Updated 2 July 2017

Knowing solar input for homes is easy. It’s done much like measuring rainfall – where rain is captured each full day in a ‘bucket’ and shown as a total.

 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 what 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 a Melbourne or London mid-winter, capturing 1-2 PSH takes a full day. (This article relates to fixed locations. For those planning to travel extensively click on Solar Input Available for Caravans.)

Solar map Australia

Solar input in Australia varies from 2.0 PSH in winter, to 7-8 PSH (in summer). Northern Australia has less variation. It is from 5.5 PH in winter to 6.5 PSH in summer. Pic: courtesy solargis.info

Solar map global

Solar input globally. The potential in Africa, Australia and much of South Africa is high. It is lower than might be expected in parts of Asia due partially to atmospheric pollution. Pic: solargis.info

What solar modules really produce

Today’s commercial solar modules are 10-20.5% efficient. A solar module of one square metre may have the equivalent of 1000 watts per square metre of sunlight shining on it. The most it can theoretically convert into electricity is thus 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.

That claimed by promotionally is not achievable in real use. Solar modules produce maximum power (volts times amps) at around 17-18 volts. A 12 volt battery, however, requires 13.0-14.7 volts to fully charge. With a basic solar regulator, that gap between solar voltage and charging needs is not accessible.

In practice you are likely to have about 70% of that seemingly claimed if using a cheap solar regulator. That can be increased to about 80% using the regulators described below.

What solar modules really produce – MPPT

So-called Multiple Power Point Tracking (MPPT) regulators assist recovering some of that input. It is mainly effective when the battery charge is low, also during early and late in the day. Most vendors claim ‘increases’ energy of ‘up to 30%’. In practice it reduces that otherwise lost by typically 10%-12.5% a day. This can justify its use. It is, however, far less than commonly claimed. MPPT is built into all grid-connect solar regulators, and also in up-market stand-alone solar system regulators

If used with a cut-price solar regulator, most polychromatic and monochromatic solar modules 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 modules are monochromatic (17-20.5%). Polychromatic modules are typically 12-16%. The now rare amorphous modules 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 modules 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 100 C). This is not an issue in temperate climates, but it 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. There, it stays hot at night as well as all day. Fridge energy usage may thus be 40% greater.

Optimising solar output

For optimum solar input, solar modules need to face directly into the sun. Mechanical tracking enables this, but is complex and costly. Solar module 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 modules 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) modules should be at latitude angle plus about 15-20 degrees.  

Exact angling is not necessary. The sun’s irradiation is far from a ‘shaft’ of light. It is often diffused. Variations of plus/minus 100 in north/south facing/tilt, cause less than 5% daily difference. Over 12 months, solar input for Adelaide with modules at the tilt optimum (30 degrees) results in an average gain of about 8% compared with horizontally. 

All solar modules have a designed maximum power output. No matter how much sun, they cannot exceed that. They lose power when hot, – typically by 4-5% for each 100 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

Every aspect of solar is covered in Solar That Really Works (for RVs) and Solar Success (homes and properties). See also: Living With Solar. For details of the author’s solar see: All Solar House.

My books sell globally. Overseas orders are sent by airmail. 

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