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Why don't people use large 400w Panels on travel trailer?

CanadianEH

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First post here.. Lurking for awhile. Thank you in advance for all help tips.

I am not 100% new to solar systems, but certainly no expert.. previously setup a very Simple solar charger setup to my last travel trailer. Which comprised of 2 100 watt panels, a victron mppt. More or less to keep batteries tippy top while sitting in storage.

I am have a new trailer now and rather than putting money into a Genny.. I am interested in setting up a much larger system including inverter and 24v life4p0 battery.

My question.. I have enough room for 1 or 2 425w q.cells peak duo L-g7 panels. Or perhaps the slightly smaller duo g7 330w panels. But I don't seem to see anyone go with larger Panels like this, everyone seems to use many 100 watt panels. These qcell panels would cost me quite a bit less then buying 4x 100 watt panels.

Any reason i shouldnt go the larger panel route?

And bonus question. Is there any reason that the panels have to be mounted very tight to the roof? Or can they be raised up 5-6 inches or more off the roof to help decrease the chance of shade from things like an ac unit?

I'm sure I'll have several more questions but this will help determine where to start.
 
Welcome to the forum.

Plenty use big panels. Often easier to tetris more total wattage with 100W panels, and smaller panels get marketed as kits.

They can be raised, but it's critical that they not be be susceptible to air coming under them and lifting while driving. A small "ramp" that connects the panel edge to the roof to act as a fairing to deflect the wind.

I can't find the post, but someone mounted two panels over his A/C as follows:

1613455425012.png
 
Thank you for the welcome.

So just to confirm.. There isn't any reason to not use a larger panel if you have the room for it? ( I'll size the rest of the components accordingly.)

Am I correct to assume that he used the one panel as the wind ramp and the other Panel flat?

My thought isnt as much to cover the ac unit as it is to minimize shade at sharper sun angles.
 
Whatever size panel works for you is fine provided you size the rest of the equipment accordingly.

Yes, the front panel was the wind ramp.
 
Whatever size panel works for you is fine provided you size the rest of the equipment accordingly.

Yes, the front panel was the wind ramp.
Just to clarify, the wind ramp itself was not a solar panel, just a piece of metal (or plastic or fiberglass) used to deflect wind from creating updraft on the solar panels?
 
In my initial suggestion, yes, if the panels are 5" or so above the roof. Another poster did as I sketched using panels.
 
In my initial suggestion, yes, if the panels are 5" or so above the roof. Another poster did as I sketched using panels.
Thoughts on that? Do you think it’s a good idea or indifferent?

if my panels are less than 5“ from the roof top, is the deflector unnecessary or even pointless? Can you reference info on how the physics work?
 
Airflow at speed getting UNDER a panel will create flexing and lift. The mounting would need to be MUCH more robust to stabilize the panels at vehicle speeds, and crosswinds or headwinds can massively increase the lift.
A steel frame welded to the framing of the vehicle would be needed, and panel damage would still be likely...
Remember... AIR is the biggest resistance to vehicle speed... and that air getting under a panel is massive. Think, tornado power...
Hurricane force winds are only 75MPH... driving at 70mph, when the wind is changing directions all the time... a normal 40mph wind heading into the vehicle, or sideways across the road can rip your roof off.
Not to mention the fuel economy hit.
 
The roof of these things can be cluttered with equipment and vents, so sometimes only small panels fit in all the available spaces.
I had room for one large 330 watt on my rv, works great!
2019, Drove from Florida to Alaska, spent the Summer, drove back to Florida.
It is stuck to the roof with 3M VHB tape
330panasonicrvroof.jpg
 
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Supervstech. That is a great point.

I was thinking of using an air ramp or wind deflector of som kind.. But perhaps it isn't worth the trouble.

I suppose my next question would be.. I wonder if anyone has done testing at what height wind becomes an issue? Even low to the roof panels would have significant airflow under them at driving speed..
 
There is a skin effect, and obstructions do break up the wind, but on my suburban my panels are about 2” above the roof, and the wind forces are pretty large.
I cannot go too fast without them getting quite squirrelly... I am planning a wind ramp, and side covers soon.
 
Seems to be a delecate balance of wind resistance vs airflow for panel cooling.

I see the benefit of adding a front air deflector. But if you cover the sides too wouldnt that be decreasing airflow to restrict cooling effect?
 
Concerns over the strength of a large piece of glass laying flat going over rough roads have been raised. Adding a rib to the underside of a large panel has been mentioned. Happy to see the 330w had no issues going to Alaska though....
I personally wouldn't mount my panels higher even if it meant going down in wattage to avoid shading. I would also suggest using screws and brackets for mounting.
 
FWIW, I've got 10 335 watt panels going on top of my 5th wheel.
I would like to see a pic once complete or a sketch if you have it.

I have a 35' RV and could not fit that much on. Perhaps 2 kw if I put them across both sides.
 
I would like to see a pic once complete or a sketch if you have it.

I have a 35' RV and could not fit that much on. Perhaps 2 kw if I put them across both sides.
I am a full-time RVer. I Have seen people with air foils upfront but MOST do not have that. Mine are off the roof about 4 inches, large panels, no airfoil.. The wind goes under and over the panels thus equalizing the pressure. It is only if a leg came loose, or some other catastrophic event that a panel could get pulled off.
 
I use one 96 cell 327w sun power on my 6x 10 cargo trailers conversion roof down to a 100/30 Victron MPPT smart solar CC. Works quite well for my needs.
I use SikaFlex to bond mounts to roof along with steel cable safety tethers for backup.
365C35E3-AE8F-457B-9742-63A2778E257B.jpeg
 
I was reading and thinking more about that as well Dhasper. The panel shouldnt act like an airfoil becasue it isnt shaped like one. An airplane wing has lift due to the airfoil shape. if the panels are close to level with all other things being equal they should have the same airflow over the top of the panel as underneath, which would equalize pressures.

I was thinking also the larger panels would have more potential force due to larger surface area. with smaller panels wind would have a shorter distance to escape without the potential for damage.

Whinny.. Good point about the extra ribbing. Although I generally dont often travel rough backcountry rough roads with my travel trailer.
 
I used 100 watt panels due to space restrictions as well as due to the roof line. I have an Airstream and both the front and rear "caps" taper off quickly. The sides are also "rounded" which means mounting panels flat requires mounts of different heights. I used mounts from AM Solar out of Springfield, OR. AM has done many, many Airstream installations and use 3M VHB tape exclusively for mounts and they assured me that they have never had a panel blow off. Even so I put screws through the front mounts on each of the panels ( except the ones between my AC units and have never had issues with 75 mph driving (I don't remember if there has ever been any wind") even though these are 2 to 4 inches above the roof.

There is one panel on each side of the forward AC unit (fore & aft), two panels mounted athwart ship between the Fwd and Rear AC and two more panels mounted end to end behind those on the starboard(curb) side. The mounts as heavy duty SS and use plastic handles screw studs to go through the mount and into matched fittings on the panels. They work great and come in various "heights" / Configurations ( but they are not cheap).....
 

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