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…
- 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.
- 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:
- Calculate the maximum current that the solar can provide in full sun (use watts divided by volts – eg 120w panel @ 12v = 10amps)
- 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).
- 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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Tags: battery, Bus Conversion, Caravan, Energy, Motorhome, Power, Solar
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Thanks and enjoy our motorhome website - Gavin & Tracey.
June 27th, 2010 at 10:49 pm
hi there gav
almost ready to get solar panels for bus
done as u said 24v
so whats best?
2x12v panels
or 24v panels
im thinking
24v
if i loose one 12v panel iv lost 2 panels?
im looking at putting 800w on roof
and can get 80w 12v panels for 260 each
so 10 panels
they are shade tolerant
but what specs should i be looking at on panels
like cut off voltage etc
and does this differ from 12v to 24 v panels??
on another subject
i never heard back from that compan y about the inverter
lost cause
but i will be taking it up with ebay
any way safe travels to u and tracy
cheers
john
July 1st, 2010 at 9:25 am
Generally 24v panels are a little cheaper per watt (they are used in large grid connect systems – and thus more of them are produced). When I look at panel specs, I normally look for a “voltage at peak power” value that is a close as possible to my system voltage.
Example:
200w Panel #1 has a VPP (voltage at peak power) of 37v. In my 24v system I calculate that it will produce 5.405 amps at peak (200 / 37 = 5.405)
200w Panel #2 has a VPP of 34v. This should produce 5.88amps at peak (34 / 200 = 5.88) – slightly higher than the output from panel #1 when connected to a 24v system.
Cheers
Gavin
July 7th, 2010 at 7:04 pm
I need clarification?
In the example above am I correct in assuming that each of the 200w panels has it’s own cable to the (12v) battery?
In other words if the example above only had one 200w panel 16mm2 copper would still be needed?
Is it fair to say if the two 200w panels (in parallel) used the same cabling to the battery at least 30mm2 of copper would be needed?
cheers
Andrew
July 8th, 2010 at 9:14 am
I see the reason for your question and I have updated the article to make it more clear.
The short answer is yes – the calculation is for EACH 200w panel.
30sqmm of copper will produce a voltage drop of 0.34v (or 2.833% of 12v) when asked to carry 33amps over 18meters.
Many people consider that it is ok to ignore the voltage drop of the return (the negative wire) if it is connected to the chassis of the vehicle (and thus half the length of the cable for the purpose of the calculation). I feel that this is generally NOT a good plan for solar panels in a mobile situation – also many solar regulators control the negative and in this case you can not connect the panels to the chassis at the panel end.
Gavin
July 17th, 2010 at 9:07 am
hi there guys
firstly the reason behind the question
as u know i had a iberter failure , not due to my doing .is in your other section
so i now have 2 inverters.one for appliances one for fridge
thus giving me added protection of back up for fridge( the main item in any home)
so now to the question
i intend useing around 800w solar/8 batteries@225 amp
if i use one regulator pl40 and it fails ive lost all solar?
can i split into 2 400w and 4 batteries???
and run 2 regulators
thus still having the abiltiy to have at least some power
i dont like splitting systems ,but im thinking back up
i dont think running 2 controllers woudl work(confliction??)
but i think with the amount off batteries it would work with split system
swicthing fridge to second bank isnt a problem.but id have to monitor closer as not to drain one bank below acceptble level
system is 24v
and fridge is 240v ac 130w
which i think would use about 1000w a day running
but if there is no prob then both banks are still charging indevidual
my aim is to make it fool proof , which is some time not the best
cheers john
July 17th, 2010 at 6:28 pm
Hi John,
Coming from the IT industry I understand the desire for redundancy. However my advice would be NOT to split the battery bank and solar. So long as they are correctly and robustly wired the PL series regulators are a picture of reliability. Worst case – you can by-pass the regulator and manually monitor the battery voltage and disconnect the solar when the batteries reach peak voltage.
Splitting is a bad idea for the following reasons…
1. What if one bank is full and the other empty – one half of your expencive panels are doing nothing
2. You will never get full use out of your battery bank and you will shortern the life of the most used bank.
3. Connecting two banks of batteries together when they are not equally charged can cause very high currents to flow between them – this generates heat, can damage the batteries and will result is a loos of energy.
My recommendation would be to put all panels through the same regulator and connect all house batteries together into the same bank.
(I would also recommend a low voltage fridge based on a Danfoss compressor – but that is a different story)
Hope this helps
Gavin
July 17th, 2010 at 7:08 pm
now how did i know u say that about the fridge lmao
well im willing to sacrafice the solar for the fridge
theres enought going up on the roof to do that
and if my fridge dies
its easier to replace than a 24v in any major town
but thats another story
ok see ur point about solar
im a mantance manager wirth big company
and as such i need back up fast
is my whole way off thinking
but i agree with u
i didnt want to spit system
i just wanted to know if it was a good idea or not
every thing on the bus is done ,over board as to say
wiring all heavier than needed
i donttttttttt wana do things twice
your web site is very helpfull
and have learned a lot
and some off your simple things you talk about gets me thinking more
which is good
ive found its always good to toss idea’s around .and hear others idea’s
well the bus is almost finnished inside
engineers cert thursday 22nd july
then rego
and then hope fully a few short trips
read ur article on buy bus from start or already done
well i agrree
im lucky
i have heaps off acces due to my job
and im still going after 18 mnths
but from srcatch well. its not for those who think its simple
cause its not
i have spent upward s of 15k and ive save about 20k doing what ive done
but the out come is looking good
any way’s
maybe see u on the road next year
we intend leaving .before next winter
get dam cold here in southern highlands
kk safe travels
cheers john
August 9th, 2010 at 7:56 pm
hi again
ok ordered 6 x80w pannels
4 x225ah deep cycle batteries
adding more as money becomes avaliable
pl 40 on the way
regadring shunt ?? is there a specila cable for the pl 40??
or is just wires from shunt to pl 40?/// im putting shunt in negative line
ok now the part i need to know
what to hook up in what oreder>>>
pl 40 to batteries
then add pannels one at a time???
24v so in series
everthing will be fused
and longest run from solar pannels to pl 40 is 5 mtrs
and if u got time
any tricky settings on pl 40???
thx
john
August 9th, 2010 at 9:16 pm
Hi John,
You need a shunt adapter to attach the shunt to the PL40 (PLS1 – I think it is called). This attaches to the PL40 via a RJ11 plug and a telephone wire.
The shunt can go in either the +ive or the -ive battery line.
Remove all fuses, then wire and check everything. Once you are sure it is all ok, put the battery fuses in first then the solar fuses.
The PL40 comes with a good diagram – follow this and you will be fine.
Make sure you have set the PL40 to 24v.
Nothing tricky on the PL40 – just make sure you set the correct program for the type of batteries you have.
Be sure NOT to overload the PL40 load terminal (5a max).
Gavin