Selecting Solar Panels for a Motorhome or Caravan

For someone wanting to equip their motorhome or caravan with solar panels to collect free power from the sun, there is an almost endless list of technical terms rumours and myths to wade through.
Terms like polycrystalline, amorphous and temperature co-efficient to name just a few.

Sales pitch aside, how is the average person supposed to decide which solar panel is right for their situation?

Aside from brand specific issues like warrantee and quality of manufacturing, there are some clear differences in the technologies used in creating the solar panels commonly available today.

The three panel technologies are :

Polycrystalline, monocrystalline and amorphous. Below is a feature by feature comparison of each type…

Polycrystalline

poly-crystalline

These are the most common and the cheapest (in terms of dollars per watt).  Manufacturers of polychrystaline panels include:
BP, SX (formally Solarex),Sharp and Kyocera.

They have a conversion efficiency of between 12% and 12.5% – this means that 1 square meter of panel exposed to the sun will on average produce between 120 and 125 watts. All solar panels are rated at a cell temperature of 25 degrees.


Unless you live in sub zero temperatures, your panels will almost always have a cell temperature far in excess of 25 degrees.
Polycrystalline panels reduce their output as the cell temperature increases. Thus you should expect your new 125 watt panel to produce about 14% – 23% less than the rated value – this is the temperature co-efficient of the panel.

Polycrystalline panels have a hardened glass surface that is polished and thus self cleaning (to some degree).

Monocrystalline

mono-crystallinel

These panels were among the first panels to appear on the market.
They have a higher efficiency at around 15% . They are typically a little more expensive but perform slightly better than their polychrystaline cousins in hot conditions with a typical reduction in output of between 12% – 15%.
Well known brands include BP and Siemens.
Monocrystalline panels also have a hardened glass surface that is polished and thus to some degree self cleaning.

Amorphous

amorphous

The best known brand of amorphous panels is UniSolar. They use the term “shade tolerant” heavily in their advertising material.

These panels are quite different from the other two technologies already discussed. They have the photovoltaic material bonded onto a stainless-steel material then costed with a rippled polycarbonate.
This has a number of advantages (and one major disadvantage) – the panels are light and very easy to handle, they are almost indestructible (I have seen a panel that has been ripped off a caravan by the wind only to be run over by a truck – testing showed it was unaffected by this treatment. Don’t try this with either of the glass panels!  These panels also reduce their output less than the other two types when partly shaded (by a twig or bird dropping for example).


The two major downside of this construction is that the surface is not smooth and the panels have a very low efficiency rating.

The small ripples in the polycarbonate have a bad habit of holding dust and dirt.
If the panels are mounted flat on the roof of a motorhome, be prepared to get up there and clean them every few weeks.

Amorphous panels have an efficiency of just 6.3%. This of course means that an 64 watt amorphous panel will be more than twice the size of a glass panel with the same rating. This is a major factor if you have limited room on the roof of your motorhome or caravan.

Amorphous panels do not reduce their output when the cell temperature rises above the rated 25 deg – this means that you are far more likely to get all 64 of the watts that you paid for when you buy one of these panels.

At a glance

  Polycrystalline Monocrystalline Amorphous
Manufacturers BP, SX, Sharp,
Kyocera and others
Siemens, BP and
others
UniSolar
Positives

Lowest cost per watt

Polished glass surface self cleans

Higher efficiency – small size per watt

Polished glass surface self cleans

Highest efficiency – smallest size per watt

Light and easy to handle

Very tough – almost indestructible

Very good high temperature performance

Negatives

Glass construction can be broken

Heavy

Poor performance in hot conditions

Glass construction can be broken

Heavy

Poor performance in hot conditions

Rippled surface holds dirt and dust

Low efficiency – very large panel for given output

 

Conclusion

As you can see, there are many factors to be considered when selecting solar panels. In our situation, we have all three technologies on the roof of our motorhome (not a good idea – it is best to install just one type of panel) . We have no more room left to install additional panels – this is largely due to the four huge 64watt UniSolar panels that take up more than their share of the roof space. With the benefit of hindsight – I would not have purchased these panels, I would install only glass panels. Your situation may of course be different.

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12 Responses to “Selecting Solar Panels for a Motorhome or Caravan”

  1. new caravan Says:

    Great reviews mate very in depth, I was wondering though; you made it pretty clear which technology you think is best but do you have any feelings on which brand is best? Is there much difference in build quality and reliability? I am looking to kit out my ‘van with some of these in the near future and wondered if you had any advice?

  2. Hobo Says:

    I have seen and installed a number of different brands and among the big names I have not really seen any major difference in build quality etc. The warrantees all seem to be about the same.

    One thing I would warn about is the large number of panels that are beginning to appear on eBay. Most of these are fine – but a few are designed to be used in large grid-connected systems. Their open circuit voltage is outside of the normal range. These are not suitable for use on 12v systems.

  3. david Says:

    I have 2x 180 watt monocrystaline panels on my shed roof which can produce 5 amps or about 86 volts o/c this then is converted in a maximum power point tracking charge controller (mppt.) to up to 32 amps at 12 volts into my 4×110 amphr ex telstra batteries . 30 amp. mppt is made in australia . 60 amp also available , they are on ebay . this allows the use of std. solar cable connection from panels to charge controller .
    Does this help ?.
    David.

  4. isis solar Says:

    isis solar…

    Motorhome and Caravan Info Australia » Blog Archive » Selecting Solar Panels for a Motorhome or Caravan…

  5. Ern Jennings Says:

    Hi,

    I wonder if ;you could give us some advise on solar power for our bus as what we want to run is fluo lights, fridge and engel freezer. What wattage would I need to run these, what size inventer, and what size panels, and how many batteries.

    As everyone we talk to has a different oppinion but you have been there and done that so we are hoping you can give us some advise. As for major power we will have a 6 kva generator in the bus.

    Thanking you for such interesting articles.

    Ern

  6. Hobo Says:

    Hi Ern,
    In order to give any kind of meaningful recommendation on sizing, I really need a lot more info. For example you say you want to run a fridge – what size? what type? what voltage? What size is the Engel freezer, how many watts is it rated at?
    Getting the sizing right is very important and well worth the effort of collecting all the required info. We need to know the wattage and voltage of every power using device that will be used and an estimate of the number of hours it will be used each day.

    There is an article here http://hobohome.com/news/?p=263 that should help in understanding the requirements for sizing a solar system for a bus.

  7. Marken Says:

    Having just purchased our caravan and tried it out over the last VERY HOT week in Vic I find that the 90litre fridge running on gas is not (as many people have pointed out) the best option. The beer was cool but not chilled and the milk was dubious in quality.
    Could I run the fridge on 12 volt continuously backed up by a 80Ah battery which is charged by a 100 watt solar panel?
    If I was to add further solar panels (say 170 watts) which have their own controller where do I connect these to the system? Is it through the controller in the caravan (thus using two controllers) or do I connect straight to the battery? I have searched through all (I think) the blogs and reports but cannot find the answer to that question. I know that there will be many more questions from me in the near future so I am watching the site very closely.
    Rgds Marken

  8. Hobo Says:

    Hi Marken,
    Let me be honest right up front – I hate gas fridges with a passion. We had one in our motorhome for what was one of the worst 8 months in my life. I have spoken to very few people who travel in the warmer regions of Australia with a gas fridge who have anything nice to say about them. I honestly feel that if you plan to spend any time in warmer climates a compressor fridge is a must.
    Now to answer your question. Running a three-way absorption fridge from electricity produced from solar is never a good idea. The amount of battery capacity and solar required is just insanity. The concept of cooling the inside of a cabinet using HEAT on the outside of that cabinet is very clever, but massively inefficient. To do what you suggest would require about four times as much battery capacity and many times more solar panels. For the sake of the beer and the milk, consider purchasing a compressor fridge, you will not regret it I promise.

  9. Russ Says:

    Hi, great info. Two questions for you. I need healthy charge and power bank for a Waeco 215l compressor on longer week stays. (7.2a on 12v) If I add an additional 150w panel to roof (total 600) it would seem from your info that my 30w Morningstaregulator would be too small (60w needed?)
    Likewise if I used a portable panel connected to Anderson plug, it would have its own regulator?
    Is this correct?
    I noted that Maxray units seem great value with seemingly good specs (http://www.ebay.com.au/itm/NEW-MaxRay-12v-160W-Solar-Folding-Panel-Kit-Caravan-Boat-Camping-Mono-Charging/291007239893?rt=nc&_soffid=100&_soffType=SaleAndClearence&_trksid=p5731.m3795)
    I use 2 x 120A batteries (AGM) that probably should be 3?
    Maybe I should add a panel and have portable too?

  10. Hobo Says:

    Hi Russ,

    You are correct – 600w at 12v will require a controller of at least 50 amp capacity.
    You could use a portable panel with its own regulator. Be aware that most of these sold have cable that is far to light for the length and current to be carried – this has the effect of vastly reducing their effectiveness.
    Solar is about $1/watt right now – portable panels are about twice that.

  11. Clem Says:

    Hi, really like your site, I have read a lot of your articles, I’m just setting up a fifth wheeler with solar etc. and I would really appreciate your input. I have 4, 150 amp agm batteries a 50amp battery charger when on power. I have all led lights, a 210lt compressor type fridge and a 90lt potable fridge freezer with compressor motor and a diesel heater. I really don’t know what they draw on power, also I have a 1000 w inverter for computer, TV, should I put in a separate circuit or hook it into the 240 v system? It needs to be wired in. Now the tricky part, I have solar on my roof, I wanted to remove 4 panels to put on the van. I have a Flexmax 80amp charge controller Max PV input wattage 12vdc 1000w. I was tolled I could use this type of panel with this controller. Features of this controller are voltage stepdown capability allowing a higher PV array voltage configuration. Just a bit of info on the panels, Sunrise SR-P660250, Standard Power 250w Tolerance 0-+3%, short circuit current 8.96A, open circuit voltage 37.33V, Max power current 8.30A, Max power Voltage 30.1 V, Max system voltage DC 1000, with a 15A fuse I don’t want my batteries hurting. Man what a mouth full. Can you help me out, much appreciated I’ll make a donation.

  12. Hobo Says:

    Hi,

    it sounds like you have everything you need. To answer some questions and make some comments…
    1. I normally recommend 200w panels as an absolute maximum on a vehicle. Panels larger than this can flex and break on rough roads.
    2. The Flexmax MPPT charge controller will handle higher voltage panels and charge 12v panels Maximum charge wattage is 960w @ 12v – but the controller will not be damaged by higher wattage’s (the controller will just limit the charge current to 80amps)
    3. You need a 240v double pole changeover switch OR an auto selection relay (simple to build) to handle switching the van between external mains power and internal inverter power.
    4. You should wire the panels in series up to a maximum of about 10v to 15v BELOW the maximum controller input voltage (using the panel VOC). In your case with the panels listed, two in series. All banks must be the same!

    If you are interested in more info and a detailed diagram, please see the advert on the site for my paid design service.

    Cheers

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