Study Finds Solar Panel Power Output Often Less Than Promised

Researchers at Germany’s Fraunhofer Institute for Solar Energy Systems ISE have revealed a recent trend of solar panel outputs often being less than nameplate rating. It may not be by much, but that’s not the point.

Solar panel nameplate capacities are often accompanied by a “power tolerance” figure, which indicates the range a PV module’s actual output might vary from its rated capacity. Back some years ago, this was usually a “±” figure expressed in Watts or as a percentage. For example, a 250-Watt solar panel may have had a power tolerance of ±5% noted, meaning the panel’s output could be 237W to 262.5W. It’s quite a range.

The industry generally has moved to positive power tolerance – particularly among brands considered better quality – meaning that if you buy a 440-Watt solar panel, then that is the minimum it would output under standard testing conditions; or perhaps a little more. It’s a selling point.

What Fraunhofer Found

Researchers at CalLab PV Modules at Fraunhofer ISE have tested 70,000 solar modules since 2012. After this data was filtered according to evaluation criteria, analysis of 1,034 performance measurements associated with monocrystalline solar panels revealed a new trend in recent years.

From 2012 to 2016, measurement deviations were always less than one percent on average and positive deviations were also frequently measured. In 2016, the deviation was 0.6 percent on average – and given this was during a period when ± power tolerance was commonly specified and clearly stated on datasheets, it was a non-issue. But since then it’s been a different story; particularly between 2020 and 2023.

“For the year 2023, this culminated in a negative deviation between the manufacturer’s specification and our review of about 1.3 percent,” said Fraunhofer ISE’s Daniel Phillip. “A positive deviation was hardly ever observed.”

The researchers saw a slight trend reversal in 2024, but still negative deviations of 1.2 percent on average Mr. Phillip said. Perhaps the slight improvement was due to manufacturers realising they were being over-optimistic – or heard rumblings the jig is up.

Which Manufacturers Are Involved?

A total of 15 manufacturers were included in the evaluation. Fraunhofer isn’t saying publicly who they were. But it provides a line-up of potential suspects as only manufacturers among the top 10 (assumed: for shipments) in the respective year under review were included in the analysis. While that incorporates all the big names in panel manufacturing, it doesn’t mean to say all of those companies have been “overly-optimistic”.

What’s A Few Watts Between Friends?

The discrepancies may not sound like much, and they aren’t really in terms of home solar given oversize systems are so common in Australia. This is where solar inverter capacity is less than total panel capacity in order to:

• Maximise the solar rebate.
• Get around inverter restrictions in some cases.
• Improve average inverter working efficiency.
• Take into account various system inefficiencies and the wide range of conditions the panels will be working under.

However, it’s not a good look for manufacturers if their datasheets can’t be trusted.

But on that point, back in 2023 SQ’s Kim revealed another trick often used – “measurement tolerance“; usually in fine print on a datasheet. Essentially, this is an allowable range of variation permitted in peak power watts measurement.

Let’s look at a 440W panel example again with a clearly stated positive power tolerance of 0~+5W. The datasheet I reviewed had a “measuring tolerance” (in tiny print) of ±3%; which is apparently a common figure.

That means this 440W panel could actually be as little as 426.8W. On the flip side, it could be as much as 453.2W. Multiply that by the number of solar panels and long-term it could translate to a significant amount of potential production either way. But again, system oversizing comes into play and some panels in a system may be a bit over and others a bit under; balancing out.

Still, if you’ve been sold a 10kW system, and there’s no accompanying documentation indicating it might be not quite 10kW, that’s what should be installed.

As Kim suggested, it’s not something Australian solar buyers or owners should lose much sleep over given warranty protections, but it’s an issue system designers should bear in mind for the same reason. It also highlights the importance of independent third-party testing by organisations such as Fraunhofer ISE; putting a spotlight on issues and reminding manufacturers they are being watched.