Micro-Inverters Useful in My Specific Install? (Pics)

cebury

Would micro's help optimize/capture more KW in the below situation?

I understand the concept & pros/cons of micro vs. central inverters. However, I need your help as determining appropriateness is beyond my knowledge: evaluation of bypass diodes on the panel + hi-quality inverters w/multiple strings, with different roofing locations & some seasonal shading, differing times of day, etc. vs. same panels w/Enphase micro-inverters. I've been told different answers by different installers, based on what they're selling.

If you'd be willing, I'd like your opinion on this actual scenario below.

Shown below is a possible 27 panel install with smaller Sunpower sized panels. If I used Kyocera-245W or Yingli 230W I'd get fewer on the South facing & put more on the West face (or possibly the E at roof peak).





There is seasonal shading during non-Summer months on the two (small) Southern facades. In the late AM it's partly shaded on the SE corner and in the PM the NW corner has partial shading due to deciduous (no leaves) tree. The W face is never shaded.

Would micro's help optimize/capture more KW in the above situation? Or is it not worth the small/added expense when using good panels like Sunpower E19x240 or Kyocera 245W w/SMA inverters? (vs. Enphase m215 with whatever SP/Kyocera/Yingli panels are compatible with it).

Thank you for your input!

mike95490

Re: Micro-Inverters Useful in My Specific Install? (Pics)

I'd save $ and forget the area labeled "reverse tilt". Plug the numbers from that area into PV Watts, and see if there is any harvest at all?

With the several different planes, you would need at least 3 string inverters, or a mix of string and micros. I'd only use micros in areas where you expect shade to interfere for more than a month.

solarix

Re: Micro-Inverters Useful in My Specific Install? (Pics)

You can't run these different orientations with a string inverter without getting clobbered. Besides Emphase you should look into SolarEdge's distributed system which is made for just this situation. I would also go with more panels on the west facing stretch rather than the "reverse tilt" batch that will look horrible from your front side.

cebury

Re: Micro-Inverters Useful in My Specific Install? (Pics)

Thanks guys. I've run PVWatts with the base de-rate factor and also with a slightly less rating (.73) if I used lower quality Yingli panels and account for my shading (I studied the default de-ratings method & how they calculated .77). But no way to account for micros vs. central. FYI: I'm in a 5.8 sun hours area according to the associated mapping site.

Reverse tilt is backyard -- since they are the only South faces (garage + reverse tilt) I figured that was best. Yeah, adds $500 extra -- whether I'd make that back vs. relocating to the West facing is questionable.

Plug the numbers from that area into PV Watts, and see if there is any harvest at all

I'm unsure what you mean. Harvest is the same as over the garage, 9 panels in both spots, both facing South. Obviously difficult to account for shade, but I do have some de-rate numbers I was provided by my 1st preference of installer. The de-rate numbers between both spots are different per month, but overall multiply out to about the same.

I'd only use micros in areas where you expect shade to interfere for more than a month.

South Face shade De-Rate: J-75,F-90,M-92,A-95,M-96,J-96,J-96,A-96,S-94,O-92,N-84,D-67. These are numbers used for CSI rebate, so not sure if they are conservative or not (to maximize rebate). The West face is little worse Dec-Feb, but better in Mar & Oct-Nov, and about the same in Summer months.

you would need at least 3 string inverters, or a mix of string and micros.

The installer was planning on using 3 separate strings plugged into the one SMA6000.

That's my big question: Isn't using ONE central inverter, a questionable decision in this scenario? Doesn't the simple facts they a) are in 3 different areas, b) face two different directions + c) have different trees to cast different shadowing (even if minor), make this a no-brainer -- go Micros?


I've felt unsure of this since Day1 of the project but the installer is adamant "central inverter is fine" -- but I haven't signed yet until I know this for sure. They'll use micros "if I really want to" but they do a lot of Sunpower, which obviously isn't all that excited about micros (I suspect at least not until their new "AC panels" arrive). Other than this, I really like this installer.

BB.

Re: Micro-Inverters Useful in My Specific Install? (Pics)

Solar Guppy in the past has been very adamant that panels on different planes (and different mounting types) should have different MPPT input controllers.

I had argued that MPPT Vmp/Imp tracking was mostly based on solar panel temperatures--and that panels have similar temperatures with similar solar/mounting conditions. (i.e., that once both east and west facing panels have similar sun--not identical--the panel/cell temperatures would be "close enough" that not very much power would be lost).

However, Solar Guppy, who actually has the design and test experience, plus the equipment to do the testing, has said that my above assumptions are wrong and that each plane needs its own MPPT input for optimum power collection.

So--that would argue against a single input large central inverter.

Hopefully, Solar Guppy will have a chance to read this thread and can join in with an informed opinion.

-Bill

cebury

Re: Micro-Inverters Useful in My Specific Install? (Pics)

that once both east and west facing panels have similar sun--not identical--the panel/cell temperatures would be "close enough" that not very much power would be lost

Thanks for your input Bill. Is your position here "panel temperature is the greatest & over-riding derate factor and thus negates angle of reflection loss." I would assume it's multiplicative, not overriding, but I'm a complete amateur. Also in my situation a minor, but different, amount of shading is involved in each facing location (tree & roof-peak shading).

BB.

Re: Micro-Inverters Useful in My Specific Install? (Pics)

No, no, no...

We are talking about different things here... Angle to sun and shading is critical for solar panels. They need full sun. Period. Stop.

Angle of sun can be estimated by the Cosine of the Angle... If you are off by 10 degrees, the Cosine of 10 degrees is 0.98 or ~2% loss... If you are at 20 degrees off axis, you get 0.94, and for 30 degrees it is 0.87 etc... If you use PV Watts or other programs, it will take angle of sun and average weather conditions into account (as well as angle of mount, fixed or 1/2 axis, etc.).

What I was typing about specifically was the MPPT function of GT and more expensive solar charge controllers...

Maximum Power Point Tracking basically allows the charge controller to adjust the operating voltage point of the solar PV array to find the optimum Power=Vmp*Imp point. It turns out, that, for the most part, the Vmp of a solar panel is roughly proportional to the temperature of the solar cell itself.

So, my argument (which is apparently wrong) was that most of your solar panels are running at nearly the same temperature (when under a useful amount of sun light and optimum angle) so that a single MPPT input GT inverter would still, on average, collect nearly (withing a couple percent) of the maximum available power from the array if you were to put a separate MPPT input controller on each separate facet/array orientation.

In general, when paralleling or series connecting different brands and models of solar PV panels--if the Vmp (parallel) is within ~10% between panels, or Imp (series) is 10% or better match, the IV curve is "flat enough" that you don't lose very much energy. If panels miss-match by >10%, you will lose much more than 10% energy (can lose upwards of 50-100% of added panel capacity in worst case).

-Bill

drees

Re: Micro-Inverters Useful in My Specific Install? (Pics)

Not to mention that as soon as any one panel gets shaded, it will cut power significantly for the whole system.

Seems like a good reason to go with micros to me, but I'm biased since I have micros.

With your west mounted array that is going to kill the whole system's output until the sun starts shining on it...

As far as the reverse tilt deal - mounting east or west instead of south will reduce production by about 25% - you can play with the numbers in PVwatts (as well as to see whether E or W produces more power in your area) to determine if you're better off doing a reverse tilt or just adding more panels your West facing string (or perhaps go with east facing string depending on the data).

solarix

Re: Micro-Inverters Useful in My Specific Install? (Pics)

Connecting three different strings into a SMA GT inverter just ties them together in parallel. There is only one MPPT input on an SMA inverter.

Solar Guppy

Re: Micro-Inverters Useful in My Specific Install? (Pics)

Him, I don't remember say BB was wrong :roll: only that there are effects that may not be obvious. I've been surprised more than once on how panels are effected in the real world operating conditions.

The key is heat and current:

Heat: Even on the same roof, on difference surfaces facing different directions one can see 20-30F differences, this simialry equates to differing panel tempatures and vmp points for the array

Current: When you have panels in series, your going to be limited by the lower performing panel current wise, so if panels are off angle from each other, that's a primary consideration for being in a series string

In the proposed installation, your only choice is micro inverters and lose the north facing panels, they will never have payback.

cebury

Re: Micro-Inverters Useful in My Specific Install? (Pics)

As far as the reverse tilt deal - mounting east or west instead of south will reduce production by about 25% - you can play with the numbers in PVwatts (as well as to see whether E or W produces more power in your area)

In regards to West vs. South, all other things being equal (which they aren't exactly, due to a minor amount shading):

PVWatts calcs 14% loss (or 16% gain) for facing West instead of optimal South, eqv. 436 Watt annually for 9 panels at my long/latt. That's about $75/year at pre-PV-TOU rates.
DC Rating: 2.2 kW
DC to AC Derate Factor: 0.770
AC Rating: 1.7 kW
Array Type: Fixed Tilt
Array Tilt: 22.0°
Array Azimuth: 180.0°

Az Month Solar Radiation(kWh/m 2/day) AC Energy(kWh)
270 Year 5.06 2706
180 Year 5.79 3142

BB.

Re: Micro-Inverters Useful in My Specific Install? (Pics)

SG--Not trying to put words into mouth... Just trying to be clear (black and white answers)... Enough shades of gray (grey?) in real life to keep things confusing.

-Bill

cebury

Re: Micro-Inverters Useful in My Specific Install? (Pics)

Solar Guppy wrote: »

In the proposed installation, your only choice is micro inverters and lose the north facing panels, they will never have payback.

There weren't any panels on North facing, all just facing South. They were on Reverse Tilt extending the South facing roofline, so my crummy MSPaint 2d drawing made them appear North facing.

Well that pretty much confirms it. Everyone I ever ask says go micros. The only ones that haven't, typically don't use them or have another motive for being against them and would never use them period.

Even if I forego all the 25yr warranty argument, my situation makes it easy. There are only 2 layers deep of panels, so replacing a micro doesn't involve tearing panels off to get at center rows which was part of another argument against. I've also read on their forums where the Enphase first generation had failure issues in high heat conditions, but that was a while back.


You guys who answer questions on forums like this are so valuable to novices/amateurs like me just trying to figure out what installers are telling us is truthful.

Thank you so much!