Max # of these panels I can safely put on SMA-8000US's?

[solarsocal]

Hi all,

I'm trying to squeeze every last bit of juice I can into a pair of SMA-8000US's without going beyond a level that would be safe or would notably reduce the lifespan of the units (excess heat?).

Panels are Canadian Solar 295W (CS6X-295M). PTC rating: 263.6w, 0/+5%.
Specs here: http://lib.store.yahoo.net/lib/yhst-...022/CS6X-M.pdf

Los Angeles (91436) location. 190 degree azimuth. 5 degree mounting angle. We have room for up to 66 panels (33 per inverter).

Side note: 54 would need to be mounted at 5 degrees to prevent shadowing between rows (roof was already pre-pegged in such a way to make planar arrays possible, but less financially practical). The remaining 12 spaces are in an area that can support up to 25 degrees but I'm not sure that makes much difference given how strings work on central inverters? Anyways, the area of 12 might need to get its own inverter depending on the answer for max # of these panels that the SMA 8000US's can safely support.

One source told me that 30 of these panels per inverter (60 total) is the max we should install. I have another installer that is still a little ancy about that and wonders if we should take it down 1 more notch; he said with 30/per we'd probably be running at max capacity for several hours a day for months.

Thoughts? Input is much appreciated.

[Naptown]

Go to SMA's website and use the string calculator It will ask you some questions but will come up with the string sizing. Usually takes about 5 minutes to do.
I would either put the 12 steeper pitch on another inverter or reduce the pitch to match the others.

Here's the link

http://www.sma-america.com/en_US/service/solar-design-tools.html

[Mike90250]

first, you have to split the array/inverters so that all the panels facing 1 direction, are on 1 inverter, and the panels facing another direction (even 5deg makes a difference) are on their own inverter. Mixed Panel Aim (I'll say this again - Mixed Panel Aim) will greatly diminish the inverters ability to perform correct MPPT tracking, because the panels will be fighting each other.

And shaded panels are not allowed either. if you plan on mounting panels in shade, use microinverters on those.

[Mike90250]

It's perfectly fine to load an inverter to 100% or 110 or 200% of it's spec. All it does is throttle back to it's max spec, and waste the excess PV as power not sold. No extra heat, it just doesn't convert the excess power. If your inverter gets too hot, it will also throttle back till it cools off, even if it's only 50% spec, it won't allow itself to BBQ, when things are working right.

(note - you MUST keep the string VOLTAGE below the kill voltage of the inverter)

[Naptown]

that module is not listed on the calculator
Looks like you're going to have to do it the old fashioned way manually.

[Naptown]

The thing to be really worried about is the cold module voltage so you are not supplying overvoltage to the inverter this will fry it and will not be covered by the warranty. Make sure the open circuit voltage at the lowest temperature on record is within the tolerance of the inverter you will have to calculate the strings and compensate for temperature using the figure on the module specs.

[solarsocal]

Thanks for the responses.

Agreed, either all panels on the same pitch, or dedicate a single central inverter to only the 12 panels that can have their pitch increased.

I went to that SMA link Naptown provided and tried the string calculator. Out of curiosity I substituted a different manufacturer's mono 300w panel for my 295w mono panel since mine isn't listed. FYI it came up with either 10 or 11 panels on 3 strings for the 8000US. (11 would support my max install size of 66 panels).

I also tried the SunnyDesign 2.0 program and manually created the CSI 295w panel. I don't know how much faith to put into the calculator vs. real world experience from installers but it allowed 2 SB8000 units for 66 295W panels (in strings of 11) as "compatible" and deemed it only marginally less efficient vs. 60 panels. 60 panels: 83.6% performance ratio, 95.4% inverter efficiency. Raise to 66 panels or lower to 54, and it goes to 83.4% / 95.3%.

Basically the Sunny Design tool seems to want you to come as close as possible to getting your panel's STC's to the inverter's max rating. If I go with 54 295w panels on 2 SB8000's and add 1 inverter dedicated to the 12-panel 25-degree array, the Sunny Design tool recommends an SB3000 for the 12 295W panels. STC difference between the panels & the inverter's max is a few hundred watts. Project file attached in case anyone's interested.

I think the only thing I've accomplished since posting this first message is reminding myself that I'm "just the homeowner", I don't have any prior experience with solar installs, and I think I've reached the limit of armchair quarterbacking .

Unfortunately I've recently had different answers to the same questions from various installers, so as it's getting time to lock down the final plan and move forward with a single installer, I'm a bit nervous about making sure it's a good plan that will work as expected.

So, I'm no further along at this point to knowing whether it's better to use 2 SB8000's on 66 295W panels or if it's justified to spend the extra money to add an SB3000 and throw 12 of the 66 panels at it. I can calculate efficiency/$$ differences easily enough, but I'm still clueless as far as what to believe on how much inverter capacity I actually safely need & where things will run best.

[Mike90250]

You want your panels PTC spec, to very closely match your inverter max spec, without going over the voltage limit at record cold temperature for your area.

Or use your panels STC spec and delete 10%, and then use that # to max out your inverter.

[Naptown]

The increased harvest by tilting a dozen modules up when offset by purchasing another inverter and costs associated with wiring disconnects etc will fall far short of the increased harvest by a long shot.

[Naptown]

You want your panels PTC spec, to very closely match your inverter max spec, without going over the voltage limit at record cold temperature for your area.

Or use your panels STC spec and delete 20%, and then use that # to max out your inverter.

Most of the modules I deal with are in the 89=91% ptc to stc ratings
best place to find ptc ratings is golsolarcalifornia.org

[solarsocal]

The PTC for the modules is 263.6 per the CA GoSolar site.

Record low is 18deg F.

Installer said he ran SMA's inverter selector for 33 panels and found they allowed 7.68 Kw AC output from the SB 8000US, whereas he said NREL predicts 8.13 with 33 panels.

Sounds like I'll need to move to 3 inverters no matter what unless I want to reduce modules/capacity.

The increased harvest by tilting a dozen modules up when offset by purchasing another inverter and costs associated with wiring disconnects etc will fall far short of the increased harvest by a long shot.

The installer already committed to labor & misc parts whether I end up with the 3rd inverter or not, so the only cost to consider is the inverter itself (out the door ~$1600 SB3000 or $2000 SB4000). With PV Watts, 25 deg vs. 5 deg for 3.54kw in panels gains approx 450kwHr/yr, so DWP picks up $729 of that & Fed gov picks up either $261or $381 depending on inverter. Worst case scenario (SB4000) has an 8 year break-even point. Not a good choice all else equal, but with it looking like I'll be forced to go with a 3rd inverter, it's nice to know the necessary added cost will actually pay for itself eventually.

The rest of the system should pay for itself in under 4 years. Not bad at all for roughly a 19.5kw DC system. I am very appreciative of the LA DWP's program and Fed Tax credit!

[solarsocal]

The rest of the system should pay for itself in under 4 years. Not bad at all for roughly a 19.5kw DC system. I am very appreciative of the LA DWP's program and Fed Tax credit!

Great, I jinxed myself. DWP just put the program on hold for now, and possibly opened the door to a change in rebate $$. http://www.ladwp.com/ladwp/cms/ladwp000787.jsp

More time to get our ducks in a row, but then my solar project is unfortunately stuck in limbo until DWP picks things up again and we can see if the financials still make sense.

[nn678]

Hey Look at ME, I'm a SPAMMER

i would either put the 12 steeper pitch on another inverter or reduce the pitch to match the others.