(Updated 16 December 2015)

Knowing solar input available for homes is easy. It’s done much like measuring rainfall – where rain is captured each full day in a ‘bucket’ and shown in inches or millimetres. Solar input available for homes is measured by the energy of sunlight (in watts) on a flat 1.0 square metre. It is expressed in units the industry calls Peak Sun Hours (PSH). One PSH is an intensity of 1000 watts per square metre/hour (1 kWh/m). In practice, that intensity is mostly confined to temperate areas for an hour or two each side of midday in places like Australia’s Alice Springs, or Houston (Texas). In a Melbourne or London mid-winter, capturing 2 PSH will take a full day. (This article relates to fixed locations. For those planning to travel extensively in camper trailers, caravans and motor homes please see Solar Input Available for Caravans.)

Solar map Australia

Depending mainly on latitude and season, solar input in Australia varies from 2.0 PSH down south in winter, to 7-8 PSH (in central and southern areas in summer). Northern Australia has less variation: from 5.5 PH in winter to 6.5 PSH in summer. Pic: courtesy of http://solargis.info

Solar map global

Solar input globally. As can be seen the potential in Africa, Australia and much of the USA is high. That it is lower than might be expected in parts of Asia is probably due to it being blocked by high atmospheric pollution. Pic: courtesy of http://solargis.info

What solar modules really produce

Today’s commercial solar panel are 10-20.5% efficient. Whilst a solar panel of one square metre may have the equivalent of 1000 watts per square metre of sunlight shining on it, the best it can theoretically convert into electricity is 205 watts. In practice there are further losses.Reality is about 175 watts (see below for why). There are inevitable daily variations, but the maps above are a reasonable guide. (Successful Solar Books can provide detailed monthly information for any place on Earth for A$175 – see Solar Success order form).

As with many products, that claimed promotionally may not be achievable in typical use. Solar panels are no exception. The technical explanation (not essential to know) is this. Solar panels modules produce maximum power (volts times amps) at around 17.1 volts, but a 12 volt battery however requires 13.0-14.7 volts to fully charge. With a basic solar regulator, the voltage in that gap between the solar voltage and the charger’s needs is not accessible.

So-called Multiple Power Point Tracking (MPPT) regulators ‘juggle’ the available energy (reducing the volts and increasing the amps to optimise watts). This recovers some part of the input otherwise not accessible. It is particularly effective where the battery is low in charge, and during the early and late hours of the day. The technology is often claimed to ‘increase’ energy by 25-30% – but it simply reduces some otherwise lost: typically 10%-15% a day. This is enough to justify its use but far less than the ‘up to 30%’ commonly claimed. The MPPT function is built into all grid-connect solar regulators, and also in the better stand-alone solar system regulators

If used with a cut-price solar regulator, most polychromatic and monochromatic solar modules (and that’s about 99% of them) produce about 70% of that seemingly claimed. If used with a Multiple Power Point Tracking (MPPT) solar regulator they produce 80-85% of that seemingly claimed.

(The most efficient solar panels are monochromatic (17-20.5%). Polychromatic panels are typically 12-16%. The now rare amorphous panels are typically 10-12%.)

Knowing solar input available for homes involves multiplying the area’s daily PSH by 70% of what you thought your solar panels produced (i.e. that claimed promotionally). Or with an MPPT unit, by 80-85%.

Heat loss

Solar modules lose power as they heat up (about 4% for every 10 degrees C). This is not an issue in temperate climates, but can be in the tropics. This should be taken into account when scaling the system.

Further, many people wrongly assume that solar input in tropical areas is higher year around. In reality that’s only so in winter. There can also be issues with solar-run fridges in the tropics as it stays hot at night as well as all day. Fridge energy usage may be 40% greater.

Optimising solar output

For optimum solar input, solar panels need to face directly into the sun. Mechanical tracking enables this but is complex and costly. Solar panel prices have plummeted by 80% since 2010.It’s now much simpler and cheaper to accept the typical 30% loss and add extra solar to compensate. Locate them facing true north (not compass north) in the southern world, and true south in the northern world. For the best year-round input they should be tilted at the location’s latitude angle. (To establish that Google the location plus ‘latitude’). To have more input in  winter (at the expense of summer) the panels should be at latitude angle plus about 12-15 degrees.  

Exact angling is not needed. The sun’s irradiation is far from a ‘shaft’ of light. It is often so diffused that variations of plus/minus 10 degrees in north/south facing, or tilt, cause less than 5% daily difference. Over a whole year, the solar input for Adelaide with modules at the optimum fixed tilt there (30 degrees) results in an average gain of about 8% compared with horizontal mounting. 

All solar modules have a designed maximum power output that, no matter how much sun, they cannot exceed. They may  heat up and actually lose power – typically by 4-5% for each 10 degrees C increase. The now less commonly used amorphous solar modules are not heat affected. All others lose some power when they become hot.

 Further information

All of my books are written much as is this article. If you enjoyed reading this you’ll also like my books. All are written in plain English and based on decades of personal experience. Click on  Review of Solar Success and also About Collyn Rivers.

A full explanation of the above, and every aspect of solar and its use, is covered in much greater depth in my two globally selling solar books. Solar That Really Works is RV oriented.  Solar Success is oriented toward larger home and property solar – including grid connect. See also Article: Living With Solar. For details of the author’s own big solar system see Article: All Solar House.

Solar Success shows how to establish typical solar input for mid-summer and mid-winter for Australia. It also has recommended tilt angles for optimum yearly input. It also shows how to optimise for winter solar input (at the expense of summer solar input) and vice versa. There are minor changes in solar irradiation over time. Solar Success is updated each print run (typically every 18 months) to allows for this. It is based on a running ten-year average.

If installing a costly system one can save thousands of dollars by ensuring you have the optimum amount of solar. To enable this Successful Solar Books can provide full monthly maximum and minimum averages for your area (of about 100 km by 100 km) anywhere in the world. It includes tilt angles for optimising input year-round or summer or winter. The price is A$175 (and includes a free copy of Solar Success) airmailed to anywhere on earth.

To order please email [email protected] and (ideally) provide your latitude and longitude – or the names of the three closest major towns. (Payment is via credit card or direct bank transfer).

My books sell globally. They are sent by airmail. 

This article is copyright Successful Solar Books, PO Box 356, Church Point, NSW 2105, Australia.