Photo-voltaic panels mounted flat??

I've noticed a lot of RVs with P.V. panels mounted flat (horizontal) on the roof. Just how (in)efficient would this be?? I guess it would be OK between-say- 11.00am and 1.00pm, but when travelling, I'm looking for a shady tree to pull under for lunch!! Also, on taller vehicles (motorhomes & caravans), would there not be a problem of the panels getting dirty and dusty- further reducing efficiency??
And the ones who mount the P.V. panel as a 'deflector' at an angle at the front of the roofrack?? This would be great if you were travelling in a Northerly direction. Completely a waste of effort if heading in a Southerly direction.

Have I got it all wrong??

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Reply By: CSeaJay - Wednesday, Aug 31, 2011 at 20:22

Wednesday, Aug 31, 2011 at 20:22
Ye you have got it right

But there is a lot to be said for it as well; (for the flat ones)
It is not as efficient as a portable would be, no. But it is non-fiddly, secure, and has some benifit. Would rather have it than not
When we are on the road and have short stays at camps they are fine.
Many (me too) have portables as well, and we do take these out for longer stays and where the roof mounted ones are under shade. We adjust the portable ones all the time (as in hourly) to face the sun. But at some parks I would not leave the portable ones unattended.
AnswerID: 463960

Reply By: Member - joc45 (WA) - Wednesday, Aug 31, 2011 at 20:25

Wednesday, Aug 31, 2011 at 20:25
Hi Wombat,
Yes you're right on those things, but in practice, they still work fine. You will get about a 15% loss by laying them flat, but there is the convenience of them being bolted firmly to the roof which might outweigh the risk of tilted panels on the ground being stolen. If you're far up north, then this isn't all that much of an issue due to the angle of the sun up there. And if you're in Tassie where the sun is lower, then it's a bit cooler and your fridge won't work as hard.
Yes, they need to be cleaned from time to time. Mine are mounted at a slight angle on the roof rack (only coz the spare tyre at the rear dictates this) and they still need cleaning. I have mine hinged, so if I need the extra grunt, I can raise them to the correct angle to the sun, and I usually pick up another amp.

AnswerID: 463961

Reply By: S Paul - VIC - Wednesday, Aug 31, 2011 at 20:35

Wednesday, Aug 31, 2011 at 20:35
I have a 140 Watt panel mounted on my Jeep - The difference in Melbourne in Winter between flat and 45 degree angle is about half power - but having the tilt option gets me max power - I also have an alternator charging option if the Sun goes away for a few days ..


AnswerID: 463962

Reply By: Echucan Bob - Thursday, Sep 01, 2011 at 13:36

Thursday, Sep 01, 2011 at 13:36
Maximum power from the panel is when the sun is directly overhead. So at 1200 on the 21 December at a place on the tropic of Capricorn a flat mounted panel would be getting maximum insolation.
By March 21 the sun would be over the equator, and at 1200 would be striking your tropical flat panel at 23.5 degrees. The amount of power is proportional to the cosine of this angle. On the same day and at the same time in Melbourne the angle of incidence is 37 degrees off vertical (cos 37 is 0.8).
Of course the above considerations are at 1200 (solar noon). As the sun moves across the sky each day the angle changes accordingly.
In Melbourne at 1600 in the above example the angle of the sun is about 45 degrees, so the amount of power hitting a flat panel is 0.8 by cos 45 (0.7) = 0.56. So at 4.00 pm on the summer solstice, a flat mounted panel in Melbourne will be running at just over 50% of what you could achieve by aiming it at the sun.
AnswerID: 463987

Reply By: Bigfish - Thursday, Sep 01, 2011 at 18:45

Thursday, Sep 01, 2011 at 18:45
Most replies mention the loss when you are stationary. Whilst driving the movement of the vehicle, hills, corners,dips, camber etc.etc means that they still run in a very efficient manner. You are much better off having an extra solar panel that helps make up the difference lost when stationary for long periods.
AnswerID: 464006

Reply By: roger ramjet - Thursday, Sep 01, 2011 at 19:03

Thursday, Sep 01, 2011 at 19:03
Ive run mine flat for all above...when in the right location, pull it down and/or tilt it up accordingly.
80w panel (plus 100amphour storage) gives me up to an extra 3 days without having to start the truck. If I leave it flat this is about halved.

I look at a lot of fixed panels on caravans and think what a waste, they've probably paid double for their real needs to compensate.

Some 'free' power is better than none.

AnswerID: 464007

Reply By: Dennis Ellery - Thursday, Sep 01, 2011 at 19:27

Thursday, Sep 01, 2011 at 19:27
I have seen some big off-roaders with 6 or 700 watts of panels on top to make up for the inconvenience of having to demount them to follow the sun. It all depends on how much cash you have I suppose.
By the way - you should have a space for an air flow, under panels that are laid flat, or you risk damaging them through heat build up.
AnswerID: 464010

Reply By: Fatso - Thursday, Sep 01, 2011 at 19:45

Thursday, Sep 01, 2011 at 19:45
I wouldn't have a clue about the efficiency of PV panels laying flat as opposed to being at right angles to the sun. But they would be far more efficient laying flat on top of a van than being either folded away inside or still sitting in the shop.
As for how efficient they are while travelling it is irrelevant if you have your 100 or 150 A/H or whatever alternator charging the battery.
I remember seeing a couple about an hour North of Mt Isa pulled over for the night on the side of the road with their van. They were sitting out beside their solar panel watching the sun go down.
I thought "what a joke".
I assumed that they had only arrived at that location in the late afternoon because it was not the type of location anyone would choose for more than an overnight stay & there were better locations in both directions.
The joke is that the drive up from Mt Isa would have put more charge into their batteries than 2 or 3 days full sun with their little solar panels.
If he were to drive on to Camooweal or further West he would again put comparatively massive amounts more charge into his battery than those solar panels would.
AnswerID: 464014

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