Quantifying sunlight is hugely eased by a measuring concept involving 'sun/hours'.

A sun/hour is a sort of 'standard-sized bucket full of sunlight of known and standard intensity'. Five sun/hours gathered in Sydney is the same amount of irradiation (as techo's call sunlight) as five sun/hours gathered in Alice Springs no matter at what time of day or year.

It's important to understand that a 'sun/hour' is rarely the same thing as an 'hour of sunlight'. It's a measure of the amount of energy in the bucket – and this may take a lot more, but rarely less, than one hour to fill. During mid-winter in Hobart it may take half a day to fill one bucket (e.g. one sun/hour). I

Sun/hour maps are obtainable from meteorological offices (and so are maps showing hours of sunshine, which is not the same thing at all). Sun/hour maps look like other maps (this is because they are like other maps) but have contour lines that delineate sun/hours. You need a different one for summer than for winter, but if you have January and July you can estimate closely enough for the months in between.

Whilst these maps are free (or at least they are from the Internet) the data is given in techo-speke (it's in milliwatt.hrs/cm.sq). This translates surprisingly easily, but the maps that I include in my books show sun/hrs directly. I've included it here in a momentary bout of generosity (but please note it's copyright, and there's several embedded things that enable me to identify it!).

To establish the most probable output from a solar module, take the number within the appropriate contour (the numbers are sun/hours for that location) and multiply it by what the module will actually produce, rather than what you'd thought it would produce because it appeared to say so in the advertising. Assume 72% for most modules in hot places, about 80% for most modules in cold places, and 85% for Uni-Solar almost anywhere – and you won't be far wrong. In fact if you look at the label on the back of almost any module it tells you what they typically produce: i.e. that which I've spelled out above.

Casino in January can expect about 7.0 sun/hours/day, and about 3.5 in July. In May (for the rally) each day is likely to have about five sun/hours each day. An 80-watt module reliably produces 58 watts. Multiplying that by Casino's typical five sun/hours (in May) results in about 290 watt/hours – or about 25 amp/hours if you think in those units. These are averages – and there's bound to be minor day-to-day variations. But over a week it's likely to be within a few per cent unless there's truly unseasonal rain, clouds etc.

Light haze makes little or no difference: in fact because sunlight is often diffused, output may even increase. It often does this close to large areas of water where the sun's energy is re-radiated and reflected back. On many days I get well over 12 kw/hr a day from my current 28-module system (where home is 300 metres from the Indian Ocean), yet the system's theoretical maximum output is 10.75 kw/hr/day.

Heavy cloud typically halves output; heavy rain reduces it to about 10%. But as it is light, not heat that does the work, if you can still see the modules there'll be at least some output (you can even get a fair solar charge by parking under sports ground or supermarket mega-lights). Smoke from bushfires cuts output substantially.

Module Orientation

Modules are best faced directly into the sun but mounting them flat results in a loss of only 10-15%, at least in most places you go to from choice. Adding 10% -15% more capacity readily compensates this.

A near-future Tech Notes column follows on from my column on tyres, and looks at some causes of motorhome overloading: prior email comments are welcomed. My address is collynr@bigpond.com