Updated 22 May 2017

Knowing the solar input available for caravans is vital, especially up north. This article shows how to know that available and increase it too.

Knowing the solar input available for caravans etc. is like measuring rainfall. It uses Peak Sun Hours instead of inches or millimetres. Imagine an open drum that ‘collects and concentrates’ sunlight (rather than rain). When ‘full’, that drum contains one Peak Sun Hour (1 PSH). It is likely to fill in one hour in Australia’s Alice Springs around noon most days. It will take longer early and late in the day. In much of Australia’s south, filling one or two drums during mid-winter takes most of one day.
solar panel on a motorhome solar energy products.
 Solar panels on a motor home. Pic: courtesy Solar Energy Products 
 

Solar input available for caravans – Peak Sun Hours

The PSH concept is based on a recognised scientific background. The term ‘Peak Sun Hour’, however, was thought up by the solar industry. It is thus not technically recognised. One PSH is a solar irradiance (received sunlight) that averages 1000 watts/metre2 on a horizontal surface for one hour. In reality, haze etc. results in a more probable 800 watts/metre2.
 
In practice even the best commercial solar panels are only about 22% efficient. Most are 14-20%, furthermore there are heat and other losses. Solar input available for caravans is thus (currently) about 70-120 watts/metre2 of surface area.
 
The daily solar input is thus 70-120 watt hours times the daily Peak Sun Hours for each metre2 of solar panel area. In other words it’s about 70% of that claimed in most sales promotion.
 
Away from the tropics, about two-thirds of that input is during the two to three hours each side of noon. Depending on season, Peak Sun Hours for (Australia’s) 12.50 to 400 varies from 2 (down south in winter), to 7-8 (in central and southern areas in summer). Northern Australia has less variation. It’s about 5.5 in winter and 6.5 in summer. This fools caravaneers assuming the opposite.
solar map reduced

Based on NASA derived original data, this map shows the most probable (averaged) mid-summer solar output (in Peak Sun Hours) currently in Australia. This map is copyright  caravanandmotorhomebooks.com

Solar input available for caravans – Peak Sun Hours worldwide

Met offices have solar input maps – but the data, however, is in scientific units. My books Solar That Really Worksand Solar Success include two maps that cover Australia. One is for mid summer, the other for mid winter. The maps are based on a ten year running average (from NASA data) and updated regularly. That for summer is shown above. There are daily variations, but the totals nevertheless provide a realistic guide most years.

Solar input available for caravans -optimising solar output

The further north or south, the lower the sun tracks east to west. To optimise solar input, solar panels should face true north in the southern hemisphere, and true south for the northern. If located horizontally on an RVs roof they’ll capture much of the day’s sun (north of a line from Sydney to Geraldton) for much of the year. You can optimise input, however, by adjusting portable solar panels every hour or two.

Dom’t be paranoid about aligning exactly. Most input is from 9-10 am to 2-3 pm – when aligning’s less critical. Furthermore, the sun’s effect is far from a ‘shaft of light’. It’s often diffused. Accurate alignment consequently makes little difference.

This is confirmed by the Australian Solar Radiation Data Book. That, for Adelaide (350 south) during January, shows difference in solar input between mounting horizontally – and optimum 100 is 0.16%. Even 200 error makes only 4% difference. Over a year, that (Adelaide) input with modules at the optimal 300 results in 8% more if mounting horizontally. Variations of plus/minus 200, in north facing or tilt cause less than 5% difference. Because of this, where space allows, accept that minor loss. Compensate for it via a more (now cheap) solar capacity.

Tilting or tracking only increases output if the solar module’s output is less than rated maximum. It thus works best in areas where solar input is below normal.

The less common amorphous solar modules are only marginally heat affected. Most are much larger (per watt), however, more recent (but costly) ones are very efficient. These are handy also for curved roofs. They are flexible and can even be glued on. If doing that consider gluing onto (say) thin aluminium sheet for easing possible repairs.

Solar input available for caravans – Multiple Power Point Tracking (MPPT)

So-called MPPT solar regulators are often claimed to increase solar output. The reality is that they cannot increase input as such at all. What they can do, however, is to recover some input otherwise lost. They do so by ‘juggling volts and amps’ to optimise watts.

The common claim of ‘ by up to 30%’ is true. But that not revealed is it’s ‘30%’ of input during early morning and late in the day – when input is tiny anyway. And/or when the battery is close to flat.

 A more general approach

The information above is of main value for those travelling north in summer. It enables you to build an optimally sized system. You need a different approach however for travelling in various areas and/or seasons. Here, it’s best to specify solar input as the minimum likely at any time. This is typically 2.5 PSH/day for all areas except Australia’s south in midwinter – where it’s too low to be effective. See below re generator back-up.

 Another good guide to solar input for caravans (of existing systems) is this. Unless batteries are fully charged by midday most year around there will be insufficient input up north excepting mid-winter.

As solar capacity is now so cheap, it’s well worth having excess. The associated solar regulator precludes overcharging.

What does not work is to increase only battery capacity. Doing that’s like opening further bank accounts for the same money coming in!

Further, battery charging/discharging loss is typically 20%. Increasing capacity unnecessarily is counter-productive. The more costly LiFePO4 batteries, however, are better in that respect.

Solar in tropical areas

Many RV owners assume that tropical solar input is higher year around. This is not so. The input during a tropical winter is typically 5-5.5 PSH/day, and 6.0-6.5 PSH/day in summer.

There are also be fridge issues: in particular that not only is it hotter all day – it stays hot all night too. Fridge energy usage is usually 40% higher.

See also Article: Living with solar

Generator Backup

The solar capacity suggested should be enough to run a well thought RV electrical system for 95% or so of the time. There can, however, be atypical periods of overcast days. Smoke from major bushfires can reduce input to almost zero.

The need for generator back-up primarily relates to your desire for frozen food. If that is not an essential, you can do without a generator. If you do need one, use it only to charge the battery/s via a high quality charger from the generator’s 230 volts output. Never from the 12 dc outlet. That outlet is intended for running 12 volt devices directly. It’s output (even if marked ‘battery charger’) is an unregulated 13.6 volts or so. That is far too low for effective charging. 

Further information

This topic is not possible to fully cover in article form. If you find this one of value, there’s a huge amount more in my books.

Every aspect of solar is covered in depth in Solar That Really Works (for cabins and RVs). Solar Success covers homes and properties. My other books are Caravan & Motorhome Book, Caravan & Motorhome Electrics, and the Camper Trailer Book.

For information about me – Click on Bio.