Question about SolarEdge inverter sizing and clipping

[J.P.M.]

So, if I had to do it again, go with 10k inverter? Is there any benefit to 7600 inverter besides price?

Installer claimed 7600 inverter would begin generating earlier in day and generate later in day versus 10k inverter...does that sound right? he said due to voltage needed to start generating being less on 7600 inverter.

Since you're asking, my preference, for single orientation arrays, and until I found out differently, would be to mimimize shading on the array, And KISS as a time tested (for me anyway) method to help avoid problems, and skip the more parts to fail optimizer design, and size a string inverter ~ = (STC rating of the array) * (inverter efficiency).

However, in the quest to find out differently, and keeping in mind that none of us is as smart as all of us, I'd consult with folks like Butch because he knows more about SolarEdge than I know about them and their products, and he might convince me that all the problems I seem to read about popping up around here in inordinate quantities about SolarEdge quirks, problems and crappy customer service are simply due to ignorant users.

Otherwise, see Butch's comments about the rest of your post.

[ButchDeal]

So, if I had to do it again, go with 10k inverter? Is there any benefit to 7600 inverter besides price?

Installer claimed 7600 inverter would begin generating earlier in day and generate later in day versus 10k inverter...does that sound right? he said due to voltage needed to start generating being less on 7600 inverter.

Nope no truth at all.
Voltage is irrelevant as the optimizers will boost the voltage anyway, however both inverters have the SAME operating voltage.
the 10000kw inverter is ever so slightly less efficient but way less so than cliping losses
the 7600 has optional EV charger and usually doesn't require a line-side tap
10000kw usually does require a lineside tap...

line side tap being a slight added cost.

[Jeff72]

Well, it's not an exact science, but yes, that's the Reader's Digest version of the story.

That 183 hrs. est./SWAG is the approx. total number of hours over a year that a shade free array's output will exceed 7.6 kW. - even by as little as 1 W, or as high as ~ 1,670 W., or anything in between, depending on how the weather and how the TMY data the model uses may match one another, again, not any more exact than the weather.

I do suspect however, that the chimney will put a bigger damper on output as the solar zenith angle increases for the winter months.

The lost production from clipping will also probably be less as the array ages due to several other reasons that will decrease max. array output and so lower the max. array power per kW irradiance input:
1.) Annual cell efficiency degradation. Roughly something like 0.2 %/yr. to 0.5 %/yr.
2.) Array fouling. No array is ever 100 % clean , at least for long. Mine fouls at a rate of ~ 0.75 %/week +/- some if it doesn't rain or I don't hose it down. Dirt will lower POA irradiance and thus lower the power input and so the power output of an array by varying amounts as f(array fouling), but expect ~ 3% or so production loss on average.
3.) Shade. Even with optimizers, that chimney will probably cause noticeable production loss in winter from lowered irradiance input. If you had a simple string inverter, that chimney would put a very serious crimp on winter production. See below.

Those 3 things and others can and will lower the input to the inverter. Some of that loss (over those 183 hrs.) will, in effect, decrease the clipping loss, so, sort of a no harm- no foul kind of thing.

On that chimney, some back of the envelope stuff:

If it's, say, 32" wide and casts say, a 7 ft. long (?) shadow over the array in winter, that's ~ 18 ft.^2 of shadow area. 30 panels cover ~ 515 ft.^2 or so. That means, as a rough guess,18/515 ~ = 3.5% of the array will be in a shadow that no optimizer or other practical scheme will be able to compensate for. With winter production usually being lower, to a rough 1st approx. the chimney might reduce annual production by a % or two or so.

If someone has a better SWAG based on better data/logic, I'd not argue with it.

So, if I had to do it again, go with 10k inverter? Is there any benefit to 7600 inverter besides price?

Installer claimed 7600 inverter would begin generating earlier in day and generate later in day versus 10k inverter...does that sound right? he said due to voltage needed to start generating being less on 7600 inverter.

[J.P.M.]

ok, so if I understand you correctly:

Theoretically, my system could gain an estimate of 183 hours of extra generation with a 10 kW inverter per year.
But, the difference is really only 14,019 kWh/yr - 13,957 kWh/yr = 62 kWh/yr lost by going with a 7600 W inverter versus a 10,000 W inverter? So 0.4% lost, I guess that doesn't sound too bad.

Is that right?

Also the chimney goes right up the far south side of my south facing roof. There is a gap behind it without panels to take into consideration some shading and very minor gap on the left and right side the chimney. However, the chimnney will shade more in the winter months due to the sun being further south and casting more chimney shade on my roof, so that will affect things more in the winter too.

2019-05-23 12.36.59_old.jpg

Well, it's not an exact science, but yes, that's the Reader's Digest version of the story.

That 183 hrs. est./SWAG is the approx. total number of hours over a year that a shade free array's output will exceed 7.6 kW. - even by as little as 1 W, or as high as ~ 1,670 W., or anything in between, depending on how the weather and how the TMY data the model uses may match one another, again, not any more exact than the weather.

I do suspect however, that the chimney will put a bigger damper on output as the solar zenith angle increases for the winter months.

The lost production from clipping will also probably be less as the array ages due to several other reasons that will decrease max. array output and so lower the max. array power per kW irradiance input:
1.) Annual cell efficiency degradation. Roughly something like 0.2 %/yr. to 0.5 %/yr.
2.) Array fouling. No array is ever 100 % clean , at least for long. Mine fouls at a rate of ~ 0.75 %/week +/- some if it doesn't rain or I don't hose it down. Dirt will lower POA irradiance and thus lower the power input and so the power output of an array by varying amounts as f(array fouling), but expect ~ 3% or so production loss on average.
3.) Shade. Even with optimizers, that chimney will probably cause noticeable production loss in winter from lowered irradiance input. If you had a simple string inverter, that chimney would put a very serious crimp on winter production. See below.

Those 3 things and others can and will lower the input to the inverter. Some of that loss (over those 183 hrs.) will, in effect, decrease the clipping loss, so, sort of a no harm- no foul kind of thing.

On that chimney, some back of the envelope stuff:

If it's, say, 32" wide and casts say, a 7 ft. long (?) shadow over the array in winter, that's ~ 18 ft.^2 of shadow area. 30 panels cover ~ 515 ft.^2 or so. That means, as a rough guess,18/515 ~ = 3.5% of the array will be in a shadow that no optimizer or other practical scheme will be able to compensate for. With winter production usually being lower, to a rough 1st approx. the chimney might reduce annual production by a % or two or so.

If someone has a better SWAG based on better data/logic, I'd not argue with it.

[barrye]

Thanks Butch
I wish it was that easy. Unfortunately I have a ground mount system 125' from my house with 4 conductor tech cable buried.
I only have 4 wires to work with . My optimizers are p400.
In retrospect I should have wired a 240 plug and purchased a 10000 H . C'est la vie (such is life)
I am still really happy with 70kwh on sunny days

[ButchDeal]

Can't do that.
each string is maxed out at 5760 watts.
Don't think I can put 5760w into a 3800 H inverter. And i can't add more modules to my 7600H to decrease one of the strings

You can break one of the two strings in half so the SE7600 has one string of 16 and one string of 8 or 8.64
and the SE3800h has one string if 8. Or 2.88kw and

or depending on the optimizers you could totally restring so that you have two strings of 11 on the SE7600 and one string of 10 on the SE3800 but that is a bit more work

[barrye]

Can't do that.
each string is maxed out at 5760 watts.
Don't think I can put 5760w into a 3800 H inverter. And i can't add more modules to my 7600H to decrease one of the strings

[ButchDeal]

Butch I have been trying to figure out how to accomplish that.
I have tried looking for a line diagram so I can do the wiring.
I have two strings of 16 panels.

You would have to break a string in half. What optimizers do you have?

[barrye]

Butch I have been trying to figure out how to accomplish that.
I have tried looking for a line diagram so I can do the wiring.
I have two strings of 16 panels.

[ButchDeal]

I used a 7.6 kw inverter because it was the largest inverter with an integrated EV charger.
when solaredge comes out with a larger integrated EV inverter I will upgrade and add more panels.
11.2 kw is the max I can connect to a 7.6 kw inverter

you could easily connect TWO inverters. One option would be the SE7600H and SE3800H both with EV chargers....

[barrye]

I used a 7.6 kw inverter because it was the largest inverter with an integrated EV charger.
when solaredge comes out with a larger integrated EV inverter I will upgrade and add more panels.
11.2 kw is the max I can connect to a 7.6 kw inverter

[foo1bar]

I still don't get why they undersize the inverter for a panel array that can theoretically do a bit higher like my 9.855 kW and your 11.2 kW.

One reason can be cost.
If the max DC power from my array is just slightly above the inverter size I don't lose much energy due to clipping. BUT I may save a bunch of upfront costs because of the smaller inverter size.
It's also possible with an array that isn't pointing in optimal direction - or an array that has multiple directions that the panels are pointing - that the theoretical/nameplate max DC power output is much less than the actual DC max seen in that installation.

Here are some of the upfront costs that can be increased by increasing the inverter size:
inverter cost itself (probably only $100-$300)
Larger wire sizes needed on AC side (<$100)
Larger breaker size needed on AC side
Replacing main service panel ($1500-$5000) or switching from using main panel to doing lineside tap ($1k?)

So anywhere from $100 to $5k+ in extra costs.
And then you compare that to the other side - you have the benefit of not having to pay the POCO for the energy that would be lost to clipping.
If the clipping is fairly small (ex. 2kwh/day average for 2 months) then the financial benefit of not having clipping is fairly small. (ex: 2kwh/day * 60 days * $.25/kwh = $30/yr)
So - is that $30/year benefit (which will shrink as the modules age and become less efficient) worth the extra $1k or whatever in upfront costs?
If it's $30/year vs. $1k I'd say no.
But if it's $50/year vs. $150 iniital cost I'd say yes.

[Jeff72]

this does not look like that much clipping. you could look at the worst day and see but keep in mind that the production is an inverter parabola that in your case is clipped at 7.6 kw.
it is not effected (with solarEdge) anyplace else other than the part above 7.6kw. Other non-optimized systems would be effected all over the curve in different ways.
as JPM points out you are likely missing very small percentage. it would cost a bit of money to attache a system over 7.6kw as it likely would require a line-side tap, thus why a 7.6kW is so common.

I think I am already a line-side tap. They connected the inverter power lines to new lugs just below where my utility meter outside connects outside the house. Everything is outside the house except the cat5 for monitoring going into the house and the main feed that goes into the house after the inverter connection outside the house.

[Jeff72]

I took your info and did a PVWatts run with a 1:1 DC to AC ratio (meaning a 10 kW inverter) and took the hourly output option. Then, I sort of bastardized the intent of the model and compared the hourly output with what you provided.

1.) The max. modeled hourly output is ~ 9.27 kW.
2.) using A 1:1 DC to AC ratio, I subtracted 7.6 kW from all hourly outputs that exceeded 7.6 kW and summed the results. That result is 83.1 kWh for the year.
3.) The number of hours per year that the model thinks your system's output will exceed 7.6 kW IF you have a 10 kW inverter is 183.
4.) PVWatts modeled your shade free annual output with a 10 kW inverter at ~ 14,019 kWh/yr.
5.) PVWatts modeled your shade free annual output with a 9.855/7.6 = 1.30 DC to AC ratio (a 7.6 kW inverter) as ~ 13,957 kWh/yr. There is some rounding/quirks in the model making the 10kW inverter annual output - 7.6 kW inverter output different than I calc'd the differences. Life is not perfect and neither are models.

So, misusing the model form iots in tended purpose but not terribly so, and doing so for the purpose of maybe getting some feel for the order of magnitude of what the losses from a smaller inverter might be vs. a larger inverter over a "typical" (and note as opposed to, and NOT, an "average") year, you'll lose maybe (83/14,000) kWh/yr. ~ 0.006 of the annual output according to a reasonably conservative model.

See the PVWatts model and the help/info screens therein for particulars of inputs and also see NREL for an explanation of a "Typical Meteorological Year", what it is, how it's used, and why it is NOT an "average" of weather conditions.

ok, so if I understand you correctly:

Theoretically, my system could gain an estimate of 183 hours of extra generation with a 10 kW inverter per year.
But, the difference is really only 14,019 kWh/yr - 13,957 kWh/yr = 62 kWh/yr lost by going with a 7600 W inverter versus a 10,000 W inverter? So 0.4% lost, I guess that doesn't sound too bad.

Is that right?

Also the chimney goes right up the far south side of my south facing roof. There is a gap behind it without panels to take into consideration some shading and very minor gap on the left and right side the chimney. However, the chimnney will shade more in the winter months due to the sun being further south and casting more chimney shade on my roof, so that will affect things more in the winter too.

2019-05-23 12.36.59_old.jpg

[ButchDeal]

Example of clipping on 5-22

So is this "normal" and to be expected? At what point should I request a 10 kW inverter instead of the 7.6 kW inverter?

2019-05-22 Capture.PNG

this does not look like that much clipping. you could look at the worst day and see but keep in mind that the production is an inverter parabola that in your case is clipped at 7.6 kw.
it is not effected (with solarEdge) anyplace else other than the part above 7.6kw. Other non-optimized systems would be effected all over the curve in different ways.
as JPM points out you are likely missing very small percentage. it would cost a bit of money to attache a system over 7.6kw as it likely would require a line-side tap, thus why a 7.6kW is so common.