Caravan DIY: How to install a solar panel
Step-by-step: how Rob Farrendon selected, installed and improved a solar-power system for off-grid and winter use of his beloved Bailey Unicorn
Words and photos by Rod Farrendon
Before deciding on a solar-power system for my Bailey Unicorn Valencia, I carried out much research, mostly online. I also talked to other caravanners who had fitted solar power and I emailed potential suppliers.
Which solar panel type?
Advice from all these sources indicated 100 watts should satisfy my needs. I'm not one for putting fixings in my caravan roof, if I can find another way. A semi-flexible panel stuck on the roof would be best, I decided.
But what about heat build-up without the circulation of air under the panel? I obtained advice that told me this was, "not a problem, the caravan roof will dissipate the heat".
I decided on a back-contact panel of 25% more efficiency. To explain, the 25% increase in efficiency is obtained in the following way. If you look at the face of a solar panel you will see many small squares bordered and crossed by thin silver foil-type ribbons.
The small squares are the solar cells and the silver ribbons are the connections between them. The greater the output of a panel, the more cells, which means the panel is larger. These silver connections take up surface area that could be used by the cells to produce electric energy.
They take up 25% of the surface area, so, by putting the connections behind the solar cells, you can free up more surface for energy production. You won't see silver ribbons on a back-contact solar panel, but you obviously don't get a 100-watt panel that produces 125 watts because it's 25% more efficient. You simply get a 100-watt panel that's 25% smaller and, of course, lighter.
Which solar panel controller?
Now for the controller. There are two main types, the PWM (Power Width Modulation) and the much more expensive MPPT (Maximum Power Point Tracking). All technical – I just wanted the maximum power from my solar panel.
PWM controllers use a rapid switching on and off method to regulate the power going into the battery, it pulses the charge.
An MPPT controller uses circuitry to control the rate of power, adapts to the battery and can vary the voltage required (like an intelligent battery charger). It can also provide power directly to a DC load connected to the battery.
In short, an MPPT controller uses circuit wizardry to extract up to 30% more energy from a solar panel, and is more efficient in its distribution of power.
I liked the reviews and the sound of the Votronic MPPT Duo; duo meaning it could charge two batteries simultaneously, the bulk of the charging current going to the leisure battery and a trickle charge keeping the starter battery topped up when needed.
I carry a spare wheel for the caravan, a spare gas bottle; in fact, I carry a few spares of this and that. A spare battery? Now there's an idea!
So far, I had the main ingredients of my solar power system sorted: a 100-watt, semi-flexible, back-contact solar panel, producing up to 5.3 amps, a 165-watt MPPT controller, extracting as much energy as possible from the panel and deciding how best to distribute it, and a back-up battery should the worst happen, like the main battery going flat in the middle of nowhere!
I didn't want a battery bank – batteries wired in parallel. If one of the batteries is duff, it will
drag down the other good battery and, of course, it would defeat my idea of a spare emergency battery.
Which changeover switch?
Next task was to source a heavy-duty battery changeover switch. I found one on eBay that switched from battery 1 to battery 2 and then to battery 1 and battery 2 combined. I didn't want the combined option, but it wasn't expensive so I ordered it and took it apart when it arrived.
I then set about changing it to my requirements. I trimmed a little off the contacts, then, using plastic weld, fixed a blocker between them. Now I had a switch that could easily change between the two batteries.
The next task was the positioning of the components. The solar panel is easy, near to one of the rear bump panels that are fitted to the Bailey Unicorn I, as this will act as the cable conduit from roof to under the caravan. The spare battery is going in the front side locker, sealed off from the gas locker and the interior of the caravan, and has its own vent.
The MPPT controller will be positioned at the entry point of the power cables from the solar panel. There are two reasons for this.
Firstly, it is close to the battery; it needs to be less than 2m (to avoid voltage drop). Secondly, it would be above the floor vent I was using for cable entry, so in a cool location as per the fitting instructions.
I fixed an extra piece of batten to the bed box frame and screwed the controller unit to it. I also fixed some plastic brackets on either side to protect the controller from items being stored under bed.
Then, using these positions, I had to accurately measure the length of the power cable run from panel to controller. This is so that I can work out the wire gauge needed, giving a minimum percentage of voltage drop. There are websites you can use to calculate this, rimstar.org being one, or you can ask a friendly supplier to work it out for you.
I couldn't easily find my chosen MPPT controller. I ended up purchasing it from an obliging Chinese gentleman in New Zealand and payment was in Aussie dollars!
I emailed this supplier several times and asked for specifications and advice. He was knowledgeable and helpful. I checked out his other items and shop; he specialised in RV solar equipment, probably big business down under, so I was happy.
I bought the MPPT controller along with a remote display unit (I didn't want to be lifting the bed to see if things were working) and a battery temperature sensor unit (this helps the controller to be even more efficient in how it supplies the battery with energy). It all arrived within a couple of weeks.
I purchased my solar panel from the website, Photonic-universe.com. Again I found it helpful in answering any queries. Incidentally, I now also see they supply the Votronic Duo MPPT controller.
Marking in pencil the position of the panel, I applied the adhesive/sealant, then positioned the panel, smoothed it over with cloth, then covered it, weighted down overnight and ran a bead of sealant around the edges the next day.
When connecting the controller, I used tape to mark the positive cable; mine weren't marked and I did not want to be connecting in reverse polarity. When I'm doing any work on the solar system, I cover the panels; a piece of carpet does the job and a couple of heavy planks if it's windy.
And when connecting batteries, I connected the controller first, then the battery, and kept these panels covered so that they were not producing electricity.
A folding solar panel kit
I have been trying to convert my old pal to off-grid caravanning. It's proving harder than converting my own caravan! I think he's coming round to the idea and may go down the folding solar panel kit route. It is a great and easy way to start off-grid caravanning. In fact, a panel tilted and facing the sun is more efficient than a horizontal panel.
But, as with any alternatives, there are compromises… off I go researching again, and discover the difference in efficiency between tilted and horizontal seems to range between 15% and 27%.
I presume to achieve the higher percentages the panel would need to track the sun all day. So let's take the middle ground of, say, 20% between tilted and horizontal panels. I reckon that could increase when the winter sun is low.
Improving a solar system for winter
For my next project, I wanted to improve my system specifically for winter use, by increasing the panel output.
I decided to add a 50-watt panel to run in parallel with the 100-watt panel I already had. If this improved output by only 25% it should make winter off-grid caravanning a better proposition.
I wasn't going to buy the more expensive back-contact panel, but I made sure that the voltages of the two panels were the same or similar. The labels on the back of the panels are where to look for this information – the maximum power voltage and the open circuit voltage.
It is also a good idea to install blocking diode connectors into the positive feed from each panel. This stops any single panel drawing current from another panel, for instance if one panel is shaded and not producing a charge. So I added another 50-watt panel to my system.
Winter off-gridding here I come!
An MPPT controller is up to 30% more efficient. It can also provide power directly to a DC load connected to the battery, so the best time to use high- energy appliances, like vacuums or even showers, is in daylight.
If you can fit a spare battery safely, it's great to have a fallback.
Be careful when buying on some websites; check out what else the seller is selling. Ask for advice from suppliers.
My complete system has cost around £470 and a good 100-watt folding panel system should cost around £250.