(Updated December 2015)
It is true that solar modules produce less than claimed – by as much as 30%. It is due to the solar industry quoting solar module output using ‘Standard Operating Conditions’ (SOC). These are only rarely and briefly achieved outside solar testing laboratories. The ‘SOC’ output is the equivalent of measuring a perfectly sun-aligned solar module on top of a high mountain near the equator on a very cold mid-summer day. There would be no issue with this for test purposes, but this ‘claimed’ output is used also in promotions and sales. It is what installers quote. If challenged they usually admit solar modules produce less than claimed – but say ‘it’s standard industry practice’. So is burglary in the thieving industry.
There is a second industry standard. Called the NOCT (Nominal Operating Cell Temperature) it indicates the realistic typical output – but is rarely (if ever) used for marketing or sales.
The NOCT data is almost always shown also in manufacturers’ specifications. It is often also shown on a small data panel on the rear of the solar module. The data reveals that for most solar modules (and systems) the output is typically 71% of that seemingly claimed. For more sophisticated systems it is likely to be 80%.
Solar modules produce less then claimed. Here is an example data panel of an actual solar module. The promoted output is as in the second column, the most probable output is as in the third column.Pic:successfulsolarbooks.com
More about this is explained in my books Solar Success (for home and property systems), Solar That Really Works! (for cabins, boats, camper trailers, caravans and motor homes) and Caravan & Motorhome Electrics (for all aspects of the electrical systems in camper trailers, caravans and motor homes).
Because of the above, a grid-connect system marketed as 1.5 kW is unlikely to produce more than 1.3 kW. This is known at regulatory levels so rebates (where applicable) are based on that lower amount (that has to be legally declared by the installer).
Many ask if this practice can be prevented – but I’m an engineer/writer/publisher – not a lawyer!
If you found this article of value -so too will you of my books. If you are building, or planning to build a solar power system, the A$42.50 Solar Success can enable you to save thousands.