Solar panels are not created equal, but to the casual observer, they all look the same. Among the good, the not-so-good, and the downright rubbish, there are also different types of panels. Here’s how to pick the best solar panel for your RV.
These are easy to recognise as they are black and uniform in colour. They are the least efficient of the solar panels. Why bother with them then? The amorphous panels work really well in low light situations and when there is partial shading. If you are planning to do a lot of camping in low latitudes such as Tasmania or Victoria in winter, the amorphous could be worth looking into.
Another positive with amorphous is that they work well in high temperatures. The Cape or Kimberley in summer are locations that will certainly give high temperatures, so again, amorphous could be worth looking into if you are travelling to very hot areas. Amorphous solar film is most commonly used in solar blankets. These are useful for topping up a 12V system but would not normally be used as the primary solar input.
Most recognisable because of their blue tint and visible crystalline structure. Each cell will be square in shape. In terms of efficiency, they sit between the amorphous and monocrystalline but come in at a cheaper price point, so they have been a very popular choice for RV owners. A 140W polycrystalline will be of similar size to a 160W monocrystalline.
The most efficient and most expensive of the solar panels available are monocrystalline. These are easy to recognise because each cell is cut at the corners, forming a diamond shape where four cells meet. Don’t be put off by their price though, as the cost has been reducing over the years. Solar panel prices are now around a quarter of the price they were 15 years ago.
Busbars don’t serve alcohol
The lines you see in each solar cell are the busbars, which carry the energy from the silicon cells to the junction box and then onto the solar regulator. Theory is that the more busbars you have, the more efficient your panel will be at converting sunlight into energy – to a point, as each busbar takes up space on the cell. Put too many busbars in the cell and you won’t have any cell left to capture the sunlight.
Commonly you will see two and three busbars in each cell. Some even have 12 busbars, but any more than that and you are reducing the solar cell size too much. The more sophisticated solar cells will not have any visible busbars as they have been incorporated into the rear of the cell. Rear busbars can make a solar panel much more efficient for the size of the panel. A rear busbar can put out 330W versus a 12 busbar panel of the same size providing 205W.
The number of cells count
The voltage of a solar panel will be determined by the number of cells that it has. The exception to this is the amorphous crystalline, as it is one large cell. Most common solar panels will have 36 cells, which makes them a 12V nominal panel so we can charge a 12V battery with a standard solar regulator. The output voltage of the panels will actually be around 20V open circuit, which gets stepped down to charge the 12 batteries.
Likewise, a 72-cell panel is a 24V nominal panel but can still be used to charge 12V. Residential panels are commonly 60-cell and have a 20V nominal output. These can be used to charge 12V systems but need to be installed in a way so they don’t get much movement, as they have been designed for a fixed roof location rather than a moving RV.
Installation and efficiency
Unlike houses, RVs have flat roofs. The best angle to mount a solar panel is perpendicular or 90° to the sun’s rays. This is why portable solar panels are best pointed towards the sun, moved around during the day and angled upwards from the ground.
Mounting solar panels on an RV generally means that the panels will be mounted flat to the roof; this will be perfect when the sun is directly overhead but sub-optimal at other times. In real world conditions, a solar panel will deliver around 75 percent of its rated voltage due to this angle, and also the range of temperatures encountered. Solar panels are tested at a standard 25°C.
When sizing the amount of solar that you will require for your system, these considerations need to be factored in. A 140W-rated panel will probably deliver 110W in real conditions in summer and even less in winter. Getting the most efficient panels and as much as you can fit, will give you the best chance of keeping up with your battery draw.
Shade is not your friend when it comes to solar
Even the shade of a tent rope can dramatically affect the efficiency of a solar panel. Shade from branches will affect it even more. Items such as TV aerials will also put shade on rooftop panels, so location of mounting is important. It’s more comfortable to set up camp in the shade in summer but this will work against using solar panels to charge your RV. When you have the time in camp, check out the input from your solar panels at various times during the day. School hours (9am to 3pm) will be the most productive hours in terms of sunlight.
Quality of panels
Not all solar panels are equal. Price is a good indicator, but not always; a visual inspection can reveal the quality of a panel. Look for joins that don’t have gaps, the rear of the panel should be flat and even, there shouldn’t be any visible bubbles between the glass and solar cell. Cables should be heavy-duty and any silicone sealing should be even and neat. If it looks to be of bad quality, it probably is.
The last word
Solar panels are a great part of your overall 12V battery system, but you need to ask questions that go beyond just “what is the wattage?” Are they the most efficient? Could you obtain greater wattage from a different type of panel? Where will you install them? Will they be shaded by anything? How many busbars do they have? Knowledge is power, and so are great solar panels.
Words and images Gary Tischer.