Boxer Symbol Solar Panel

Hi
I'm embarking on research for adding panel(s) to my 2002 Symbol.
the prob I've got is that the leisure battery isn't used often and especialloy in the winter.
I don't have theluxury of a hooking up to a charger where I'm parked and I would like to run my Eberspacher heater for around 4 hours every month in the winter.
Anybody out there been faced with a similar problem?
What size wattage panels shold stand up to re-charge the battery in winter for the amount of use I need.

Thanks in advance!

ffisher wrote:
What size wattage panels should stand up to re-charge the battery in winter for the amount of use I need.

The easy answer is as big as you can afford.

The problem with solar is it's dependent on the weather and time of year. As you say winter will be the problem when it's overcast and not many hours of daylight, so you need a panel that is high efficiency and high power.

Generally 100W is probably the minimum I'd go for in this circumstance. That should keep your battery easily topped up during the winter months when you're not using the van. If you want to use the van and need the solar to not just top up, but fully charge a battery from a previous nights use, you'll probably need a fair bit more and to be honest in winter I think you could struggle on grey murky days anyway.

I helped fit a 140W flexible panel to my friends VW T25 poptop roof recently, and there was easily room for a second one alongside so the options are pretty varied.

Thanks for your reply. Unfortunatley I've onlhy a small area on teh roof due to roof lights. However I can get two 50w panels up there.

For just keeping the battery topped up, that should be fine.

Hi, Any tips on the type of panel, ie. semi flexible or rigid. My Boxer roof is flat but has ridges running along it's length?

There are pros and cons to each. It depends for a start on how deep the ridges are. If they are deep (as on an unconverted T5) then it can cause issues. If not, the Sikaflex (or whatever) will probably bridge no problem.

A flexi panel will typically be much neater and will probably save a small amount in fuel consumption. However, you pretty much have to accept that if you commit to a flexi panel, it will be a lot more aggravation if it fails than removing and replacing a rigid/framed panel.

There is also a suggestion that rigid panels can be slightly more efficient in really hot conditions than a flexi panel, though I suspect that this is conjecture rather than measured - I have never measured the difference either, so I don't know.

-mojo- wrote:There is also a suggestion that rigid panels can be slightly more efficient in really hot conditions than a flexi panel, though I suspect that this is conjecture rather than measured - I have never measured the difference either, so I don't know.

This is true. Solar panel efficiency decreases with increases in temperature, which is ironic since you want direct sun to get the best production, which in turn heats up the panel.

A solid panel mounted with room for air to flow beneath allows better cooling and so works more efficiently than a panel glued straight to the roof that is then not only difficult to cool but will probably suffer from being in contact with the raised temperature of the roof itself. As you say though, in practice I doubt you would really suffer any noticeable lack of power from it unless it's a metal roof.

That said, my friend has a flexi panel we fitted to his T25 poptop roof (also has ridges and had no problems) and he gets very good voltages on sunny days.

You can also get brackets to mount flexi panels so they are effectively removable, but it sort of negates the reasons to get a flexi panel if you see what i mean.

If fitting a flexi to a roof with contours and ridges, just get something of a decent weight to sit on the panel until the sikaflex cures - we used some old car batteries that were lying around, and as above.

We also used some polystyrene to pack any larger gaps between ridges and then fill with sikaflex. The polystyrene (or sponge would do) just allows a surface to be present to build the sikaflex up against, otherwise it will pile in behind itself and you end up using tons of the stuff just for gap filling which ends up costly.

Noggin wrote:
This is true. Solar panel efficiency decreases with increases in temperature, which is ironic since you want direct sun to get the best production, which in turn heats up the panel.

A solid panel mounted with room for air to flow beneath allows better cooling and so works more efficiently than a panel glued straight to the roof that is then not only difficult to cool but will probably suffer from being in contact with the raised temperature of the roof itself. As you say though, in practice I doubt you would really suffer any noticeable lack of power from it unless it's a metal roof.

Agreed, but... I suspect that a lot of people's views about solar panel cooling are based upon what they ~think~ they know, rather than what they ~actually~ know!

Specifically, I strongly suspect that people who comment on this may be over-estimating the effect of air cooling, and underestimating the effect of heat conduction into an underlying surface that the panel is bonded onto.

In particular, I don't really understand the fact that people pick out metal roof materials as being especially bad, when metals such as steel are known to be good at conducting heat, and therefore should be good at conducting heat away from the panel...

Unfortunately my panel is bonded onto GRP and I don't have a setup which allows me to test the temperature of this versus one mounted on steel, but my money would be on the difference being a few degrees at most in real world full UK sun conditions - so, given that the efficiency decrease is typically only 0.4% per degree above 25 degrees C - it's probably not as big a factor as it's often given credit for, and can probably be ignored by most users.

You're right that it can be pretty much ignored for the common man fitting a panel to a MH roof.

With a metal roof, while you mention conducting heat away which would be ideal on cooler days, especially in winter so as you say actually not a bad thing, on a hot day in direct sun it's more likely to conduct to the panel and between the two of them reduce efficiency a fair bit since metal roofs could easily be getting over 50 degrees if not more on a hot day.

To be fair, it's not really worth going into for our purposes as there's little we can do. I agree I wouldn't take it into account for my own choice of panel, but some think it's worth considering.

It's still a puzzle to me, because although a small number of panels these days are on a plastic substrate, most are not. Most are already bonded onto an aluminium base plate, as shipped from the factory.

So I really struggle to understand why putting a solar panel which itself has a metal base onto a metal roof would somehow cause the heat gain to be worse, or the temperature be any hotter, than placing the same metal-based panel onto a GRP roof. But, as I said above, not everyone thinks about this in quite the same way...

If your van roof is white steel, it probably won't get as hot as the black solar panel, so there might be some benefit to having a cooler heat sink for the panel, though it won't then radiate the heat away as well as a black roof would...

If the van roof is GRP, then mounting a flat black solar panel directly to it must make the panel hotter than if there was some air flow under it...

It'd be (vaguely) interesting to check these suppositions, but our panel kept the batteries fully charged even in the rubbish weather we had in France a week or so ago, so I don't care much

This discussion is like the "would a bullet fired vertically kill you as it came down" or "can I survive falling into the sea from an airliner" perennial arguments.

meanchris wrote:
This discussion is like the "would a bullet fired vertically kill you as it came down" or "can I survive falling into the sea from an airliner" perennial arguments.

Except that people are deciding (at least partly) on the everyday issue of which type of solar panel to use, based on these suppositions. Hopefully your life is not so much more interesting than mine that you need the answers to either of the above in the course of your day-to-day decisions...

I would not mind if it were not so common for these suppositions to be presented as fact, over and over again.

I should add that I'm not referring to any thread on the subject here - but I have seen threads on other forums where people write things like "Do not bond a flexible panel to the roof of your van or it will overheat and be damaged". It's very rare for people on these threads to acknowledge that they don't actually ~know~ this for a fact... typically their response is just "that it is obvious".

I found that on a sunny day my panel had topped up the battery before breakfast, so the heat was not a problem. 
Surely increased sunlight will give increased output that should outweigh any fall off in performance.
Presumably the panels are made to withstand the hottest sun they are likely to encounter. In strong summer sun the panel will be way too hot to touch, even in UK, 
I haven't seen any reports of panels failing because they've got too hot, however they have been mounted. As it is in the fresh air they should be OK, but ventilation to keep them a little cooler can only be a good thing.
For me the critical thing is the integrity of the fixing, driving into a headwind and passing a truck could conceivably give effective gusts of well over 120 mph in random directions.