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By Collyn Rivers

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I have written this piece in two parts. The first is an overall view. The second part contains technical explanations and data to back up that overview. I would appreciate feedback - as to whether it aids, or hinders access. As it will be reproduced also in the USA I have used the term 'RVs' throughout.



Solar modules can and will only produce the output claimed by their manufacturers if they are used in specific circumstances. Typical RV usage is not one of them.

Outputs vary from type to type, but in typical RV installations most solar modules produce a bit over 70% of their apparently claimed output. Many modules have a small panel on their rear face that shows what they actually produce. For an '80-watt' module this is usually about 58 watts.

Whilst this may seem misleading it is defendable. (The explanation is technical and is provided in the second part of this piece).

Two Main Types of Module

There are two main types of solar module: for the purpose of the first part of this explanation they may be seen as Uni-Solar and the Solarex Millenium series, and the rest. One type is not inherently better than the other. They simply have different characteristics that cause them to be more effective in some conditions than in others.

Most solar modules produce less power as they heat up - as much as 20% in seriously hot places like the north of Australia, and they start losing this power from only a few degrees above freezing. Uni-Solar and Solarex Millenium modules use a different technology. They produce slightly more power as temperature rises.

Because the power output of most solar modules falls as they get hot, from around 35 degrees C (95 F) onward, a 64-watt Uni-Solar module produces much the same output as an 80-watt most anything else. The same ratio is probably true of the Millenium modules, but I have no direct experience. As they are about 15% cheaper in most countries this can sometimes be a considerable benefit

Uni-Solar modules are also less affected by partial shadowing. If most panels are shadowed by an area even as small as a human hand, they lose virtually all of their output. Uni-Solar modules lose only that area shaded. It is claimed that some (but not all) Kyocera modules are also more shadow tolerant.

No solar module will work in complete shade. I need to spell that out because after I stated (elsewhere) that solar modules produce a small output under high-intensity full-spectrum light, this rapidly turned into 'Collyn says they even work under street lights'. They don't.

The above would appear to be an overwhelming argument for Uni-Solar and Solarex Millenium. This is not necessarily so. Their heat advantage is only really worthwhile above 25 degrees C (77 F) or so. And a big downside for many is that, because they are less efficient, they are much larger (40% or so) than other modules. That's why I use two Solarex 80-watt modules on my OKA truck, and 28 by 64-watt Uni-Solar modules on my 5 hectares (about 10 acres) at home.

In practice it is safe to assume that you will get 58 watts from an 80-watt most-anything module, and about 55 watts from a 64-watt Uni-Solar module.

The amount of energy you will capture each day can be readily worked out by taking the true output of the modules (or about 72% of what it says on the marketing brochure) and multiplying that by so-called the Peak Sun Hours typical for where and when you are going. Maps showing this can obtained from meteorological offices, but their's need translating, and (for Australia) from all of my books. Peak Sun Hours are explained below. It is also obtainable for most areas from various Internet sites, but whilst there if you search hard enough for it, it is surprisingly hard to locate for the USA. (If any American reader is having problems re this, email me your latitude and longitude plus nearest city and I'll try to assist).

Ideally solar modules should face the sun, but flat roof mounting is surprisingly effective. Whilst modules can be carried loose, they are readily stolen. In my experience it is not worth arranging for tilting or tracking systems except in the extreme south of Australia, or the more northern part of the USA. Adding about 20% more module capacity will make up for any loss.

Heavy cloud and rain cuts output by 50% or more. The highest output is typically on bright days with scattered low cloud. The sun shines down, is reflected from earth and bounced down again from those clouds.

Because most modules are heat-sensitive it pays to mount them so there's an air space beneath. Less important with Uni-Solar and Solarex Millenium, but air space provides useful heat insulation in the vehicle.


Technical Stuff

Solar modules are tested using 'Standard Operating Conditions.'

These specify cell temperature of 25 degrees C. Note that this does not mean ambient temperature. It is the actual temperature of the cells, and in sunlight this corresponds to about 5 degrees C.

The output of solar modules is obtained by plotting curves of voltage and current and from these using whatever combination of those two parameters gives the highest 'number'. In practice this tends to between 17.0 - 18 volts. Thus a module that produces 4.7 amps at 17 volts is rated at 80 watts.

Power being P = IV, that module will produce 56.4 watts at 12 volts - and 65.8 volts at 14 volts.

In other words module output is partially a function of the voltage developed across the load.

The industry's 'Standard Operating Conditions' (SOC) measures output at a cell (not ambient) temperature of 25 degrees C at an irradiance of 1 kW sq.m. As this equates to an ambient temperature close to 0 degrees C, the SOC may better be regarded as 'Standard Test Conditions'.

Manufacturers do however also quote a separate NOCT (Nominal Operating Condition Temperature).

This gives an indication of the actual cell temperature at 20 degrees C ambient, but at 80% of the irradiance of SOC operating conditions, a wind speed of 1 m.s, and the back of the module enclosure open to atmosphere. Under these conditions the NOCT is typically 47-49 degrees C. temperature - or looking at it another way, the cell is likely to be 25-30 degrees C hotter than ambient temperature.

Mono- and poly-crystalline modules lose output at a rate of approximately 0.4%-0.5% per degree C above about 5 degrees C (voltage drops considerably, current rises slightly). Thus at 30 degrees C ambient, output of modules using that technology is likely to be 15% down (ie. over and above the loss due to working at 12-14 volts).

Amorphous technology modules (Uni-Solar/Solarex Millenium) tend to be increase output very slightly with rising temperature.

The current output of modules is usually shown in the technical data. It may be shown as ISC (short circuit current), or as operating current. The latter is the figure to use. True output is the operating current times the operating voltage. It must then be corrected for temperature.

Whilst it is rare for modules to produce their full rated output (Pmax) it is possible to achieve it (or very close to it) when driving a load (such as a water pump) that will operate at the voltage at which peak output was tested. It can also be achieved by using multi-point tracking systems that, in effect act as an 'electrical torque converter' and 'swap volts for amps' (actually a dc/dc converter). So whilst people tend to be confused by the rating system, it's both technically credible and legal.

The term Peak Sun Hours (PSH) is not my invention! It's used extensively in the photo-voltaic industry but does not seem widely known by engineers in other disciplines. In effect it's the number of hours of midday sun on a clear day equivalent to the irradiation for that day. One Peak Sun Hour equals 1 kilowatt/hour/square metre.

Full details of what can be run from solar, required module and battery capacity, installation (probably rather more than you wanted to know!) are in my books - and in brief on my website.

This article is copyright Collyn Rivers, Broome 2003. Email: It may reproduced on non-commercial Internet sites free of charge providing this last paragraph, including addresses, is included in full.



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