Updated December 2015

Solar modules lose energy when hot. They lose 4% to 5% energy output for every 10 degrees C rise from only a few degrees above freezing  – not the 25 degrees C that almost everyone believes. Here’s why – using evidence from the industry’s own data. 

A solar module’s power output is rated by makers at a claimed 25 degrees C. But they do not make it clear that 25 degrees C is not ambient air temperature around the solar module. It is the temperature of its glass covered black solar cells under a hot sun.  At an ambient of 25 degrees C those cells are at 47-49 degrees C and the module has already lost some 10% of its output.

That solar modules lose energy when hot is revealed in solar industry specifications, and often on a data panel on the solar module itself. But when asked whether solar modules lose energy when hot, most resellers and installers and any number of articles and books assume that 25 degrees C mentioned relates to ambient temperature. It does not. The proof that it relates to cell temperature is shown below.

solar kyocera sign

This data panel, on the rear of a 120 watt rated Kyocera solar module, is typical of solar modules generally. As can be seen from the third column (that indicates the module’s most probable output), the cells are at 47 degrees C and the power output is 87 watts. Pic: successfulsolarbooks.com

Another standard, Standard Operating Conditions (SOC) uses tests under controlled conditions that are not likely to be encountered in real life. The test is akin to having the solar module facing directly into the sun a 12 noon on a cold winter day on the top of a mountain close to the equator.

This information is revealed, but in tech-talk, in the makers’ full specifications and often on a data panel on the rear of most modules.

The SOC is used for design and development – but the results are also used for promotion and marketing. Not realising this when buying can present major issues for those aware of physics but not such ‘curious’ marketing. For those interested fuller details are included in my (ongoingly updated) books Solar Success, Solar That Really Works! and Caravan & Motorhome Electrics

Solar reality

Due to heat and other losses, the typically promoted output of most solar modules in temperate climates is (via a low price solar regulator) likely to be 70%-72% of that seemingly claimed – i.e. what it says on the box. 

If used via a genuine MPPT solar regulator the output in temperate climates is likely to be 80%-82% of that seemingly claimed. 

So-called amorphous solar modules are far less affected by heat but are less electrically efficient (i.e. they are much larger per watt). Some are available in flexible form – handy for mounting onto curved roofs etc. 

Every aspect of the design, installation and use of solar for homes and properties is covered in my constantly updated book Solar Success. That for camper trailers, caravans and motor homes is covered in my also constantly updated book Solar That Really Works!. My associated Caravan and Motorhome Electrics covers every aspect of that topic. It is also used extensively for training courses and by auto electricians in Australia and New Zealand. 

See also our associated website: caravanandmotorhomebooks.com

This article is copyright (2015) Successful Solar Books, Church Point, NSW Australia.