Guide to caravan solar panels - Practical Caravan

Free power whenever there’s daylight? Go solar, says John Sootheran in this guide to caravan solar panels

It really doesn’t get much greener than solar power. Modern photovoltaic solar panels take natural light and turn it into electricity. The more light, the more power you get, but even cloudy days create some current.
Solar panels are ideal for keeping your leisure battery topped up in summer, or providing the power for low-draw devices, such as alarms and trackers, when a ‘van’s in storage for months.
They’re a boon for off-gridders, too, providing enough current to keep your leisure battery charged when you have no hook-up. So to sum up, solar panels are very effective and relatively cheap, easy to use and eco-friendly.

How do they work?

Photovoltaic solar panels are covered in a thin layer of silicon. When sunlight strikes the panel, photons are absorbed, which causes electrons to separate from the silicon atoms and move about. This creates a DC electric current, which is ‘collected’ and directed, via a controller, to charge your leisure battery. Typically, a motorhome solar panel creates 17-18V of charge.

Types of solar panel

Silicon solar cells are currently available in three main types, which are known as monocrystalline, polycrystalline and thin-film amorphous. Monocrystalline cells are made up of a single silicon crystal; polycrystalline comprise fragments of silicon.
The mono versions are more efficient, because they allow electrons to move more freely, but they also tend to cost more. These crystalline panels work at efficiency levels of 15-20%.
Monocrystalline panel cells appear as a single, flat colour; polycrystalline versions have a grain and visible edges.
Thin-film panels use a completely different technology to that of mono and poly panels. At present, they are considered new tech and, while they are less efficient than the other two (except in low light), great advances are expected in the next decade.
Already, researchers have achieved 23.4% efficiency with them, although commercially available versions typically operate in the range of 10-15%.
Owing to their lower production cost, thin-film panels are expected to see the biggest growth in future. These types of panel are flat-black in colour.
Solar panels have an anti-reflective coating, which increases the absorption of sunlight, allowing the maximum amount to reach the silicon cells.

Freestanding panels

Freestanding panels can be moved and used for different applications. They often comprise two panels, hinged in the middle, which fold up into a briefcase-style portable unit.
They can be moved throughout the day to maximise the angle of the sun, so they can be the most efficient.

Rigid frame panels

Rigid panels are flat, so ideal for roof mounting on a ‘van, which optimises exposure to the sun. They are strong and durable, but can be heavy.
Heat build-up lessens efficiency, so it’s a good idea to leave a gap below the panel to allow for increased cooling airflow. Roof-mounted panels should be cleaned on a regular basis, because any deposits of grime or bird lime will affect their efficiency.

Flexible panel

Thin, flexible panels are robust, light and low-profile, and can easily be bonded to the roof of a leisure vehicle.
They’re 100% waterproof and some have a ‘self-healing’ top layer, where scratches and abrasions are filled in.

Choosing a panel

Experts say that minor differences in efficiency are not as important as buying a respected brand of panel, with good build quality.
Solar panel warranties vary. Some provide just one year, although we would recommend a five-year product warranty (especially on flexible panels). Some firms, such as Solar Technology International, offer their customers a 20-year performance warranty.
Bonded panels are hard to remove, while framed, bolt-on ones can be moved to your next motorhome, but there’ll be holes left to fill.
Many of the better panels will have IEC 61215 quality certification.

What else will I need?

All panels over 18W require a voltage regulator/control panel, fitted between the panel and the battery. This regulates the flow of charge and indicates the level of solar power being generated.
Typically, solar kits have cheaper pulse width modulation controllers. These switches connect a solar array to a battery and manage the voltage of the array to match that of the battery. We recommend buying a maximum power point tracking (MPPT) charge controller. These are more expensive, but they create a solar power system that’s up to 20% more efficient.
They modify voltage from the panel to match the battery’s requirements, so whatever the weather, your panel will deliver maximum power.
You’ll also need cabling, a fitting kit or bonding agent and a waterproof gland, to protect the hole where the cable enters.
Solar charge is stored in the leisure battery. We recommend fitting the largest battery you can, so you have power for those times when the sun isn’t shining. A 100Ah lead-acid/AGM battery is ideal.
Alternatively, you might consider a lithium leisure battery, such as the Lifos LiFePO4 lithium iron phosphate battery. These are more expensive, but deliver many advantages over traditional batteries. As well as being 33% smaller and 75% lighter, they are far more efficient and can be 95% discharged with no damage.
A 68Ah Lifos battery contains the same amount of charge as a 120Ah lead-acid battery. Manufacturers reckon they also last up to seven times longer than traditional leisure batteries.
Lifos provides a five-year warranty, and they are available in two sizes, 68Ah and 105Ah (equivalent to a 200Ah lead-acid battery).

Panel power

The power-generating potential of a solar panel is calculated using the Standard Test Conditions recognised by the industry.
Solar panel efficiency depends on many variables, including the intensity and angle of the light, and temperature (excessive temperatures can make them up to 25% less efficient).
This means that a 100W panel is unlikely to create 100W of power for every hour of daylight. At 8am to 9am, for example, the light intensity might be half that of noon to 1pm.
The angle of your panel has a significant effect on the power it creates, and the optimum angle will depend on your location and vary during the year.
Fortunately, you can find out which are the ideal panel angles wherever you are in the world, on any date, at http://www.solarelectricityhandbook.com/solarcalculator.aspx 

Calculating your power consumption

This is important data, especially if you like to go off-grid. There are several ways to approach this calculation.
If you flatten a 110Ah-rated leisure battery in two days of camping, you’re only actually using about 55Ah of power, because you can’t oompletely discharge a traditional leisure battery. Divide that by two and you’re probably using about 27Ah of electricity per day. That is the amount you need to top up.
Alternatively, calculate how much electrical charge each appliance you use will require. For example, an Avtex 24in TV uses 35W, which divided by 12V is about 3A. If you watch for three hours, that’s 9Ah of power used.
Likewise, an LED light strip draws about 5W of charge, which equates to 0.4A. So if that light is on for five hours, you’ll use 2Ah of charge.
Tot up all of your power usage in this way and you’ll end up with an Ah figure that you need to replace each day.
Alternatively, you can calculate your power consumption using watt/hours. In our example, the Avtex TV will use 3 x 35W – 105W/h, while the lights will use 5 x 5W – 25W/h of power.

Size of panel required

Panels have a rating in watts, typically 5-200W, or higher. To estimate charge created, multiply this figure by the hours of daylight/sunshine exposure.
Wattage rating x hours of daylight/sunshine = Total watts per day. For example: 150W panel x five hours of sunshine = 750W.
It is very important to remember, however, that the intensity of sunlight varies massively throughout the day, and depending on the time of year. On a clear summer’s day, you could easily create 750W of charge, but in the depths of winter, you might only get the equivalent of one hour of useful light.

Where to buy

Cheap solar panels are available online, and if you’re lucky, you might end up with a reasonable installation at a budget price. But all of the experts recommend buying your panels from a recognised, reputable supplier.
As with many such major purchases, in most cases, spending a little more on carefully sourced solar panels will almost certainly yield dividends in terms of power output and longevity – and just as importantly, in customer care, should anything go wrong, or if you have any queries.

Solar panel contacts

Solar supporters

Mark Williams recently fitted a solar panel to his ‘van: “My new solar panel has amazed me. We stayed up very late on the first night, with the ‘van interior and exterior lights on, phones charging, then washed the crockery, and ourselves, making a fair demand on the 12V system. The battery had discharged to below 11V when we went to bed.
“We woke at about 8am, and the leisure battery was fully charged, on less than two hours’ sunlight. Even in winter, the panel is capable of keeping my 110Ah leisure battery fully charged when we are staying off-grid.”
Rod Farrendon is a keen off-gridder and relies on solar power to keep his leisure batteries topped up: “I’m over the moon with my solar panel set-up! This is despite the fact that my first one, which was a 100W flexible model, began to delaminate after only a few months (the company just told me to glue it together).
“I had bonded it to the roof, so I managed to peel it off and instead, bought a 150W rigid panel, which I mounted to the roof with brackets that I bonded on. Id say, yes, price is important, but in the long run, build quality definitely trumps it.
“I also fitted a Votronic Duo MPPT controller, to maximise the charging potential. This allows me to charge both of my leisure batteries.
“The performance is excellent. I’ve never, ever had a problem. Although the experts say that a 100W panel will be ample for off-gridding, a flat panel, mounted on the roof, loses up to 25% in performance, so I decided to increase the panel output by 50%. That way, I’ll definitely get sufficient charge.
“This system keeps my batteries charged, and I now have a Lifos lithium battery, which charges a lot quicker and regulates itself.
“I would definitely recommend solar panels to anyone who prefers touring off-grid.”

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