Solar Panel output: Rating versus Reality

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Solar Panel output: Rating versus Reality

Post by rhills » Thu, 20 Dec 2018, 21:54

Hi All,

Last year I bought a couple of lightweight, flexible solar panels that are rated at 134W (vmp 23.1V, imp 5.8A) each for use as additional chargers for our motorhome house battery system.

After having had concerns about their output, I've recently had them tested and established that in close to ideal conditions (Sunlight > 1000W/m2, Ambient temp 24°C, cooling breeze, close to 90° to the sun, no shading), under load they generate about 4.1A at 19V, or ~78W, which is a little under 60% of their rated output.

I have been given to believe that lightweight panels don't perform to spec because they heat up more quickly. I had always naively believed that a solar panel would be expected to produce close to 100% of the rated output in ideal conditions. What level of derating is reasonable to accept?
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Re: Solar Panel output: Rating versus Reality

Post by jonescg » Thu, 20 Dec 2018, 22:29

That is pretty bad. I have 12 x 190 W (nameplate rated) panels on my shed - 2280 W as a maximum. In early November after a cool rainy day the sun shone gaily upon the panels. They pushed out 2800 W for a few minutes before they heated up too much.
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Re: Solar Panel output: Rating versus Reality

Post by brendon_m » Thu, 20 Dec 2018, 22:36

In the automotive world of panels you normally expect to get 80% of the rated output as a maximum and shading /angles etc degrade the output from there. 60% would be a pass if I was testing it.

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Re: Solar Panel output: Rating versus Reality

Post by brendon_m » Thu, 20 Dec 2018, 22:56

https://www.redarc.com.au/solar-faqs
How many solar panels do I need?

The amount of solar panels you require will be determined by the amount of charge that you need to return to your system during the sunlight hours of each day. For a 12V system, if you need to replace 100A/h of charge to your batteries each day, and you have 8 sunlight hours in each day you will need...

100AH x 12V = 1200WH
1200WH / 8H = 150W of solar panels.

In reality it is recommended that you always overrate your requirements by at least 20%, therefore you would need 180W of solar panels.
What can I run on my 120 Watt Panel?

A 120W solar panel can supply between 6 and 7.5 Amps on a sunny day, for most of the sunlight hours of the day. If we assume that only 1/3 of the day is sunlight hours then it is safe to assume that the panel can supply between 6 and 7.5 Amps for this period.

Given that there are 24 hours in a day, a 120W panel can effectively run a load of around 2.75A continuously throughout the day/night running at no loss.

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Re: Solar Panel output: Rating versus Reality

Post by rhills » Thu, 20 Dec 2018, 22:57

Hi Chris,
jonescg wrote:
Thu, 20 Dec 2018, 22:29
That is pretty bad. I have 12 x 190 W (nameplate rated) panels on my shed - 2280 W as a maximum. In early November after a cool rainy day the sun shone gaily upon the panels. They pushed out 2800 W for a few minutes before they heated up too much.
My rooftop system is rated at 5.7KW but our angles are sub-optimal (facing NW rather than N and about 15° from horizontal) and we get up to 4900W peak out of those, I think we've flatlined our 5kW inverter a couple of times since we installed about 7 years ago.

I've been told that it is "usual" for rooftop panels to produce closer to their rated output than lighter, flexible panels (these aren't amorphous crystal cells, BTW), not sure how true that is.

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Re: Solar Panel output: Rating versus Reality

Post by rhills » Thu, 20 Dec 2018, 23:05

Hi Brendon,
brendon_m wrote:
Thu, 20 Dec 2018, 22:36
In the automotive world of panels you normally expect to get 80% of the rated output as a maximum and shading /angles etc degrade the output from there. 60% would be a pass if I was testing it.
Thanks for this and for your more detailed subsequent posting.

I actually go to a lot of effort to ensure my (mobile camping) panels aren't shaded and are as close as possible to perpendicular to the sun, hence my concerns about their performance. On the day I had one of the panels tested, the people testing it for me had one of their own panels (an 80W polycrystalline, rigid panel) side-by-side with mine. On the same MPPT regulator their panel produced pretty much the same output as mine, ie 75W, which was much closer to its nameplate rating.

Would you really rate 60% of nameplate as a pass if testing under ideal conditions?
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Re: Solar Panel output: Rating versus Reality

Post by rhills » Thu, 20 Dec 2018, 23:12

brendon_m wrote:
Thu, 20 Dec 2018, 22:56
https://www.redarc.com.au/solar-faqs
How many solar panels do I need?

The amount of solar panels you require will be determined by the amount of charge that you need to return to your system during the sunlight hours of each day. For a 12V system, if you need to replace 100A/h of charge to your batteries each day, and you have 8 sunlight hours in each day you will need...

100AH x 12V = 1200WH
1200WH / 8H = 150W of solar panels.

In reality it is recommended that you always overrate your requirements by at least 20%, therefore you would need 180W of solar panels.
I guess the problem was I'd thought I had overrated quite heavily by buying 2 x 134W panels when I'd calculated my needs as being about 200W x 6 hours per day (there may be 8 hours or more of sunshine in a day, but in my experience by the time you allow for intermittent shading, sub-optimal angles because you don't spend every minute of the day tweaking the panels, etc, 6 hours is probably safer).

So, I'd hoped to get 1200WH a day from my panels, but we're struggling to get over half that in practice. Hence my concern.

Cheers,
Last edited by rhills on Fri, 04 Jan 2019, 23:43, edited 1 time in total.
Reason: Removed my reply from inside the quoted previous post.
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Re: Solar Panel output: Rating versus Reality

Post by brendon_m » Thu, 20 Dec 2018, 23:18

If it was a flexible panel, yes but we never get to test in ideal conditions either. Its always cloudy when people bring their panels in...

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Re: Solar Panel output: Rating versus Reality

Post by T1 Terry » Fri, 21 Dec 2018, 10:59

Been doing this stuff for around 15yrs, doing other people's set ups for a bit over 10yrs and lithium house batteries for a bit over 7 yrs. The basic mud map info is, any thing more than 70% of the genuine solar capacity is a bonus. To get the genuine Imp or the max amps you can expect, load the panel output using halogen bulbs until you pull the voltage down to the expected battery voltage you want to charge. Now change the halogen bulbs into a series/parallel combination and slowly remove load until you see the max amps start to drop off, then load it up again to the point you see the max amps and the max voltage, this is the Vmp or the point you could get the most output in watts.
Why do you need to go through all this when there is a label on the back of the panel that gives you all this information? That information rarely has any verification that it isn't a work of fiction. Those test results if carried out properly were the result of a light flash from a light box built to imitate the sun. Only a flash because the panel must remain at 25*C throughout the test. In real world conditions, the panel can't possibly remain at 25*C if it remains at the perfect angle to the sun, 70*C is the more likely operating temp in an Aust summer.
As far as semi flexible panels, correctly mounted to allow the heat from behind the panel to dissipate, will out perform a similar surface area rigid panel.
This is a 5th wheeler van with 19 x 100w panels fitted using PWM control for battery charging. Even though the panels are flat mounted, a charge current of 120 plus amps is frequently seen flowing into the 400Ah lithium battery
1900w semi flexible solar completed job small.jpg
1900w semi flexible solar completed job small.jpg (80.65 KiB) Viewed 2042 times
Solar output 1.jpg
Solar output 1.jpg (62.83 KiB) Viewed 2042 times
It you do the sums, that is 87% efficiency from solar advertised capacity to actual charging watts, I've never achieved that from flat mounted rigid panels.

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Re: Solar Panel output: Rating versus Reality

Post by rhills » Sat, 22 Dec 2018, 00:11

Hi Terry, thanks for the detailed post.

Your test with Halogen lamps sounds like you're replicating what a good MPPT regulator should do.

I'm guessing you have a test board set up with halogen bulbs and switches you can use to manipulate the load. I'd imagine trying to do it with bulbs strung together manually would be a nightmare.

It would be good to have something you could use to test panels with before taking delivery of them. I need to replace the panels on our yacht but after this experience, I'm very reluctant to buy something from interstate again no matter how reputable the vendor seems to be. For that matter, I'm reluctant to buy anything I haven't tested to ensure it produces a decent proportion of the claimed output.

I should mention these panels were bought from a well-established business in Melbourne, not cheapies from FleaBay!
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Re: Solar Panel output: Rating versus Reality

Post by T1 Terry » Fri, 28 Dec 2018, 11:45

I should mention these panels were bought from a well-established business in Melbourne, not cheapies from FleaBay!
I can guess you bought these panels from Solar4RV, they aren't the cheapest supplier but they do stand by the products they sell so talk to them about your problems and I think they will have a solution that you are happy with and can set up a system that does what you are hoping to build. One important point is the "these panels can be walked on" myth, they can be walked on if you are trained like that "grass hopper"bloke on the tv series from yrs ago and can walk across rice paper without tearing it. Heal strike will destroy any solar panel because the pressure is far greater than most realises and this cracks the modules and ends the panel output, just one cracked and that's it, the voltage reduces to the point you get no current output at battery voltage.

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Re: Solar Panel output: Rating versus Reality

Post by Paul9 » Sat, 29 Dec 2018, 07:37

Just a bit of a warning.

About 3 years ago I purchased a flexible panel to put on the top of my EV to keep the aux battery topped up. Saves me having to use the DC-DC converter. The panel conked out after about 2 and a half years. When I purchased the panel I also researched methods of attaching it to the roof of the car, primarily in an attempt to prevent someone coming along with a screwdriver and helping themselves to a new solar panel.

The most secure method was to use high bond glue and I used plenty of it!

It turns out the reason my panel only lasted less than 3 years was because there was no "give" between the panel and the roof of the car. Constant heating and cooling of the panel and the roof, which are made of different metals, actually pulled the connections in the panel away from the panels modules. It also resulted in slight "bowing" of the panel which again helped pull the connectors apart.

Most flex panel suppliers no longer advise glueing to a roof but to use a Very High Bond adhesive tape which, while giving a very high bond, maintains a gap between the panel and the surface on which it is mounted. This allows air to circulate between the surfaces and the tape has "give" in it allowing the panel to expand and contract at its own rate.

I can also advise that the rigid panels on my house give closer to their rated output than the flex panel on my car.

Regards
Paul

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Re: Solar Panel output: Rating versus Reality

Post by T1 Terry » Mon, 31 Dec 2018, 09:44

Hi Paul, If you look at he photo you can see how we mount these panels. After 3 plus yrs of bleeding money replacing failed panels we have now come up with a method that does work.
You need a sheet of 10mm Lexan Thermoclear to go under the panel, 6 feet (or eight feet if you have the 8 eyelet type) made from 3mm aluminium plate around 30mm square and a counter sunk 25mm x 4mm stainless screw through it so the edge of the foot will line up with the panel edges when the screw passes through the eyelet, make sure you add a lock nut to hold the screw to the aluminium plate if you ever want to remove the panel, but it would make an excellent anti theft device if you didn't. Put the panel and feet on the roof and mark out where the feet will sit, apply some glue and set the feet. Once the glue dries lift the panel off and fit the sheet of Lexan Thermoclear over the 4mm studs, then the panel and last a triangular off cut from the 3mm aluminium as a washer or with hole tapped through it to act as a nut, tighten this down until you can just move the panel eyelet, a lock nut on top and a bit of the same glue over the nut and thread. If the leading edge faces into the oncoming wind you will need to add a bit of aluminium angle across the front to act as an air deflector.
The secret is to allow the panel freedom to move and the hollow thermoclear will dissipate the heat from the back of the panel. This cools the panel enough for it to outperform a rigid panel of the same rated output and/or the same surface area ... as long as the Vmp is around the 16v to 18v, the 22v Vmp and 12v Vmp panels aren't really much good for anything and that is why they are so cheap.

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Re: Solar Panel output: Rating versus Reality

Post by Johny » Mon, 31 Dec 2018, 11:28

Thanks Terry. We will be getting a couple of 80 watt semi flexible panels to mount on the yacht hatches this summer and I've been wondering how to not-cook them.
I need enough power to support 24ah each day (fridge) and one may be shaded by a sail a lot of the time.
Bunnies sell a product similar to the Lexan product.

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Re: Solar Panel output: Rating versus Reality

Post by T1 Terry » Tue, 01 Jan 2019, 11:45

If you find your local Lexan supplier/warehouse they have off cuts and protection cover sheets they are happy to cut to size for you at a very reasonable cost I discovered. The 100w panel are just the right size to get the max number of pieces out of one full sheet but also seem to match up to the most common off cut sizes as well, no idea about the 80 panel sizes though. If you are fitting more than one panel you will need to add a blocking diode or the current backflow when the solar controller disconnects the panels (because the battery is fully charged) will flow into the weakest module in the panel and destroy it, generally appearing as brown areas or jiggered cracks through the module. Loose one module and the panels is as good as dead.

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Re: Solar Panel output: Rating versus Reality

Post by Johny » Tue, 01 Jan 2019, 12:15

Thanks Terry. Understood about the diode - I've followed most of your posts on panels over the years.

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Re: Solar Panel output: Rating versus Reality

Post by rhills » Wed, 02 Jan 2019, 19:07

Hi Terry,

Thanks for your detailed info re mounting and your and others' posts re possible issues. Our panels are from the supplier you mentioned and they actually look very similar to the ones you pictured in your earlier post of the top of a bus (?), except that our panels are nearly square. Ours are 6 x 7 cells, custom made to that shape so the two panels cover our motorhome windscreen. They have not ever been walked on, though some flexing may have occurred early on. Each panel is now permanently attached to a frame made of 25mm square aluminium box-section with the back completely open. We either mount them on the windscreen if we're forced to park in the sun, or preferably somewhere away from the van (because it's in the shade of course), in the sun, tilted up and angled as closely as possible to perpendicular to the sun.
T1 Terry wrote:
Mon, 31 Dec 2018, 09:44
... as long as the Vmp is around the 16v to 18v, the 22v Vmp and 12v Vmp panels aren't really much good for anything and that is why they are so cheap...
Our panels actually have a rated Vmp of 23.1, so your comments about the 22v Vmp and 12v Vmp panels are interesting. What do you make of 23v Vmp? In fact on several different occasions, I've measured the Vmp at 19V, substantially below their rating.

At the end of the day, their rated peak output is 134W per panel. Our recent trip away to the SW of WA has confirmed our previous field observations of a maximum of 130W from both panels connected in parallel. They actually start at up to 130W (ie 65W per panel but then fall away to 110-120W (55-60W per panel) after about 5 minutes. This is of course with a cloudless sky, panels unshaded and at 90°to the sun.

Terry, you mentioned in an earlier post contacting the panel seller. I've emailed him with a detailed description of my problem but his reply was to check the Voc and Isc values (rated at 27.72V and 6.26A respectively). I've measured these values and found them to be very close to spec so his response was that this suggests there's nothing wrong with the panels. He's pointing the finger at the MPPT controller saying he has no control over how they perform. The problem with that theory is that I've had the same results with two completely different controllers, one being the Votronic controller he sold me with the panels. For unrelated reasons, that controller is now connected to another set of panels mounted to our van's rooftop (rigid panels that came with the van and deliver very close to their peak output rating under ideal conditions BTW). The flexible panels are now feeding through an Intervolt BCC (DC-DC charger with inbuilt MPPT controller). I've actually been up to Intervolt (an excellent WA company with a superb product and enthusiastic support in my experience) and had their guys test my panels with various instruments which also show what I have observed, that their Vmp is 4V below the rating and at the panel's maximum power point on the day it was producing half its rated output. I should point out that the Intervolt techs had an insolation meter on the day showing 1100w/m[sup]2[/sup] and a rigid panel rated at 80W that produced 76W measured on the same equipment.

What I don't understand is how the Voc can be 27V but the Vmp then drops to 19V. These measurements are through the same connectors BTW, so I believe that excludes excessive resistance through the connectors. FWIW, the connectors don't seem to get any hotter than the cables do in the sun.

I think my main problem here is that I can't easily take the panels back to the seller and say "here, can you please test them on your equipment and show me that they perform as expected". I'm in WA, he's in Victoria and freighting the panels back to him will be expensive. I'd pay the freight if I could be confident that I'd get back panels that perform to a fair expectation.
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Re: Solar Panel output: Rating versus Reality

Post by T1 Terry » Thu, 03 Jan 2019, 13:08

Voc is volts @ zero current, Isc is amps @ 0v, test figures but meaningless otherwise because the only power measurement is watts and that requires a mix of both parts. That 23.1v Vmp is at 25*C on a flash light box and also only a test figure and really only useful for comparing other panels of the same type on a light box calibrated to the same specs.
As soon as the panel goes in the sun the internal temp goes up and the power coefficient drops by 0.46% per *C over 25*C according to Kyocera's charts and they seem to be the only manufacturer that gives out this sort of information. Even Kyocera won't give the losses per *C over 25*C for Vmp or Imp, but they do supply an interesting chart that shows at 25*C and around 18v the current starts to drop off rapidly, at 75*C that voltage is down around the 12.5v mark when it starts to drop off.
Kyocera 140w panel 1.jpg
Kyocera 140w panel 1.jpg (47.43 KiB) Viewed 1853 times
If you do the rough math, 75*C - 25*C = 50*C x 0.46% = 23% 23.1v x 1% = 0.231v x 23% = 5.313v. The Vmp @ 25*C is 23.1v - the temp coefficient loss of 5.313v = 17.787v. I know the percentage loss is for total power, not just the Vmp, so the math is vague at best, but you get the general idea.

Unless you have some method of moving the heat out of the panel the temp will easily climb over 75*C inside the panel. This is why we put the Thermoclear under the panel, so it can absorb the heat and disperse it via the natural airflow that develops as the cool air replaces the hot air through the hollow sections. Even better would be to water cool the panels, but that starts to get a bit technical. By building a frame behind the panel, the air trapped within the frame can only rise and that traps the heat in the panel rather than moving it away from behind the panel.
We did make a few of the 100w panels into portables by mounting them on a piece of that open aluminium screen they use on the outside of flyscreen doors, with a narrow bit hinged as a leg to stand it up. They worked quite well as long as the wind didn't blow them away because they were so light.
Can you modify the attachment of the panel to the frame to allow an air gap between the two so the heat can get away?

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Re: Solar Panel output: Rating versus Reality

Post by rhills » Thu, 03 Jan 2019, 15:36

How solid is Thermoclear Terry? The main reason for my home-made frame was to prevent the panels bending too much while being handled (or wind-blown) but also to leave them light enough to make for easy handling. The frame has turned out to be much bulkier than I would like anyway so I'm happy to ditch it. Might the panel + thermoclear backing be rigid enough for reasonably gentle handling + propping up to the right angle?

Do you do mobile panels for your clients? If so, how do you frame them? Or do you use rigid panels for this purpose?

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Re: Solar Panel output: Rating versus Reality

Post by T1 Terry » Fri, 04 Jan 2019, 10:29

We actually made portable semi flexible panels on request using either Thermoclear or the aluminium mesh as a backing. Thermoclear can be walked on so it is strong so plenty strong enough to use as a panel backing.

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Re: Solar Panel output: Rating versus Reality

Post by rhills » Fri, 04 Jan 2019, 23:53

Hi Terry,

Thanks for that, I've emailed Ampelite's Perth office to see if they have someone that sells Thermoclear near us.

Now I just have to see if I can find somewhere in Perth that sells the glue, I gather you recommend Simson 70-03 or Selleys Armourflex.

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Re: Solar Panel output: Rating versus Reality

Post by T1 Terry » Sat, 05 Jan 2019, 12:41

rhills wrote:
Fri, 04 Jan 2019, 23:53
Hi Terry,

Thanks for that, I've emailed Ampelite's Perth office to see if they have someone that sells Thermoclear near us.

Now I just have to see if I can find somewhere in Perth that sells the glue, I gather you recommend Simson 70-03 or Selleys Armourflex.

Cheers,
Selleys Armourflex is by far the easiest to use, just roughen up the surfaces and clean down with a metho soaked rag. Armourflex is available in the big sausage tubes from Bunnings both in NSW and here in SA, so I'm guessing it would be on the shelf over there as well.
Simson 70-03 is a brilliant high temp adhesive, requires the special cleaner/prep solution applied first much like Sikkaflex, but once it's on you need to cut it to get it off. It isn't that easy to find a stockist, it isn't cheap and in this application I think it's over kill.

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Re: Solar Panel output: Rating versus Reality

Post by rhills » Thu, 10 Jan 2019, 21:57

Hi Terry,

Well, got my Thermoclear, got my Armourflex and have lightly sanded and cleaned down the back of a panel and the Thermoclear, spread around the Armourflex and stuck down the Thermoclear. I've got a plywood panel on it and some weights to hold things together without damaging the cells (the panel is face down on its original packing foam).

What I can't find anywhere is how long Armourflex takes to cure. Nothing on the product container, nothing on the Selley's website pages about the product. The only info I can find is that an opened container should keep for 6 months in a cool, dark place. I'm hoping I don't have to keep the weights on my panel for that long!

Any thoughts?
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Re: Solar Panel output: Rating versus Reality

Post by T1 Terry » Fri, 11 Jan 2019, 10:13

rhills wrote:
Thu, 10 Jan 2019, 21:57
Hi Terry,

Well, got my Thermoclear, got my Armourflex and have lightly sanded and cleaned down the back of a panel and the Thermoclear, spread around the Armourflex and stuck down the Thermoclear. I've got a plywood panel on it and some weights to hold things together without damaging the cells (the panel is face down on its original packing foam).

What I can't find anywhere is how long Armourflex takes to cure. Nothing on the product container, nothing on the Selley's website pages about the product. The only info I can find is that an opened container should keep for 6 months in a cool, dark place. I'm hoping I don't have to keep the weights on my panel for that long!

Any thoughts?
Ummm.... you weren't supposed the glue the panel to the thermoclear because the expand at different rates. We use something to act as a big washer and countersink a 4mm screw through it so it passes through the thermoclear and the eyelet in the panel, then a washer of some sort to stop the eyelet coming off the screw and a nut and lock nut the hold it in place yet loose enough for the panel to move separate from the thermoclear. As long as the airflow through the thermoclear is good like the ribs flowing from top to bottom with the panel standing up as a portable panel it should be ok. If the ribs go across the panel it should probably stand on its side so the airflow is at its best.
Thermoclear will skin dry in an hr if its humid but take up to a week to cure all the way through if it is thick. Turps will remove cured Armourflex by frying up the edges, metho will remove uncured Armourflex. It actually requires cutting through the Armourflex if it has fully cured and fully bonded to both surfaces.
We use 2 ltr milk bottles full of water as weights to hold the solar panels down till the glue dries because each one only weighs 2kg and if the fall off the roof of an RV they just make a mess rather than damaging things.

T1 Terry
We use the Armourflex to glue the feet (washers under the thermoclear) to the roof so they stay where they are put but easy to remove if needed.
Green but want to learn

rhills
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Real Name: Rob Hills
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Re: Solar Panel output: Rating versus Reality

Post by rhills » Sat, 12 Jan 2019, 00:28

Ah, I'd misunderstood obviously. I'd thought the differential expansion problem was between the metal of the vehicle roof and the plastic base of the panel. I'd assumed that the expansion/contraction rates of the plastics in the panel base and the thermoclear would be similar.

As you say, hopefully it will work out. I'd planned to mount the panels with the channels running vertically (or "up hill" at least) to facilitate convection currents.
Rob Hills
AEVA Webmaster
2014 Mitsubishi Outlander Aspire PHEV
Jul 2014 - Mar 2019
Total Petrol: 646.6L
ODO: 47979
Av Consumption: 1.35 L/100km

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