Updated May 2017

Knowing how much solar input is coming in like measuring rainfall. It uses units called Peak Sun Hours instead of inches or millimetres. Here’s how and why.

Much as a rain gauge shows a day’s rainfall in millimetres, knowing the day’s number of Peak Sun Hours indicates how much solar input there is each day. This article, by Collyn Rivers (Caravan and Motorhome Books) explains all.

How much solar input

 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 Alice Springs around noon most days, but takes longer early and later in the day. In a Melbourne mid-winter, filling one drum takes most of one day.


solar lake titticaca

Solar is even used on the world’s highest lake (Lake Titticaca) in Peru.

A Peak Sun Hour is a solar industry unit. It is that amount of sunlight that averages 1000 watts per square metre for one hour. For example, 4 PSH/day is as if there were four hours at 1000 watts per square metre. About 75% is likely to be over the two/three hours each side of noon, and 25% over remaining daylight hours. Translating that into solar module input is (relatively) simple.

In theory, a solar module of one square metre captures 1000 watts per PSH a day. In practice it’s far less, because solar modules are only 14-21% efficient.

Depending on latitude, season and weather, PSH in Australia varies from 2.0 (south in winter) to 7-8 (in central and southern areas in summer). Northern Australia has less variation: from 5.5 in winter to 6.5 in summer. This fools visitors assuming the opposite and wondering why there’s less than expected.


solar map reduced

Based on NASA data, this map shows probable (averaged) mid-summer output (in Peak Sun Hours). copyright © caravanandmotorhomebooks.com

How much solar input – worldwide

Meteorological offices worldwide have maps that show how much solar input – but in scientific units. My own are based on a ten year running average (from NASA data) and updated when needed. The current summer data is shown here. There are inevitable variations, but they provide a reasonable guide to how much solar input for most years.

Optimising solar output

The further north or south, the lower in the sky the sun tracks east/west. To optimise solar input, solar modules face into the sun at midday. They face due north (in the southern hemisphere) and south (in the northern), tilted at the location’s latitude angle. To establish that angle, Google your location plus ‘latitude’. In Australia’s far north, the sun tracks close to, or overhead part of the year, so close to horizontal captures most sun. For homes, avoid having modules totally flat – or dust settles. And so do birds and small animals.

Tracking mechanisms enable solar modules to face directly into the sun. They work but are complex and costly. It is cheaper and simpler to accept the loss. Adding 10%-30% more solar capacity compensates. Further, the sun’s effect is far from a ‘shaft of light’. It is often diffused – so minor non-alignment makes little difference. Tilting or tracking increases output if/when the module is producing less than its capable maximum.

The Australian Solar Radiation Data Book has full data. It shows that, for Adelaide (350 south) in January in solar input differs, between horizontal mounting and the optimum 100 tilt by only 0.16%. Even 200 error makes only 4% difference. Over a year, solar input there, with modules at the optimum 300 provides an average gain of about 8% compared with horizontal mounting. Variations of plus/minus 200  in north facing or tilt cause less than 5% difference.

The now less common amorphous modules are less heat affected, but all others lose power when they become hot. This loss is typically by 5% for each 100 C.

Solar input for marine and RV use

Data above is for fixed locations, or those travelling mainly in summer. A different approach is needed whilst travelling extensively in Australia, or at varying times of year.

For distance travelling you need solar capacity that is excess much of the time. Or if preferred, using solar to complement a generator-based system (or vice versa). An effective rule is assuming 2.5 to 3.0 PSH for all areas except Hobart and Melbourne’s midwinter. There, under 2 PSH is common in June/July.

A reliable guide (for existing systems) is that, the batteries are fully charged by midday most days year- around there’s insufficient up north except in mid-winter.

What solar modules really produce

The promotional output cannot be achieved in typical use. Just why is explained below. In practice, if used with a cheap solar regulator, most solar modules produce about 70% of that seemingly claimed. A Multiple Power Point Tracking (MPPT) solar regulator lifts that to about 80%.

Your most probable input, is daily PSH (for the area times 70% of what was claimed. Or with MPPT unit, 80%-85%.

Technical explanation (why solar makers do not get sued)

Typical solar modules produce maximum power (volts times amps) at around 17.1 volts. Charging a 12 volt battery however requires 13.0-14.7 volts. With basic solar regulators, all between that 17.1 volts solar output and the charger’s needs is not accessible.

An MPPT regulator ‘juggles’ available volts and amps thereby optimising watts. This recovers of that otherwise unavailable. It is particularly effective when the battery is low in charge, and during early and late hours of the day. Dismiss claims of MPPT ‘increasing’ energy by 25-30%. It recovers about 10%-15% over a day of that otherwise lost. Enough to justify its use but generally less than claimed.

How much solar input in tropical areas

Many owners assume solar input in tropical areas is higher year round. This is not so. Input during tropical winter is typically 5-5.5 PSH, and 6.0-6.5 PSH in summer. There can be major issues with fridges that run from solar because it stay hot all night as well. Energy usage is up to 40% higher.

Further information

For a full explanation see my Solar That Works (for cabins and RVs). Solar Success (for homes and properties. See also Living With Solar.  Details of the author’s own (Broome) system see All Solar House .

All aspects of RV usage is in the Caravan & Motorhome Book. For RV electrics – Caravan & Motorhome Electrics. See also the Camper Trailer Book. For author info click on Bio.

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