose that model because it is designed for the higher voltage of 96 cell panels. However as I reread his post it was a prospective question based on his interpretation of the AC output. He did not compare the voltage of two panels in series to the voltage spec of IQ7s as you suggested earlier.
    In my case I will never approach the max output of the IQ7s because they are powered by used Sunpower 305 Watt panels that are west facing in a sub optimal orientation.




    I might be wrong but IQ7+ cannot handle voltage of Panasonic panels. The spec has 60V max DC voltage for + and 80V for X. Most 60 cells panels will have more than 60V for 2 connected in series.

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  • Ampster
    replied
    Originally posted by SunEagle View Post
    ............
    Strange that the spec indicate a maximum input of 440watts but only a 295w output. Seems like a waste of panel wattage.
    At the risk of hijacking this thread into a discussion about clipping, high DC to AC ratios are a more common aspect of system design.They have gone unnoticed in the microinverter environment because the shape of the production curve may be less pronounced than that of a string inverter. High DC to AC ratios don't get that much comment until someone sees the results in term of a "Fez" shaped curve instead of the "somewhat" sinusoidal curve that they expect. They feel gyped, but the result of higher DC to AC ratios result in a steeper ramp up and a longer production period than with less capacity in panels. For sure, just increasing the inverter size would give more overall production but in this case that would double the amount of inverters to get a marginal increase. It may not be cost effective and it is not a simple calculation for many because it reminds them of their calculus teacher in high school.
    Ironically the comment of @gbynum did offer a solution because with two panels on one microinverter with different orientation one could avoid the clipping and possible increase the amount of total production from that inverter. That is essentially what Bruce Roe has done with his multiple orientations. Conceptually it would be harder to do with micro inverters with paired panels but one could imaging them on a ridge contiguous to each other one slightly south east and the other slightly southwest. As the cost of panels has come down these kind of outside the box ideas seem to have a life of their own.
    Last edited by Ampster; 08-09-2020, 12:16 PM.

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  • SunEagle
    replied
    Originally posted by Ampster View Post
    I looked up the specs of the IQ-7+ and it clearly states that the maximum continuous power output is 295 Watts. That means a DC to AC ratio of 1.35 to 1 if the OP uses two panels per inverter. I would be curious what the cost benefit would be of using one panel with less expensive reqular IQ-7 versus two panels with more expensive IQ+7+ with clipping.

    My system was designed with a ratio of 1.5 to 1 and it performs to the PV Watts model and installer's estimate.
    Thanks for the update. At least that should provide the OP with some data to make a decision to maybe not use 2 panels wired to a single IQ7+.

    Strange that the spec indicate a maximum input of 440watts but only a 295w output. Seems like a waste of panel wattage.

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  • Ampster
    replied
    I looked up the specs of the IQ-7+ and it clearly states that the maximum continuous power output is 295 Watts. That means a DC to AC ratio of 1.35 to 1 if the OP uses two panels per inverter. I would be curious what the cost benefit would be of using one panel with less expensive reqular IQ-7 versus two panels with more expensive IQ+7+ with clipping.

    My system was designed with a ratio of 1.5 to 1 and it performs to the PV Watts model and installer's estimate.
    Last edited by Ampster; 08-08-2020, 07:18 PM.

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  • Ampster
    replied
    I use some IQ7+ and specifically chose that model because it is designed for the higher voltage of 96 cell panels. However as I reread his post it was a prospective question based on his interpretation of the AC output. He did not compare the voltage of two panels in series to the voltage spec of IQ7s as you suggested earlier.
    In my case I will never approach the max output of the IQ7s because they are powered by used Sunpower 305 Watt panels that are west facing in a sub optimal orientation.





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  • SunEagle
    replied
    Originally posted by Ampster View Post
    We don't know if 400 Watts are being inputted to the IQ7. It could be the two panels in series are the issue like PV Andy asked. Perhaps the voltage is out of the range of the inverter and the MPPT algorithm is not able to optimize the output?
    Someone did suggest putting the panels in parallel but that may put the current out of range.
    Agree. The IQ7+ may see the panels wired in series much differently then wired in parallel. I am hoping for someone else that uses the IQ7+ equipment chime in.

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  • Ampster
    replied
    Originally posted by SunEagle View Post
    .........
    While the input to the IQ7+ allows up to 440watts why is it only showing less then 300w when over 400watts are being inputted to it?
    We don't know if 400 Watts are being inputted to the IQ7. It could be the two panels in series are the issue like PV Andy asked. Perhaps the voltage is out of the range of the inverter and the MPPT algorithm is not able to optimize the output?
    Someone did suggest putting the panels in parallel but that may put the current out of range.
    Last edited by Ampster; 08-08-2020, 09:54 AM.

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  • SunEagle
    replied
    To get back on topic. The question from the OP is why is the wattage of 2 panels wired to a single micro not adding up to what they expect.

    While the input to the IQ7+ allows up to 440watts why is it only showing less then 300w when over 400watts are being inputted to it?

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  • Ampster
    replied
    Originally posted by J.P.M. View Post
    .............
    Can you tell me how and in what ways any observation of yours that you are referring to illustrates any point(s) relative to what I have discussed in this thread relative to residential applications ?
    No problem. You specifically said, " unless proved otherwise by some number crunching,"
    For example if all the available southern facing roof space is taken, then a west facing array may be worth some investigation. In that situation if the cost of an installation divided by the kWh production over a reasonable period of time is less than the cost of energy that would otherwise be purchased it would make economic sense. Simple economics supersedes optimal engineering theory. Do you want a real life example?

    I made my original comment to support the comment that @gbynum made about considering some other orientations to mitigate clipping before you made any comments on this thread. My analysis does not depend on mitigating clipping but that is a whole other topic worth discussing
    Last edited by Ampster; 08-08-2020, 12:37 AM.

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  • J.P.M.
    replied
    Originally posted by Ampster View Post
    The array orientations I observed illustrate that point. We will never know what assumptions went into the number crunching of those installations.

    It all depends on where you are standing.
    I aim to stand on the firmament of knowledge and experience verified by observation, measurement and experiment.

    I'm writing about using models as design aids that seem to have - at least to my experience - reasonable and verifiable agreement with measurements to be used as a GUIDE in design of residential grid tied PV systems and T.O.U. billing tariffs.

    If some such application negates an optimum orientation or winds up requiring a split array, such as for limited area or shading, etc., that's one thing. But to go into a design with the idea that splitting an array is the best way to go when it's not necessary or that it'll somehow be more cost effective is starting off on the wrong foot. That, BTW, applies to just about any application, residential, commercial, on/off grid, whatever.

    The numbers and methods I used in my analysis and the precis I used to describe the method are examples only and meant to illustrate the idea that, contrary to SunEagle's point, the idea that a 270 deg. azimuth is mostly or always a good first choice for use with a T.O.U. tariff in a residential application, a 180 deg. azimuth for those on T.O.U. rates will often and probably yields a greater annual bill reduction per installed STC kW, and so yield a more cost effective installation. AND, the usually greatest annual bill offset is with an orientation that's often relatively close to equator facing. Point is, it ain't a slam dunk and closer to equator facing orientations will probably be more cost effective more often than less equator facing orientations.

    The same applies to your idea of splitting an array. First off, I'm referring to residential applications, not large ground mounts or ag/commercial installations you seem to be referring to. There may well be good reasons for splitting an array. As I and others harp on, every application is different. I can sure find or think of good design reasons to split an array - such as for ag water pumping and to minimize storage requirements (for both water and power) and associated costs for example.

    Second, I've got some numbers behind my examples. You point at non residential installations you know nothing about except where they're located and call it confirmation of your unverifiable opinion.

    Can you tell me how and in what ways any observation of yours that you are referring to illustrates any point(s) relative to what I have discussed in this thread relative to residential applications ?

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  • Ampster
    replied
    Originally posted by J.P.M. View Post
    ...........
    Bottom line: For many, if not most orientations of arrays operating under a T.O.U type tariff, unless proved otherwise by some number crunching, an azimuth orientation closer to equator facing will probably be more cost effective than one farther off of equator facing.
    ........
    The array orientations I observed illustrate that point. We will never know what assumptions went into the number crunching of those installations.

    It all depends on where you are standing.

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  • J.P.M.
    replied
    Originally posted by SunEagle View Post

    Unless your system is clipping a lot, more than likely as it ages you will tend to lose production and go below clipping. IMO the best reason to point the panels West instead of South is because a TOU rate makes it better for the homeowner to generate later in the day when the rate is higher.
    On orientation, it's most usually not an all one or the other thing. As an example, around here, a 270 deg. azimuth at a 20 deg. tilt array will produce ~ 1,530 kWh/yr. per installed STC kW while a 180 azimuth, 20 deg. tilt will produce ~ 1,740 kWh/yr.

    BUT, (the less informed often counter, with I'd note usually with some indignation) "because of T.O.U. pricing and timing, it's more cost effective to face an array west @ 270 deg. azimuth."

    Well, that's probably not true for most applications. I got curious and ran the current and common SDG & E T.O.U. tariff for residential PV customers by hour over a year and combined that with PVWatts hourly outputs in 10 deg. increments for azimuths from 90 to 270 deg. and tilts of 20, 30 and 40 deg. FOR UNSHADED ARRAYS sized for <= 100 % annual usage offset.

    For my example above, again, provided the array is not oversized or shaded, using PVWatts hourly outputs and current hourly rates for SDG & E tariff DR-SES, every installed STC kW of PV at a 180 deg. azimuth and 20 deg. tilt will produce ~ $479/yr. of "income" that can be used to offset an electric bill, either as a direct offset against usage or banked at the T.O.U. rate for future use. That's a per kWh rate of ~ ($479/yr.)/(1,740 kWh/yr) = $0.27530/kWh.

    The similar numbers for the 270 deg. azimuth, 20 deg. tilt are 1,530 kWh/installed STC kW and $0.28105/kWh value/kWh generated for a $430/yr. bill offset per installed STC kW.

    So, for folks in my area on T.O.U, while an unshaded west facing array will give a slightly higher value for the per kWh of generated electricity, the product of (output*offset value/kWh) of the unshaded 180 azimuth, 20 deg. tilt array = that $479/yr. bill offset per installed STC kW, is greater than the $430/yr. bill offset value per installed STC kW for the unshaded 270 deg azimuth, 20 deg. tilt array.

    As it turns out, south facing is usually better than west facing with respect to annual bill offset.

    As it works out for my location, the most bill offset on T.O.U happens when the array is at ~ a 205 deg. azimuth and about 30 deg. tilt which gives about a $490/STC kW bill offset/yr., but the $ offset #'s are not real sensitive to change for maybe +/- ~ 20 deg azimuth and maybe +/- 10 deg on tilt on either side of the optimums.

    Bottom line: For many, if not most orientations of arrays operating under a T.O.U type tariff, unless proved otherwise by some number crunching, an azimuth orientation closer to equator facing will probably be more cost effective than one farther off of equator facing.

    As for max. output to an inverter going below inverter clipping output as the system ages, at an array degradation loss of, say, 0.3 - 0.5 %/yr., unless an inverter, string or micro, is sized really tight to the array STC, I'd think it would take array degradation quite a while - something like 10 yrs. or so, maybe longer, to knock an array's (or panel's) max. output below an inverter's capacity, especially given how seldom that happens in a reasonably well designed system.
    Last edited by J.P.M.; 08-07-2020, 09:54 AM.

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  • SunEagle
    replied
    Originally posted by gbynum View Post

    You are unquestionably correct. What I meant, and said poorly, was that IF CLIPPING WAS EXPERIENCED from too much panel, directing the panels at other than absolute maximum production to distributed production would give more Wh out of the inverter. Kinda like this COVID "flattening the curve".
    Unless your system is clipping a lot, more than likely as it ages you will tend to lose production and go below clipping. IMO the best reason to point the panels West instead of South is because a TOU rate makes it better for the homeowner to generate later in the day when the rate is higher.

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  • gbynum
    replied
    Originally posted by J.P.M. View Post

    The "optimal" array orientation and layout depends on the application and also on the climate - including cloud patterns and clearness index of the location.
    You are unquestionably correct. What I meant, and said poorly, was that IF CLIPPING WAS EXPERIENCED from too much panel, directing the panels at other than absolute maximum production to distributed production would give more Wh out of the inverter. Kinda like this COVID "flattening the curve".

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  • J.P.M.
    replied
    Originally posted by gbynum View Post
    You have some very good comments ... let me add another. ASSUMING your layout supports it, you can get greater harvest by pointing one of the 2 panels somewhat east of south, and one somewhat west of south. You'll have to do the research based on your conditions and location.
    The "optimal" array orientation and layout depends on the application and also on the climate - including cloud patterns and clearness index of the location.

    For most, but not all residential applications with grid tied net metering and without a T.O.U. type tariff in effect, if the goal is to minimize the annual electric bill in the most cost effective way (most bang for the buck), that's usually accomplished by maximizing the annual array output per installed STC kW. In such cases there will be one array orientation (array azimuth and tilt) that that per STC kW annual output. For such situations, other orientations will produce less output per installed STC kW. That will decrease the output/installed STC kW and so reduce the array cost effectiveness and so not meet the goal of being the most cost effective design.

    For most residential net metering applications subject to a T.O.U. type tariff, there will still be one array orientation that minimizes the array cost for a certain level of annual electric bill reduction and that orientation will in all probability be different than the orientation that maximizes array production. That "optimum" orientation will be the one that produces the greatest amount of annual electric bill $ offset per installed STC kW.

    For most applications, splitting array orientations will reduce annual total array output per installed STC kW and so require a larger total STC array size.

    In other words, if the goal is to get the most bang for your buck (assuming array cost is generally proportional to array electrical size) splitting an array orientation with the idea that doing so will produce more annual output per installed STC kW will almost always produce the opposite effect.

    That's not to say that application goals and site constraints may mean splitting an array is necessary or even desirable, but intentionally splitting an array with the idea that it will increase output per installed STC W is most always incorrect.

    Do this: Do a PVWatts run on 1 STC kW equator facing array of some reasonably optimal tilt (say, at latitude) for any location. Then, do the same for any two 500 STC W arrays of different orientations and combine their outputs.

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