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RV Solar Basics

Introduction

Until 2010, solar modules were costly, so people did complex sums to minimise the amount of solar input required. Those days have gone: solar is now so cheap 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.

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 that 180 watts a square metre input 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.

Solar input in tropical areas (when not raining) is rarely more than that above. As it stays hot all night in tropical areas, fridges often 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 polycrystalline and about 19% for premium quality monocrystalline modules (a few are now much the same). Despite this, solar module makers may 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 the few top solar cell makers assemble complete modules. A vast number of small companies assemble most from cells manufactured by others. Quality can only be assured by buying a major brand product.

Where to Install Solar Modules

Solar modules can be located on an RV’s roof. All except the less efficient, but not heat-affected amorphous, 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 June/July. Horizontal mounting also enables the solar modules to keep the RV’s interior much cooler.

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 –its benefit is that it 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.

Pic: Redarc

Pic: Redarc

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.

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