NREL slashes uncertainty rate for PV power measurement

February 27, 2019 // By Nick Flaherty
NREL slashes uncertainty rate for PV power measurement
Researchers at the National Renewable Energy Laboratory (NREL) have reduced the uncertainty in measuring the power of a photovoltaic (PV) module to just  ±1.1%, the lowest in the world.

This compares to a 0.4% tolerance on measurement of primary reference cells and 0.6% for secondary reference cells at NREL.

"This is over a $30 billion industry now," said Keith Emery, a consultant to the U.S. Department of Energy's (DOE's) ) and, before he retired in 2016, the point person for the laboratory's work to measure the efficiency of PV technology. "That $30 billion industry is weighted in units of watts, so a 1% uncertainty in a $30 billion industry is a big deal."

"Manufacturers need some kind of yardstick at the end of their production line because they're cranking out thousands of modules a day," said Dean Levi, Emery's successor and manager of the Cell and Module Performance group in the National Center for Photovoltaics at NREL.

Manufacturers price PV modules based on watts generated and typically provide a power tolerance of -0% to +3%. This means a 300W solar module, which sells for 30 cents per watt on the global wholesale market, has a margin of error for this module equates to 9W.

"Because of the way PV modules are rated for power, there's a pretty good chance they're selling you a 309-watt module for the price of a 300-watt module—and they don't like that," said Levi. "Manufacturers have been pushing for a long time for labs like ours to reduce our uncertainties because then they don't have to sell a 309-watt module for the price of 300 watts. If their uncertainty is plus or minus 1%, they can instead sell that 309-watt module as a 306-watt module, so they've made that much more money."

Although that only amounts to $1.80 more per module, large PV manufacturers produce more than 30 million 300-watt modules per year, so this small reduction in uncertainty can result in a $50 million improvement in annual revenues, he says.

To ensure proper calibration, manufacturers usually send at least two modules of the same model type

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