Installing Solar Panels on a Motorhome or Caravan

Installing Solar Panels Correctly on your Motorhome or Caravan

So you have decided to install solar panels onto your caravan or motorhome – great! Here are some important tips that will help you to get the most out of your expensive solar panels.

Wiring size

Take care with the size of the wiring. Using cable with not enough copper will seriously affect how well your solar panels will work.

There are a number of reasons why people fail to install panels with appropriately sized cables…

  1. Auto electricians and manufactures of cable for automotive use take a different (and not very useful) approach to specifying cable sizes. For example, if you purchased a length of 6mm cable from an auto electrical outlet, it would fit through a 6mm hole (ie its outside diameter would be 6mm). This clearly tells us nothing about the amount of copper inside that cable. If it has thick plastic insulation – then clearly it has less copper and is therefore able to carry less current before an unacceptable amount of voltage drop becomes likely.  On the other hand a household or industrial electrician will provide a cable that has 6mm2 (that is 6mm squared) of actual copper. This is a useful description of the cable and can be used to calculate the current carrying capacity of that cable.
  2. When you ask an auto electrician or auto electrical retail outlet for “a length of 10amp cable” – they will offer you a length of cable that will carry 10amps without melting. There is no consideration given to how much voltage will be dropped over the length of that cable – thus it is another useless way of specifying cable size.

The correct way to select a cable for solar installations:

  1. Calculate the maximum current that the solar can provide in full sun (use watts divided by volts – eg 120w panel @ 12v = 10amps)
  2. Measure the total length of cable from the solar to the battery – now double this figure (because there are two cables – (positive and negative) that the current must travel over).
  3. Now work out the voltage drop over this length of cable using the following formula

Voltage drop equals (cable length (in metres) X current (in amps) X 0.017) divided by cable cross-section (in mm.sq).

Example – you have 2 X 200w panels and want to connect them  to the battery bank that is 9 meters from the panels – you are considering a cable that is 4mm sq of copper.
200w divided by 12v = 16.66amps – this is about the maximum current for each of the two panels.

18 (two times 9m) X 16.66 X 0.017 = 5.09 now divide that by 4 (mm sq copper) – this tells us that we will have a voltage drop of 1.27 volts. The highest acceptable voltage drop is 3% of the supply voltage (so in a 12v system,  0.36v). Clearly our 4mm cable is far too small.
For this installation we are going to need length of cable that has at least 16mm sq of copper in each conductor for each panel. That is a sizable lump of cable!

As in interesting note – you would need just 4mm of copper if you were installing the same panels on a 24v vehicle – just one reason why I suggest that 12v is NOT suitable for larger vehicles (where the cable length is an issue).

One last word on wiring … while you are running the cable for your new solar panels, consider running an extra cable or two. You may think you have enough solar now but experience has shown that many people add extra panels at a later date – much simpler to do if the cable is already there!

Mounting the Panels


Take a look at the photo of the roof of this caravan. These panels were installed by a person professing to be a “professional solar installer”. Both poly-crystalline and mono-crystalline solar panels decrease their output as the cell temperatures rises. Thus it is clearly important to keep the panels as cool as possible – not an easy thing to do with something that has to be exposed to the full sun. Air flow plays an important part in keeping the cell temperature down and it is for this reason that all manufactures of these types of panels specify that the panels must be mounted in such a fashion as to allow cooling air to flow beneath the panels. There is not going to be a lot of air flow on the underside of these panels.

Mount your solar panels so that there is an air gap of 75mm – 100mm between the panel and the roof or your caravan or motorhome.  If you have selected the far less efficient amorphous type panels (and why would you?) – this air gap is not necessary (because these panels perform slightly better are they get warmer).

Got a question about solar power on a caravan or motorhome?  Got an issue with yours? Or have you really got a great system that you would like to tell us about? Why not leave a comment in the reply box below – we would love to hear from you.

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59 Responses to “Installing Solar Panels on a Motorhome or Caravan”

  1. Hobo Says:

    Hi Chris, The short answer is yes, that should work just fine.
    The long answer involves a discussion about differing set points on the two regulators and the possibility of a conflict – but I honestly don’t think that will be an issue for you.

  2. Eddy Says:

    I have caravan pre fitted with 4mm dia cable over 5mt distance. Is this cable sufficiently heavy enough to support 2 x 120 watt panels. If not what size cable is required. Many thanks.

    Eddy H

  3. Hobo Says:

    my calculations suggest that a minimum of 8mm of copper would be required in this case.

  4. Phil Says:

    Just came across this site – most informative. I wonder if I could get an opinion on a system that has been recommended to me. In my camper I currently have a single Projecta 120W folding panel connected via a Redarc BCDC1240 charger/MPPT solar controller to my two 105amp AGM batteries. The solar panel is connected to the Redarc via 5 metres of Narva sheathed twin core cable (6mm = 4.58mm sq) and a 50 amp Anderson plug. I want to add more solar capacity. So the plan is to get a second, identical 120W Projecta folding panel, with the same type and length of cable, and feed this cable into the same Anderson plug on the Redarc as the cable from the first panel via a “double adapter” – the two Anderson plugs terminating their respective cables are fed into a single Anderson plug that connects with the Redarc. The double adapter has pos to pos, neg to neg, so in effect joining the two panels in parallel just prior to connecting with the Redarc controller. Is all this OK?

  5. Hobo Says:

    Yep – this sounds fine to me (assuming the Redarc controller can handle the additional panels).

  6. Kevin Says:

    Hi – as I will not be travelling in my motorhome for some months I intend to disconnect my batteries (ignition and house). I have solar panels installed (230w) with a MPPT regulator and would like to know in which order (if any) should I disconnect the cables to the regulator and the battery – can I simply disconnect a battery terminal without causing damage to the regulator and solar panels Thanks

  7. Hobo Says:

    Just remove the positives – the order does not matter.
    However … be aware that all batteries have a self-discharge. This is fairly low for AGM batteries (~3%/mth) – but quite high for flooded batteries (~5-10%/mth). This will over time cause the batteries to become damagingly low. A better long term storage plan is to attach a smart charger or a trickle charger – this will keep the batteries at near full and prolong their life.

  8. Norman Timmins Says:

    I already have a panel on my van don’t know wattage if it is a 90watt panel does it matter if I add a 150 watt panel to it

  9. Hobo Says:

    While not ideal for maximum efficiency – mixed wattage panels will work ok.

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