How to choose solar panels and get the most out of them

By Wes Whitworth 12 Min Read

Solar is one of those things that you’re going to need if you plan on pulling up stumps at one campsite for more than a few days, or have your fridge running all day while you’re at work. If you’re staying in one spot camping, so long as you’ve got food and water, power is the next most critical thing (next to beers, of course).

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How solar panels work

Solar, or photovoltaic (photo = light, voltaic = voltage/electricity) panels, whether monocrystalline, polycrystalline or amorphous all work in essentially the same way.

Energy and light from the sun knocks electrons loose from silicon atoms (the most common construction material used) on the top side of the cell to the bottom, creating an overbalance on the bottom of electrons. The only way those electrons can get back to the other side is via the positive wire, through your battery (charging it on the way through) and back up the negative wire to the panel. Throughout the process nothing is used up; the electrons continue to travel around the circuit, equalising themselves out as more are knocked through, charging your batteries as they go.


Panel efficiency

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Roof-mounted panels are an easy way to have the solar always charging; however it is never as efficient as being able to face a panel directly into the sun.

Panel efficiency boils down to how much electricity you can extract from a panel of a given size. Clear, direct sunlight overhead supplies around 1000 watts per square metre. A good quality solar panel will run at around 16-17 per cent efficiency, meaning a one square metre panel, in direct, clear sunshine, will generate approximately 160-170 watts of energy. Cheaper panels will often generate less than that, but we’ll talk more about that below.

However, it goes beyond watts per square metre, and for those of us trying to reduce weight from our four-wheel drives, another way to look at it is watts per kilogram. Where the amorphous cells, like those used in some solar blankets, really shine is in their weight difference. Your average alloy framed monocrystalline or polycrystalline panel that you can bolt to the roof or unfold and face to the sun would be likely to generate approximately 10 watts for every kilogram. When you put that against the amorphous panels, they will generate upwards of 25 watts per kilogram. From a weight perspective, amorphous panels are much more efficient; however, are quite a bit more expensive.


Size of Shadow

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The 260W panel on the roof offered up 17A with the sun directly overhead, on a perfectly clear day.

Further to the efficiency of the panels is what is known as the ‘Size of Shadow’. The amount of power generated by a solar panel is proportional to how much sunlight is shining on it. What this means is that the bigger the shadow the panel makes on the ground behind it, the more energy it will generate – a panel at 45 degrees to the sun (with a smaller shadow than at 90 degrees) will not generate as much as a panel perpendicular to the sun.

To get specific with the maths, at 45 degrees it will generate 70 per cent as much power – Cos 45 degrees = 0.7 = 70% (My year 12 maths teacher was right… I did end up using trigonometry as an adult!). This also does not consider reflected sunlight off the glass or dirty panels, which reduce efficiency further again.

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The same panel, however late afternoon where the sun was at around 30 degrees to the panel, offering a much smaller ‘size of shadow’.

Where you can maximise the size of shadow is if you’re setting up a foldable panel, position it so the shadow it makes on the ground behind it is as large as possible. You will achieve this by ensuring it is perpendicular, or at right angles to the sun.

In so far as shade over the panel is concerned, having the panel in full direct sunshine is critical. Ten per cent shading over a monocrystalline panel will reduce the energy generated to near zero, whereas the same shade over an amorphous panel will still reduce the output, but not as much.

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The shadow from the swag severely limits the amount of power able to be gained from the panel.

Regulated or unregulated panel

When utilising your solar set-up with a dual input DC-DC charger (like the set up I’m going to use in the HiLux), you will need to run the panel directly into the charger, without using a regulator. All of REDARC’s BCDC dual input chargers have a built-in MPPT solar regulator so, like any regulator, it requires an unregulated panel on its input. Should you attempt to connect a regulated panel into a dual input charger, it’s likely the BCDC solar input won’t even turn on. If your panel has an inbuilt regulator that you can’t bypass, you will need to connect the panel directly to your battery, or alternator/main battery input on your charger.


How much solar power do I need?

First off, you’ll want to work out how much you’re going to draw. You’re going to need to use a bit of maths here, beyond just adding it all up. What you’ll want to do is work out how many watts you’re going to use for all your accessories, convert it to amps and put that against your battery bank/auxiliary battery capacity.

For example, let’s say I’ve got a 12V fridge, some LED lighting in the canopy and a USB phone charger plugged into a 100Ah auxiliary battery. So we know the phone/USB charger will draw 12W, the LED light strips will use about 20W and the fridge I’ve got is rated at 3.5A.

To work out the conversion, we divide watts by volts to get amp draw or multiply amps by volts to get watts (Look back at our 12-Volt Fuse Guide for a refresher – it’s down the bottom).

We already have our fridge amp draw (mind you that’s when it’s cycling), but our USB converts 12W ÷ 12V = 1A. Our LED lights convert 20W ÷ 12V = 1.6A. Add our fridge and we’re going to draw 6.1A in an hour. Say we run them on average for 6 hours a day (fridge cycles 15 minutes/hour, phone/iPad isn’t always on the charger and you’ll turn your lights on only when it’s dark); we’re going to draw 6.1A x 6 hours = 36.6Ah in a 24 hour period.

Now we convert our amp draw to watts 36.6Ah x 12V = 439.2Wh. So we need to put back at least 439.2 watts of power over 24 hours just to maintain our batteries. Let’s assume we’re getting eight solid hours of direct sunlight onto our solar panels, for this specific example, with everything in a perfect world and conditions, we’ll need – 439.2W ÷ 8h = 54.9W of solar to maintain the battery in perfect conditions. But conditions aren’t always perfect – it may be cloudy, you could have a tree cast a shadow over your panels, the kids might throw a towel over it without you noticing, or you could be in a valley and only get six hours of direct sunlight. The rule of thumb is to have at least 20 per cent more input than you’ll use. So with just one charge, one fridge and some lights (not thinking about everything else you want to run, an 80W panel would be perfect. Add to your power draw the everything else, or the kids in the fridge every five minutes, and your requirements are going to go up.

If you don’t want to sit there and run the numbers, there are many calculators out there to help you work out how much input you’ll need. Within the calculator, there are different examples of appliances you may own to tally up your power requirements, from fridges to stereos and LED lighting. There are also apps available for iOS and Android devices that will help calculate your solar requirements. The simple answer is that you can never have too much solar!

You can also use this when deciding how big a battery, or battery bank you’ll need in the camper, caravan, or back of the four-wheel drive. I’ve run two 110Ah AGM batteries in the back of the 80 Series, and that has covered me pretty well, with the Travel Buddy drawing 15A while it’s on, and charging camera gear and laptops on trips. You may not need this much, or you may need more. The calculators above will help you work it out.

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Are cheap panels just as good?

Simply put, no…

  • A decent panel will put out the rated power or better – not just claim arbitrary numbers to sound the best – We’ve all seen the eBay ads “OnE mIlLiOn MeGa WaTtS!!!!!!!”;
  • They use only the highest efficiency ‘A’ grade cells, meaning maximum cell size with minimum imperfections – not from the ‘factory seconds bin’;
  • The portable black blankets use top-of-the-range SunPower cells;
  • They utilise genuine industry-standard Anderson plug connections and are wired and terminated properly; and
  • You can get on the blower to someone from a reputable company and pick their brain. Good luck trying to track down tech from ‘Solar Panels Are Us!’ on eBay when your panel is not working as it should.

As with everything, the poor man pays twice, and when you’re looking to get proper off-grid for weeks at a time, you really don’t want to have to run your 4X4 for a few hours every day to stop the fridge falling over and wiping out your food (or beer).


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