Solar Power

This article covers every aspect of solar and its design and installation in RVs (camper trailers, caravans and motor homes). It shows how much power solar panels really produce, and why so many systems (particularly) fridges do not work as well as hoped. It explains why adding more batteries only works if you have adequate solar or other generating capacity to charge them. From this article you will be able to tell how much solar you can expect when and where. The article looks also at battery and battery monitoring, plus a thorough explanation of cabling – vital because many RVs are inadequate in that respect. It also explains why this situation came about.

Introduction to Solar for Campers

There are major and widespread misunderstandings about many aspects of solar. As a direct result some systems work superbly, some less so, and quite a few are silicon/lead acid disasters. There are various causes: particularly that the solar industry calculates power output in a manner likely to catch out those who do understand electrics – but do not understand the solar industry’s way of doing its sums. An example of how this can confuse is an article in a 2012 issue of a major RV magazine in which the writer overestimated the most probable daily solar output by over 200%.

A further cause has been that, until 2013, solar was not included in any auto electrical course. Furthermore, very few domestic solar installers know much about RV electrics. Most know nothing.
The best installations are done by a few specialist companies and knowledgeable private owners.

Redarc began to address this, in late 2012, by providing specialised training. TAFE intend to introduce as soon as they can for auto electricians. From August 2012 – December 2013, the author is writing and publishing an RV solar course for auto electricians (in serial form) specifically commissioned by Automotive Electrical & Air Conditioning News. Meanwhile his book (now the third edition) Solar That Really Works – sold by – is used both by TAFE and the auto electrical industry as its main text.

Solar and its installation in RVs is complex and are hopefully outlined in this article that has been rewritten – in semi-overlapping parts. This article, totally rewritten in March 2013, is presented here in two main parts. The first part is primarily for non-technical readers, but contains material that many technical minded people may not be aware of. The second part explains the issues in more depth – using technical terms where deemed necessary.

Part 1 - The Non-Technical View

Solar Input Obtainable

A major misunderstanding relates to the concept of ‘sunlight hours’. The output of a solar module is a generated by sunlight (not heat) – but not on a ‘direct hours of sunlight’ basis. It cannot be, as a few hours of sun in much of Finland is very different from the same number of hours in Australia’s Alice Springs.

The solar industry gets around this in much the same way that we measure rainfall. That is by measuring how many ‘standard bucket fulls’ there are each day. Think of sunlight as rain and you get the general idea. Each bucket full is called a Peak Sun Hour (PSH). There are typically seven or eight such PSH a day in Marble Bar (WA) in mid-summer, but only 1.0-1.5 in a Hobart mid-winter.

Daily output varies considerably from summer to winter and also from south to north.
In summer, across much of Australia, one is likely to have 4.5-5.0 PSH. In mid-winter it is not safe to rely on more than 2.0-2.5 PSH.

No solar module works at all in full shade (their energy input is light, not heat). Most lose nearly all output if more than a quarter or so is shaded. So-called ‘amorphous’ solar modules however lose output more or less proportionally to the area shaded.

For the highest average yearly output, solar modules must face directly into the sun but, except at (numerically) higher latitudes than about 26 degrees, flat mounting loses only 10% or so during winter – and may gain in mid-summer. As solar module cost has plummeted, adjustable tracking is no longer worthwhile: it is cheaper and simpler to accept the loss, and add 20% or so more solar capacity (if roof space permits). All but, the now less used, amorphous modules need an air gap of about 30 mm to limit heat build up beneath them.

The amount of solar energy you can gain is also limited by the space available for solar modules, but is typically 120-140 watts for each square metre of solar modules across most of Australia around noon.

Way up north (above a line from Cairns across to Broome) solar input is less than most assume. For much of the year it will be between 5.0 and 6.0 PSH. This catches out any number of RV users unaware that not only is there less solar input way up north in summer (than down south), but the temperature tends to stay above 28 degrees C throughout most nights. This causes grief to those who do not know about either, because fridges use about 5% more energy for each degree C higher in ambient temperature (and likewise for each degree C a fridge is set colder). That typical 15 degrees or so nightime increase results in a probable 50% increase in energy use. The PSH map shows what to expect in mid summer.
Confusion exists also over seemingly claimed solar module output because the industry has historically used a method that misleads even the electrically knowledgeable. In essence a module rated by its maker at (say) 120 watts is likely to produce only 84 watts or so in low priced systems using a basic solar regulator (see below). It may produce up to 100 watts if used with a more costly (MPPT) regulator – because that partially limits the losses. It is unlikely to produce more than that in typical RV use. (The technical explanation, to do also with heat, is given in the second part of this article).

The amount of energy that you have each day is thus about 70% of the wattage you thought you had paid for, times the number of Peak Sun Hours for the location/s involved. A 120 watt solar module in mid-winter in Sydney is thus likely to produce 200-360 watt hours.

Another way of thinking about this is that if you live more or less south of a line from Brisbane to Geraldton, unless your batteries are 100% charged by at least 1.00 pm on most summer days, your existing solar has zero chance of coping with an electric fridge up north. Nor will a gas fridge cope unless it is EU ‘T-rated’, (see also under subheading Fridges).

What You Can Run From Solar

It is usually feasible to run things from solar that do not generate heat over long periods as their main purpose: for that, gas or diesel is a better source of energy. Electric jugs are border-line feasible in big rigs, but not otherwise. Also out are 12 volt incandescent globes (legally banned since 2010 in 230 volt form), older CPAP machines (recent CPAP machines using a heating cycle still need a lot of energy), hair dryers etc.

The main electricity draw (typically 70% of the total) is an electric fridge. Late models use less, but their draw is mainly related to how well they are installed – most are done badly, some appallingly so.

It is also essential to have adequate wiring - again, most is not - and an adequate charging system (if powered also via the alternator). See later in this article re wiring.

If solar capacity is too low, adding more battery capacity makes things worse! It’s like opening a second bank account for the same money paid in. This may seem obvious (once it’s pointed out) but many people who routinely run out of solar power add more batteries. Not only does it not assist, it results in the battery capacity being chronically undercharged. This damages them and they will not last as long. (Batteries are like lead-acid Labradors – they like being fed but are not that keen on strong exercise).

Cost apart, it’s impossible to have too much solar – there is zero risk of overcharging (as that’s caused by over voltage – not over capacity). Having a lot of solar capacity helps on overcast days as there is often only 20% of normal charge coming in. It is also affordable – as solar cost has plummeted since 2010. (Prior to that, most RVs had too little solar, yet many owners have not since added more).

Many commercial RV solar systems are scaled for users who spend most nights in caravan parks. Few will cope with more than one overnight stay away from 230 volt power. Worse, many have only little better than so-called float charging, i.e. that maintain the charge of already well charged battery between uses. They can recharge a deeply discharged battery but as major one supplier advises, if deeply discharged as many are, it takes about 18 hours to recharge a 120 Ah battery recharge to 80% and a further twelve hours to charge from that to close to 100%. One major supplier quotes no less than an overall 70 hours. It is not possible to modify these existing so-called ‘Converter’ systems. This is too major a topic to discuss here.


Solar regulators ensure that charging is speedy and safe, meanwhile protecting batteries against overcharging, and appliances against over-voltage. With a genuinely high quality solar regulator installed (typically $250 plus), it is safe and beneficial to leave batteries on permanent charge - but for those who still use wet batteries (those that need routine topping up with water), it is essential to check their level at least every 8-10 weeks. Some owners claim solar regulators are unnecessary - but it's odds on their system is so badly designed/installed that even omitting the regulator makes little further difference! Or the solar input is very low.

Energy Monitoring

Instantaneous measurement of battery voltage is so totally meaningless it may lead to sound batteries being discarded and worn out ones retained. Why? It is because after 30 minutes engine running, a battery that's almost flat may measure as close to fully charged (12.6-12.8 volts). A close to fully charged battery may measure as close to 'flat' (less than 11.6 volts) after running a microwave oven for ten minutes - a total energy draw some twelve times that of starting a big 4WD diesel engine.

Further, because the energy interaction between a battery’s lead plates and the so called water/acid ‘electrolyte’ is very slow, a reading taken from a big deep cycle battery that is totally off-load for 24 hours is likely to have at least 10% error.

The only meaningful measurement is to check the energy that goes in, and the energy drawn out - deduct a bit for system losses: that which is left is more or less what you've still got. Such energy monitoring is built into upmarket solar regulators, but it can also be done by stand-alone units that, in some instances, can be easier to read.

Without monitoring, you risk wrecking batteries through constant undercharging and/or over-discharging. Over-charging is less common but occurs. It is usually caused by leaving a battery across a poor quality battery charger too long.


An electric-only fridge gobbles 60%-80% of daily electrical draw. Chest-opening types are the most efficient, door-opening types use a few per cent more power. A realistic maximum is 170 litres and that will need three (ideally four) 120-watt modules. (Around 2003, members of a well known 4WD ‘van club poured ongoing forum scorn on that contention – until they experienced a hot weekend rally - and warm beer).

An alternative is a Dometic Climate Class 'T' three-way fridge. These run on 12-volts whilst driving, 230-volts when available and gas at all other times. The 'T' indicates the fridge is designed to run in ambient temperatures up to 43 degrees C. Use this type of fridge and you may need only 160-250 watts solar (see later for specifics) for everything else.

In Australia, Dometic has four or five such in its range, but do not be fobbed off by vendors who claim that all Dometic fridges are T-rated. They absolutely are not – the confusion is probably due to Dometic upgrading all their products in the 1990s – and using the term ‘tropicalised’ for all of them. That company has never claimed that tropicalised implies ‘T-rating’ but not all sales people appear to be aware of that. In the author’s view, if a three-way fridge is used, it needs to be ‘T-rated’ in travelling in Australia’s North and North West.

All fridges must be installed correctly to work as intended. Most are not - and hence don't, but three-ways will disappoint more than most if ineptly installed. Cooling performance and energy usage of almost any RV fridge can be hugely improved via the simple changes outlined in most of the author’s books.

Microwave Ovens

A microwave oven draws more energy than many assume. The rated wattage is the equivalent heat produced - not the energy drawn in doing so. Via an inverter, a (2013) '800-watt' microwave may draw 1500 watts (around 125 amps at 12 volts). Ten minutes usage may draw a full day's mid-summer output from an 80 watt module. Use a microwave only where there's mains power. But many find long term travelling changes living styles – and that may not use a microwave at all.

[SH]Lighting]/SH]The most efficient lights are LEDs and, but less so in terms of usable light, compact fluorescent globes. Some of the latter have an inbuilt inverter for 12-volt connection. You can use 230-volt fluros via a remote inverter but that necessitates mains-voltage wiring.

LED lights draw little power mainly because their light is tightly focussed. They are good for reading and, (in headband form) for campfire cookery, but have no energy saving advantage for general lighting. Halogen globes use twice the energy of fluorescents at up to 700 degrees C. They can be very uncomfortable if close overhead. They also produce a lot of UV.


Solar is not feasible for major heating applications. Use gas or diesel for cooking and water heating water. Water pumps need to be 12 volts - ditto cooling fans. TVs, VCRs and DVDs are only a problem if used for hours on end. If you really need a computer, use a laptop. Some can also double as a TV.

Typical Energy Draw

List and total how amount of energy you are likely to use each day.

Enter the data in watts (where it is shown in amps and volts, just multiply those units and the result is watts – e.g. a 12 volt, 5 amp device draws 60 watts). If run for one hour that is 60 watt hours. If used three times a day, that is 180 watt hours/day. It is the number of watt hours/day that one needs to know.

The following is the typical consumption of various electrical equipment (in watts).
  • CD/DVD player - 30
  • Coffee grinder - 50
  • Computer (laptop) - 20 to 50
  • Computer (desktop) - 300 to 500
  • Computer printer (ink jet) - 40 to 70
  • Computer printer (laser) - 1000
  • Fans (230-volt) - 30 to 100
  • Food mixer - 350 to 45
  • Juicer - 35
  • Lights (compact fluro) - 5 to 18
  • Lights (LED) – 3 to 8
  • Macerator pump - 300 to 350
  • Microwave oven (‘800-watts’) - 1350*
  • Portable radio - 5 to 15
  • Sewing machine - 75 to 100
  • Stereo - 50 to 60
  • TV (LED/LCD 16-20 inch) - 20 to 30
  • TV (LE/LCD 26-32 inch) – 100 to 130
  • VDC/DVD – 30
  • Washing machine (cold cycle) 225-300
  • Water pumps (12/24 volt) - 50
*For microwave ovens run via an inverter - add 15%

Allowing for Inverter and Battery Loss

Typical daily consumption allowing for inverter and battery losses - in watt hours/day and ( for 12 volt systems in amp hours/day). Note: as more electronic equipment is now typically used, the totals (rounded off) are higher than shown in the previous version this feature.
  • Trailer/small caravan/campervan with three-way fridge - 150 to 250 Wh/day, (13 - 25 Ah/day).
  • As above with 40-70 litre electric chest-opening fridge – 550 to 750 Wh/day, (45 - 65 Ah/day).
  • Medium caravans/small motorhomes with three-way fridge - 450 to 650 Wh/day, (37- 55 Ah/day).
  • As above with 110-130 litre electric only fridge – 1350 to 1500 Wh/d, (110 - 125 Ah).
  • Large caravans/medium motorhomes with three-way fridge - 650 to 850 Wh/day, (55 - 70 Ah/day).
  • As above with 110-130 litre electric only fridge - 1500 to 1800 Wh/day,(110 - 150 Ah/day)
  • Large motorhomes with three-way fridge - 750 to 1000 Wh/day,(60 - 85 Ah/day).
  • As above with 220-litre electric only fridge - 1750 to 2500 Wh/day, (145 – 200 Ah/day).
  • The above illustrates the savings made by using a three-way gas/electric fridge.

Battery Capacity

A battery is like a bank account. It is not possible to bank more than comes in. Having excess storage is not only pointless, but will cause you to lose some of what you have - because battery losses relate to their number and size.

The maximum battery capacity must never exceed that which you can readily fully charge most days. Ideally have about 250 watts of solar per 100 Ah battery capacity. Large solar capacity is fine (and also now cheap). Excess battery capacity is bad (and costly).

Example: A nominally (i.e. what it said in the advt) 250 watts of solar with 5 Peak Sun Hours/day will produce about 175 watts (70% of 250 watts) for five hours/day (that is about 880 watt hours/day). Allowing for charging losses, that will bring a 12 volt 100 amp hour (1200 watt hour) battery from about 35% charge to close to 100% charge in one day. As a good system should be designed to discharge by less than 30% overnight (i.e 70% charge remaining), that gives a healthy margin for occasional deeper charges – and days with less sun.

The best working systems are like this – a lot of solar capacity, not that much battery. This way, the battery will substantially recharge on days of little or next to no sun. And the battery/s will last years longer. (If you need more storage, you must increase solar capacity in at least proportion – ideally higher.)

For many purposes (strongly recommended in my books and used on our previously-owned OKA and Tvan for years) is to run from solar alone. This is simple and works very well. The Tvan and 4.2 litre Nissan Patrol tow vehicle each had totally independent systems, but interconnectable if required (which it never was). This works well if either part requires service.

Part 2 - The Technical View

It is far from uncommon to find well written but flawed articles from those who truly know their physics or electrics - but not the curious ways of solar marketing. Electricians, electrical engineers, and particularly physicists know (by basic definition) that one volt times one amp equals one watt. Thus, a 120 watt (12 volt) solar module must, they reasonably assume, produce 10 amps at 12 volts. But none does. The ‘explanation’ can be legally defended, but misleads all who do not know how and why there is a discrepancy.

Standard Operating Conditions

The solar industry has two different Scales. One set, called SoC (Standard Operating Condition) is used for marketing and selling. SoC output is measured in a laboratory environment that, in effect, simulates the solar irradiation on top of a high equatorial mountain around a 5.0 degree C noon in peak summer.

Peak voltage and peak current are graphed separately. The quoted wattage is where peak voltage and peak current coincide. This is typically at 17 or so volts. The current at that voltage is 7.1 amps (hence the 120 watts). At the RV’s typically required 13.5 or so volts (averaged over the charging cycle), that is about 96 watts.

There is then heat loss. All but the (less common) amorphous technology modules lose around 5% output for every 10 degrees C increase above a cell (not ambient) temperature of 25 degrees C. On a moderate 25 degrees C day that cell temperature is, according to the industry’s own data – is 47-49 degrees C. So bang goes another 10% or so output – and hence a ‘120 watt’ module that produces only 85 watts or so at useful voltage.

The industry’s justification is that a 120 watts module can indeed generate that if the load will produce that equivalent in work at 17.1 volts. This can be done by dc motors commonly used for water pumping, but that 120 watt output cannot be directly utilised in most 12 volt RV usage (but some of the loss can be regained – see MPPT below).

Nominal Operating Cell Temperature

The industry has a second set of scales, called NOCT (Nominal Operating Cell Temperature) that whilst not used for selling, closer reflect climatic and user reality. The NOCT output is about 70% of that apparently claimed.

All is revealed in the technical specifications, but in terms non-technical people are unlikely to understand. It is also on a data panel on the rear of most modules. As can be seen from that shown here the NOCT output of a typical 120 watts module really is only likely to be less than 85 watts – it is in that third column of the example data panel shown.
The industry has used the SoC rating since it began. Most solar pioneers were engineers so this ‘rating’ rarely confused. But it now fools typical buyers. Few buyers realise that their ‘1.5 kW’ grid connect system is likely to produce only 1.2 kW in most places.

Most basic ‘12 volt’ appliances are designed to run on 12.0-12.8 volts, and cannot withstand, let alone utilise any more. (Battery charging requires 13.2 -13.8 volts for much of the charge, and up to 14.7 or so volts only towards the very end). Very few systems except water pumps and heaters can access that ‘full’ (17.1 volt) power.

Multiple Power Point Tracking

More commonly called MPPT, this is a dc-dc conversion technology that optimises volts and amps to best match the load requirements, and in doing so recovers some of that energy that is otherwise not accessible by the load.

In a charging application, MPPT may attempt (say) to convert 17.1 volts and 7.1 amps to a bulk charge 13.8 volts and 8.7 amps – (that otherwise elusive 120 watts). But outside Internet forums, (where perpetual motion with a power take off is all but routine) no energy transfer process can ever be 100% efficient. The reality is likely to result in around 100 watts – a worthwhile saving with big systems.

Multiple Power Point Tracking is built into up market solar regulators. It can recover 10%-15% of otherwise non-accessible energy. Some vendors claim 30% or more, but do not reveal that only happens for a few minutes after sun-up and just prior to sunset.
For any solar input over 300 watts or so an MPPT regulator is worth buying. Below that it’s cheaper to accept the loss and buy more solar capacity instead. This technique is also built into dc-dc battery chargers.


This too is widely misunderstood. Solar irradiation is measured in Peak Sun Hours (PSH) and is the equivalent number of hours when solar irradiance averages 1000 w/m2. If for example a given location experiences 5.0 PSH, that is the amount of irradiance had that been 1000 w/m2 for five hours.


A battery is charged by applying a voltage across it that is higher than it already has. The greater that voltage difference, the faster and deeper the charge.

If, as in basic mains battery chargers and basic alternator charging, that charging voltage is fixed, as the charging battery’s voltage rises that difference in charging voltage, and hence charge rate, automatically tapers off. Initial charging is thus fast, but it may take days for a battery to fully charge.
This situation is changing rapidly. Since 2000 or so some alternators have operated as low as 13.8 volts and EU regulations post 2013 may result in only 12.7 volts (far too low for charging).

Charging voltage charge is rarely an issue for engine starting because starter motor and now starter batteries are designed accordingly (and especially those on cars with automatic engine shut down when at rest in traffic).

Unfortunately, 'house' batteries charged by the same system are likewise limited. This has now been addressed by dc-dc alternator charging that, by using MPPT, results in charging being independent of alternator voltage. This works so effectively that it is virtually pointless to use anything else – including for any major refit on RVs of any age.

A sufficiently large dc-dc alternator charger will charge house batteries up to 150 Ah or so from 50% discharge to close to 100% within two/three hours driving. They are perceived by computer engine management systems as just another load (much as a pair of spot lights). Gell cell and AGM batteries charge reasonably fast from standard vehicle alternators but they too are better used with the dc –dc system.

A minor drawback of such systems is that the smaller output units will not charge a deeply discharged batteries to (say) 50% as quickly as will an alternator but, from there on, the dc-dc approach is massively faster. Some units have a bypass such that it cuts in above that 50% or so charge.
Dc-dc charging is very much a topic of its own.

Cable Ratings

Cabling has a trap that even catches out those who know about electrics - but do not read specifications sufficiently closely. It concerns the way cables are rated. Unless you know about it, it is odds on that you will use cable half that required (often a mere quarter). If you do this you will build in a fault that will plague you and future owners for years to come.

Electrical appliance makers and electrical engineers worldwide, except in the USA, specify cable by its copper core’s cross-sectional area in square millimetres. As that’s the bit that carries the current this makes every sense.

Americans mostly use American Wire Gauge (AWG) and the all-but identical B&S (Brown & Sharpe).

Most countries however use so-called auto cable. When you shop for the 3.0, 4.0 or 6.0 mm² cable that the appliance maker specifies, what you will almost certainly be sold is 3.0, 4.0 or 6.0 mm diameter auto cable. This, for reasons that defy sanity, uses similar numbers to indicate something quite different.

An auto cable vendor's 4.0 and 6.0 mm is not a measure of the copper core that carries the current. It is the diameter of the hole the cable will pass through!

As a result, 3.0 mm auto cable is about 1.0 mm²; 4.0 mm auto cable is about 1.8-2.0 mm²; and 6.00 mm auto cable is about 4.6 mm². The above can only be 'about' because auto cable 'rating' includes the thickness of the insulation – and that varies from brand to brand – not least as plastic is cheaper than copper.

Few if any auto parts stores are aware of this issue, let alone the problems it causes. Ask for 4.0 mm² cable (or even spell that out) and you are almost certain to be sold 4.0 mm auto cable (that is 1.8-2.0 mm²).

High quality auto cable is fine, but if you have used it unknowingly, (and if you have done any vehicle wiring it's odds-on you have) you are likely to have voltage drops two to four times that acceptable.

Energy will be lost as heat and things connected to those cables (especially 12-volt fridges) cannot work as intended. Lights will be dimmer, dc motors will run slower, produce less power, and overheat. Fridge cooling, in particular, is dramatically reduced.

Some auto cable has the sq. mm. data (in small type) on the drum. It is usually also in the technical literature. You can substitute 6 mm auto cable (typically 4.59 mm²) for 4.00 mm² cable, and 4.0 mm auto cable for 1.5 mm². Apart from 8.0 mm auto cable, which really is 7.0 - 8.0 mm², these are the only 'safe' conversions. Auto cable smaller than 3.0 mm has so little copper you may as well use wet string.

Ditto cheap auto store jumper cables: these have hugely thick plastic insulation but next to no copper conductor.

Current Ratings

Auto cable is also 'rated' as '30 amp' - '50 amp' etc. This is a fire rating only. It provides no guide whatever to usage, nor any indication of voltage drop. It indicates the maximum current that cable can carry before its insulation begins to melt.

Very high quality tinned-copper cable in square millimetre ratings can be obtained from specialised marine electrical suppliers. Another way (for RV lighting) is to use 1.5 mm and 2.5 sq mm multi-strand 230-volt lighting and power cable. Don't use so-called 'building cable'. It is insufficiently flexible.

About the Author

Collyn Rivers is an engineer/writer/publisher. His major books in the RV area are the recently totally rewritten, Solar That Really Works, The Camper Trailer Book, and the all new Caravan & Motorhome Electrics (the successor to Motorhome Electrics); and the Campervan & Motorhome Book. These books are written in everyday English, yet recognised globally as technically impeccable. They cover every aspect of their subject matter in detail. All are available from

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