Updated July 2017

Solar modules produce less than claimed by up to 30%. It is due to the solar industry quoting solar output using methods not encountered in real life. Such output is only (briefly) achievable in a testing laboratory.

Solar modules produce less than claimed – SOC

The so-called Standard Operating Conditions (SOC) due not reflect reality. They are akin to a horizontal solar module atop an equatorial mountain, at noon on a very cold mid-summer day. There is no issue with that for test purposes. That output, however, is used for promotions and sales. It is what installers quote. If challenged, they may admit it’s higher. They may say ‘it’s standard industry practice’. So is burglary in the thieving industry.

There is a second industry standard. This, the Nominal Operating Cell Temperature (NOCT) indicates typical output. It is not, however, used for marketing or sales.

Solar modules produce less than claimed – NOCT

NOCT data is revealed in the technical specifications. It is often disclosed on the solar module. NOCT output is typically 71% of that seemingly claimed. If used via a Multiple Power Point Tracking (MPPT) solar regulator it may be about 80%.

There are many areas where SOC and NOCT vary considerably.

Firstly, NOCT  irradiance is a more realistic 800 watts per square metre. Secondly it reveals that 250 C  is not ambient. It’s the typical 47.50 C of glass covered cells under a hot sun. That alone reduces output by 10%. The most probable output of this ‘120 watt’ solar module is thus 87 watts. An actual example is shown below

solar kyocera sign

Solar modules produce less then claimed. Here is a data panel of an actual solar module. The SOC and promoted output (of 120 watts) is shown in the second column, the most probable output NOCT (of 87 watts) is in the third column. Pic: successfulsolarbooks.com

A grid-connect system marketed as 1.5 kW is likely to produce 1.2 kW. This is known at regulatory levels. Rebates are based on that lower amount. Moreover, that lower amount must be declared by the installer.

Many ask if this practice can be prevented – but I’m an engineer/writer/publisher, not a lawyer!

Solar modules produce less than claimed – further information

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