PV panel selection? (long)

[Ductape]

Hello, all. I’m looking for some feedback on PV panels before embarking on the next upgrade to our electrical system.

To establish the background, we will be mostly boondocking, and moving frequently, so the panels will be flat on the roof. Batteries are 600 ah AGM. I anticipate using 50-100 ah per day depending on the weather. If the batteries need some feeding we always have the gen and a PD 9280 converter to stuff some amps in. Performance in partial shade is important for us, we don’t want to select campsites without trees all the time. I would prefer to supplement with a little generator from time to time if needed.

I’m thinking in terms of ~400-600w for now, with the option to upgrade later if we swap to a residential fridge or ...

Q1: brands to include / exclude? I am the type to buy a Honda, so I don’t mind paying for quality, so long as it’s not just branding. I want reliability. Kyocera is uppermost in my list right now.

Q2: Poly / Mono? I have read a number of times the poly respond better with partial shading. This gets complicated in my mind because I can fit a smaller mono panel between the roof obstacles at equivalent wattage… therefore the mono may get full sun more hours per day by virtue of the smaller footprint.

Q3: panel size. For instance I could go with two KU265-6MCA or four KD 140SX-UFBS. The second scenario the panels cost about double (at least at the prices I was looking at today). Downside is the 265w panels are slightly longer in the longest dimension, and will be partially shaded a bit longer by the A/C units. And if I want to expand later, I will have room for more of the 140’s. Not so with the 265, I would have to mismatch panels and maybe do a second controller. I am also thinking the four panels will be somewhat more shade tolerant (more bypass diodes in total). I’m planning to run the array in series.

Your comments much appreciated.

[Ductape]

Just to close out the loop, I decided to go with two of these:
LG 315w

Fit the space just right, and only two panels to install. If I do need to upsize later, I can't use this size for the remaining space, but I decided not to worry about that now. Five years from now panels may be $0.25 per watt.

I'll report back on performance once it's up and running which will probably be in a couple of months considering other projects demanding time.

I appreciate all the input and advice.

[Almot]

westend wrote:
If you position modules correctly and raise them the required height, it should be possible to have no shade on the modules.

...and with 24V modules.

[westend]

If you position modules correctly and raise them the required height, it should be possible to have no shade on the modules.

Can you share the layout diagram of the installation?

[Almot]

I wouldn't worry about 600 AH battery bank with 500-600W solar. With your stated use of 50-100 AH max, your charging cycle will be shallow, in the morning the battery will be 75-85% (even with some non-linear high draw of inverter). Your solar will charge it fully if there is 4-5 hours of unobstructed sun in summer.

3-dimensional CAD of panels layout that would show shading at different hours, can be complicated. Cardboard is more "visual", you see the shade and what happens when you raise it 3", and how much space is left for walking around.

[Ductape]

Anyone with experience on Grape Solar? I see the GS-S-160-Fab8 is available with free shipping for 229.99. Dimensions are slightly more favorable for my install.

Four right now would have me at 640w and I can easily add two more later to get to 960w. I might just go ahead and buy the extra panels now and store them to ensure later compatibility.

I will have in the worst case 6 hours per day unobstructed light, if the sun moves fore and aft on the rig. If the sun moves side to side, then no shading at all. So most shading issues will arise from trees etc.

There is no residential frig in the plan any time soon, quite happy with the propane for now. The purpose for the big battery bank is so that if we are in a shaded spot for a week, we can run for days between recharge events.

[scrubjaysnest]

Series panels in partial shade is not an option, way to much loss in performance over all. You will be very disappointed. If you can make the panels tilt to get them out of the shade then go for it.

I choose a different route after testing series vs parallel in shade. Two roof mounted panels with 4 way tilt system and two portable panels. If I can't get sun to the roof panels I just shut off their breaker. All panels are wired in parallel. This requires larger wire. 8 AWG for the portables since it is a 25 foot run.

For a 600Ah battery bank, 600 watts in full sun will be very marginal if you run the refrig off an inverter. We run ours off propane. Your battery bank will most likely be happier with 700 to 800 watts in full sun.

We use solar 99% of the time for 6 to 8 months. The other 1% is shore power and a portable smart charger.

[Ductape]

I've got good measurements of the roof, (former engineer and CAD user). The limitations on space are due to the many objects already in place. Two a/c, satellite, vents...

If I were the one designing the roof layout, I would have shifted a couple of things around to have more space clear. The a/c are low profile so I may decide to get up closer to those.

[Almot]

mchero - OP says that big panels won't work well. Only 2 would fit, and there will be some shading from A/C.

Though it's hard for me to imagine that a 29ft Class A would have less roof space than my 26ft trailer - where I put 2*250W with zero shading from (single) A/C, and still have room left for one more 250W. It is easier to install a few 24V than a bunch of 12V.

If you are in doubt, cut a few cardboard templates the size of 250-260W panel, and see how those pieces will fit when raised 3" above the roof. This is where panel surface will be, more-less. Most of them are about 40" by 65" by 1.5", and you need at least 1" space below the frame, for heat dissipation. When raised, it is enough to move 14"-16" away from A/C to avoid shading in all but very early and very late hours when there is so little sun that it doesn't matter.

You want some walking space between the panels and other roof structures in case if you need to do some work later.

[mchero]

Picked up 4 250W panels mounted flat on roof. PV wire to Midnight Solar 200 Classic light into 440ah of battery AND LOVE IT!

[2oldman]

Ductape wrote:
From what I understand if it's partial shading across multiple panels it's a crapshoot, and the MPPT controller may struggle to figure out which of the panels it's optimizing for.

Those controllers must be a lot smarter than I give them credit for. All it sees it voltage and amperage..right?

[Almot]

Ductape wrote:
I am planning at this point to go all series to MPPT. So the 140w panels will be 88.4 voc and 70.8 vpm.

My thinking is the controller can make sense of it and charge even if only one panel is lit and three are shaded and in bypass. From what I understand if it's partial shading across multiple panels it's a crapshoot, and the MPPT controller may struggle to figure out which of the panels it's optimizing for.

If 4*140 panels, 560W total, results in a less shading from roof A/C than 2*265, 530W total - I would go for 4*140W. A no brainer to me.

Partial shading in series can be difficult for any controller. It's all about sufficient number of diodes. I don't think MPPT will handle this any better than PWM. With this wattage you're more likely to find a decent MPPT than PWM though, not to mention #6 or #4 cable and rooftop box - in case of PWM.

Of course, there is always an argument that 4 panels in parallel will handle partial shading better than 4 panels in series - don't know how much truth there is to it ;)... Hear it from times to times.

[Ductape]

Thanks for the responses, everyone. Almot, I am planning at this point to go all series to MPPT. So the 140w panels will be 88.4 voc and 70.8 vpm.

My thinking is the controller can make sense of it and charge even if only one panel is lit and three are shaded and in bypass. From what I understand if it's partial shading across multiple panels it's a crapshoot, and the MPPT controller may struggle to figure out which of the panels it's optimizing for.

[Boon_Docker]

My preference would be the mono panels over poly.

They are a bit more efficient, are smaller in size for the same wattage and they are more heat tolerant (perform better when they get hot).

[Almot]

#1: Doesn't matter much. I chose quality (European). Roof panels is a long-term solution and solar is my only energy source now, so I need to be able to rely on it.

#2: Poly is better overall. Performance in partial shade has nothing to do with poly/mono, it depends on the number of diodes, the more, the better.

#3: Major difference between 265W and 140W is not "size" but "voltage". 265W panels are 24V, 140W are 12V. Different creatures and require different type of controllers. A long topic in itself. With that much wattage like you want, people "normally" go for 24V panels. "Occasionally" they go for 12V.

24V are easier to wire, can be in series with mere #10 or #8 wire.
12V are usually wired in parallel, sometimes series-parallel. With wattage over 400 it means a beefy wire to controller AND a rooftop junction box. They cost more per watt.

Once you've chosen either 12 or 24V, it's difficult to add another type into this mix later. Mismatch is a pain, and 2nd controller is more pain.

Big residential fridge + parking in partial shade = daily generator, IMO.