Once again, no warranty or responsibility is given, offered, or implied from information contained in this article.
For those of us in sunny (ish – I live in the UK, there are limits) countries, who like to go away in our vehicles and who use 12 volt electrical items, solar charging is now possible. In fact, it is quite easy with a small amount of reading and hopefully this article will help.
The theory

• You have a battery that I’m going to call the leisure battery but you can call it a second battery or camping battery. It powers your various camping related items but is not designed to run large appliances like heaters or disco units!
• You want to charge that up for free from the sun so that you are not tethered to an electrical socket on each camp site.

The calculation

• List all equipment that you have in your vehicle that needs powering from 12 volts
• Check on the equipment or in the manual (which is probably online too and you can magnify the text easier!) for the wattage or the amperage. Make a note of each one in a list.
• Think about how often you use each item in each day and be realistic.
• Think about when you camp.

Now, convert the watts to amps by dividing by 12 if you only noted the watts.
Otherwise list each item like this:

• A compressor fridge uses 5 amps and will be in use for 8 hours per day on average when in the UK. That makes 40 amp hours (Ah) per day.
• Lights are 2 watts each and I use 4 of them making 8 watts. In use for 2 hours each. Total is 16 watt hours or 1.33 Ah.
• I use 2 USB sockets that are 7.2 volts and 2 amps which is a maximum of 14.4 watts. Total of 3 hours per day. That is 44 watt hours which is 3.66 Ah.

Then add up all those little amp hours to get a total for the day. In this example I get 45 amp hours. It doesn’t really matter what that means, just get the number!
In the UK I think we can get 6 hours of sunshine per day in the summer and 4 hours in spring and autumn (fall). Your country may be different and your feeling might be different.
You then take the total amp hours, in this case 45 amp hours and divide that by the number of hours of sunshine (aka charging) to get the number of amps needed.

45 amp hours needed across 6 hours in the summer means that I need 7.5 amps. Out of season I need 11.25 amps.

On a full battery running at 14 volts, that’s 14 x 11.25 or nearly 160 watts of solar panels. If you need more than 100 watts, consider more than one panel.
The solar panel itself
They come in 3 main flavours, choose the type you prefer!

• Briefcase – these pack away and get stored unused when travelling. They can plug into the cigarette lighter that connects to the correct battery.
• Solid frame free standing panel – these can either be inclined towards the sun and stood on the grass when you are pitched or could be mounted on the roof of your vehicle. They tend to be the least expensive.
• Flexible and semi-flexible – these tend to be the most expensive and will be the focus of the rest of this article. They can be permanently attached to your vehicle.

Attachment
We purchased a 100 watt solar panel that is “semi flexible” meaning it is about 2mm thick and bends a little but bending too much will break it. We used a Biard panel but other manufacturers are available and the price was around the £130 mark for the supply only of just the panel but this was in 2015. Since then we have upgraded the panel and have used Ecoworthy to get success.
We then purchased a used roof rack and connected it at the back of our bay window to the roof gutters with a little modification as roof racks are generally lower and narrower than a Bay window needs! It did cost £4 ($6) though!
As an expedient, we laid the panel on the grass upside down, laid the roof rack on the panel also upside down and gaffer taped the two together. It was a plan for a couple of months before glueing the panel to the Dormobile roof and 3 years later it was still gaffer taped!
There are two cables coming out of the solar panel and remember they are live except in the dark so tape them up / ensure the ends are not able to create a short circuit
The clever box in the middle
You cannot just wire the solar panel to the leisure battery – bad things will happen. You need a box in the middle that will stop the leisure battery sending its charge out through the solar panel into the night. It stops the solar panel over charging the leisure battery so far that it explodes. You need to avoid that. This clever box is called a regulator or solar controller and comes in 2 main types – PWM (pulse width modulation) and MPPT (maximum power point tracking). The PWM is less efficient at getting power out of the panel into your battery but is generally a lot cheaper. MPPT would be recommended if you camp outside summer as it is more efficient in less than full sunny days or if you have more than one panel. Do not buy an MPPT controller under about £60 as they tend to think they are MPPT controllers but are just expensive PWM controllers.
The clever box for us was a cheap PWM controller that was about £10 / $15 and has just 3 lights. Charging, discharging and battery health (green is good, flashing is full, red is bad). This has since been replaced with a Renogy MPPT controller.

Let’s say that you’re solar panel is 100 watts maximum and on a dummy day is generating 60 watts of power with the panel running at 20 volts. Remember your maths at school, amps is watts over volts, in this case 3 amps.

A PWM controller will take those 3 amps and charge your battery at 3 amps. This will add 3 amp hours every hour of sunshine.

The MPPT controller looks at your battery voltage of 12.0 volts and bring the solar charge down from 20 volts to just more than the battery to charge it. For mathematical simplicity, we will say 12.0 volts. The same 60 watts of sunshine adding 3Ah on a PWM controller is now 5 amps at 12 volts and the MPPT controller sends 5 amps to your battery instead of 3.

Note that a decent MPPT controller such as the Renogy or a Victor will cost you £120+.

Wiring it
We dropped the solar panel cables down the air vent at the back so that there are no new holes on the van and therefore no chances of a water leak! Then got the wires up next to the spare wheel inside the vehicle as the controller is easier to see and that engine bay can get quite warm.

The leisure battery has two wires, leisure battery positive to the hole on the regulator for battery positive. And then battery negative to regulator battery negative.

After the battery has been connected to the controller, connect the solar panel cables to the controller.

Next I connected the solar cables – positive first as it is always live in daylight – to the solar controller / regulator. Now my controller had a light saying it was charging.
Then I checked the voltage coming from the controller to the leisure battery – I was getting over 13 volts!

Next step
Take the “Light out” positive and negative terminals and use those to power your leisure items – DO NOT run anything directly from the leisure battery, then the controller can shut off power if the battery gets low. Erm, that’s it!
Now I have a solar panel permanently on the roof, always charging from dawn until dusk even on the move. The regulator means that even with the engine running and the split charge working, we do not over charge the leisure battery and it works even under the car port in the winter keeping that battery going until spring time.
Total cost of solar system £160. Cost of replacing the leisure battery every year £120. Saving on electricity per night at a camp site is up to £10 per night. This system seriously pays for itself in no time.