RV Solar Basics -updated June 2019
Until 2010, solar modules were so costly that intending users did complex sums to minimise the amount of solar input required. Those days have gone: solar is now so cheap (10% of that 2010 price) that the only (RV) limitation today is the space available for solar modules. It is not possible to have too much: ample solar prolongs battery life and ensures at least some output during overcast days. There is no risk of overcharging, nor overloading the associated solar regulator: that regulator simply blocks the excess current.
How Much Power Will Solar Generate?
Excluding Australia’s mid-winter down south, you can realistically expect an input of 140 watts per square metre of solar module area (over a typical 3-5 hours a day) during most of summer. The solar irradiation of Sydney and north of Sydney may exceed 180 watts a square metre. Daily input varies – but for RVs used mostly outside the three winter months, assume 180 watts a square metre for 3-5 hours. Excluding the lower part of the southern island in mid-winter New Zealand (and Tasmania) has about 160 watts a square metre during the same time period.
Solar input in tropical areas (when not raining) is, to many peoples' surprise, much the same as above - but most of the year round. This is because atmospheric humidity absorbs part of the irradiation. Furthermore, almost all solar modules dislike heat. As explained in greater detail below, they work best when very cold
A further issue in tropical areas is it also stays hot all night. Electric fridges only barely work in these areas because there is insufficient power to drive them. Moreover, warm beer on a hot Darwin night is unthinkable.
Solar Module Types
Solar modules convert light into electrical energy. A good quality 12 volt solar module has 36 cells. Each cell’s efficiency is 14% - 21%, but when all are interconnected, the resultant module’s overall efficiency is around 17% for poly-crystalline and about 19% for premium quality mono-crystalline modules (a few are now much the same). Despite this, most solar module makers claim that higher (cell) output.
Both types are heat sensitive: they lose about 5% for each 10 degrees C increase in ambient temperature. Amorphous cells are not heat sensitive but are less efficient, hence larger per watt. Most solar modules weigh about 1 kg per 10 watts but the latest hybrid modules weigh only a third of that.
Apart from claimed efficiency, due to heat and other losses, solar module output is usually 20%-30% less than claimed on the packing. The maximum output is revealed but in technical units.
Only a few solar cell makers assemble complete modules. A vast number of small companies assemble most from cells made by these companies. Quality can only be totally assured by buying a major brand product from any of the widely recognised top ten. They are - in alphabetical order:
Canadian Solar, EGing PV, Hareon Solar, JA Solar, Jinko Solar, ReneSola, Trina Solar, Suntech Solar, Tunto Solar and Yingli Solar.
Where to Install Solar Modules
Solar modules are best located on an RV’s roof. All except the less efficient, but not heat-affected amorphous modules, need a 25-50 mm air gap beneath them. In much of Australia and the upper part of New Zealand’s north island, horizontal mounting is fine for the summer months. The main loss is only during June/July. Horizontal mounting plus that air space also enables the solar modules to keep the RV’s interior much cooler in summer.
For camper trailers and caravans there is a good case for locating as much solar as possible on the roof of the tow vehicle, as well as on the trailer. This not only increases available capacity, but also enables one or the other to be in the shade. Within reason it is almost impossible to have too much solar – as that will provide ample energy even during times of virtually no visible sun. As your solar regulator precludes overcharging it will simply block any surplus input with no risk to the system or its solar modules.
Portable Solar Modules
There are two types of portable solar modules: these can be one, two or three standard rigid modules hinged like a freestanding billboard, and flexible units (sometimes called ‘solar blankets’) that fold out and (often) lie flat on the ground or the roof of a tow vehicle. Both work well enough technically but may need to be re-oriented from time to time and are very easy to steal. The best ones are expensive – but worth the price.
Solar Module Tips
- No solar module produces power when 100% shaded
- Although solar module input is reduced when not facing directly into the sun, horizontal mounting is fine except in the south during the three/four mid-winter months
- Having your RV in the sun makes no sense if you then need the air conditioner to cool it
- Except the now rare and inefficient amorphous modules, roof-mounted solar modules must have air circulating beneath them to prevent heat build up and reduced output
- Portable solar modules are attractive to others also. Do not leave them unattended.
- Our companion website solarbooks.com has many articles on caravan, motor home and home & property solar systems.
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