Peak Sun Hours
To indicate solar input, the industry invented ‘Peak Sun Hours’ (PSH). A PSH is not an officially recognised scientific unit, but corresponds to an irradiation of 1000 mW per horizontal square metre. The concept is like filling a barrel with sunlight of ‘standard known brightness’.
That barrel may fill in 45 minutes in close to the Equator, but may need six or more hours in say Finland. A ‘full barrel’ is what the industry calls 1 PSH. Multiplying PSH for location and time of year, by the solar module’s real output gives watt hours/day. Dividing by the battery voltage gives amp-hours/day.
Peak Sun Hours (top) July, (bottom) January. Multiplying true module output by the relevant Peak Sun Hours results in the module output for that day. These maps, based on Australian Bureau of Meteorology data, show typical irradiation averaged over 2008-2018. Irradiation is more or less linear between the marked points.
Module mounting etc.
Light/medium cloud cover cuts input by half or so, rain by more, but it is rare to have none. The highest is on bright days with broken white cloud, particularly near sand dunes, or expanses of water that reflect light up and down again, reinforcing direct rays. Mounting modules close to flat loses 20%-30% in high latitudes, but only a few per cent otherwise. In some areas, mounting them flat is preferable on a year-round basis. In most areas, it is now simpler and cheaper to add more solar capacity than to track the sun.
In areas that are 30 degree or more from the Equator (e.g. less than 2.0 PSH/day) it is currently not practical to attempt to run from solar alone. Supplement charging by running a generator for an hour or two a day in mid-winter months.
What solar really produces
The industry has two scales (Standard Operating Conditions [SOC] and Nominal Operating Cell Temperature (NOCT). Both measure volts and amps separately. Standard Operating Conditions (SOC), used mainly for marketing, is whatever combination of volts and amps gives the most watts. It used regardless of whether the voltage at which it is measured is ‘usable’ by most loads. The SOC rating can mislead. It not illegal, however. Some loads, such as heaters and water pumps, accept that higher voltage and do more work as a result. Nominal Operating Cell Temperature (NOCT) indicates output in realistically typical RV and home usage.
This map shows typical solar input. For camper trailer solar, anywhere not dark green or blue is fine. Map: solargis.info.
A non-recoverable 0.5% output is lost for each degree above 25º C cell temperature – not the 25º C ambient temperature that many vendors claim. Under the sun, even in temperate climates, that cell temperature is about 47º- 48º C. Vendors rarely reveal this, but solar module makers show it in their specifications and often on their products. Many do so, however, in terms that only technically-minded buyers understand. That shown (below) confirms that probable NOCT output of a ‘120 watt’ module is about 87 watts.
Broadly speaking most solar modules produce 70%-75% of that seemingly claimed by the vendor.
Solar module types
Of the two main types of solar modules now used, of the best known brand monocrystalline and polycrystalline, the former generally costs more. Amorphous solar panels can be made in a thin flexible form, but are only 10-12% efficient.
From left to right monocrystalline, polycrystalline and amorphous solar panels. Pic: solarbooks.com.au.
In 2021, a high quality solar module is 18%-21% efficient. It produces about 120 watts per square metre, weighs about 10 kg per 100 watts and costs $2-$3 per watt. A few makers have some about a third of that weight but cost more.
A solar regulator controls the output from the solar modules to the batteries. It enables charging to be quick and deep, but not to overcharge. Cheap regulators ($50-$100) are voltage-sensing only.
MPPT (Multiple Power Point Tracking) solar regulators work much as does dc-dc charging. They ‘juggle’ incoming voltage and current to optimise energy output. Vendors may claim ‘up to 30%’ more input. Solar reality is that there is no ‘gain’ as such. MPPT saves 10-12.5% otherwise lost. Worthwhile, but not as usually promoted.
All solar regulators need initial adjusting of time, battery voltage, type and capacity. Vendors invariably say it is quick and easy to do. At that point demand they show you how.
This is an actual data plate of a 120 watt solar module once owned by the author. The third column shows that the most probable output of this ‘120-watt’ module is 87 watts. Pic: rvbooks.com
Approximate guide to solar capacity
If you have a chest fridge/freezer of 80 litres or less, plus a few LED lights, radio and small TV, 240 watts of solar will cope much of the time.
Increasing solar input to about 300 watts allows you to stay on site without alternator or generator input, except for extended periods of rain and overcast sky.
Less solar is needed if you have a three-way fridge running on gas whilst camping. It is, however feasible to use solar on an RV to power a conventional 40-50 litre chest-opening fridge. It is also feasible to drive for a few hours each day – thereby charging via the alternator.
To run bigger fridges add an additional watt or so of solar capacity per extra litre of fridge capacity.