Quantifying the Impact of Module Binning

As the solar industry evolves, rigorous analysis and cost-benefit scrutiny are replacing rules of thumb. In this article, we investigate the relationship between module binning, or grouping modules based on specified power or current tolerances, and its impact on energy yields.

PV Module Binning

Output current and voltages vary slightly from one module to the next. Even best-in-class manufacturing techniques result in differences in module output values. A North American EPC recently flash-tested 90,000 300 W modules from a Tier-1 manufacturer. The results showed a variation in output voltages of 34.5 V–38.0 V and in currents of 7.89 A–8.73 A.

To account for output variations, PV modules have a power tolerance specification stating the potential deviation in actual power at STC from the module’s rated maximum power (Pmax). In addition, manufacturers group modules based on the power or current from each module’s flash-test results. This process of module binning groups modules with similar output characteristics based on a specified maximum percent variation. It enables manufacturers—and installers—to further differentiate modules with a smaller percent of variation in output tolerances, which reduces the losses associated with module mismatch.

Current versus power binning. Manufacturers generally group modules based on their power rating, which is also known as  power binning, although some sort modules based on their maximum power current rating (Imp), which is known as current binning. Power binning and current binning are not equal. Figure 1 shows that module output can vary in both current and voltage. Modules connected in series should operate at near-equal current output levels to reduce losses due to mismatch. Within a PV source circuit, variation in current—not voltage—results in mismatch losses.

Within a PV source circuit, different module voltages do not negatively impact one another because the sum of the voltages from each module connected in series equals the source circuit’s voltage. When the individual module voltages are normally distributed (as they are in Figure 1), the higher-voltage modules offset the lower-voltage modules in each string. This netting effect results in essentially no mismatch losses due to the voltage differences between modules.

However, binning modules based on their rated power can result in wide differences in module output current. Binning modules based solely on power results in a mix of relatively high-current with lower-voltage modules, and relatively low-voltage with high-current modules. So even the tightest power band of modules can have significant current mismatch. In Figure 1, for example, the entire sample has a power range of 12% and a current range of 6.5%. But as we tighten the power ranges to 1%, that reduces the current range to 3.5%.

Depending on the manufacturer, the binning tolerance (the range used to bin modules) typically varies from 2% to 10%. Most manufacturers use a 5% tolerance, meaning they group together modules that are within 5% of each other’s output values.

In addition, some manufacturers charge a higher price for modules grouped with narrower binning tolerances. For example, Trina Solar (trinasolar.com) offers current-binned modules that it has binned to a range of 2% for a premium of $0.01 to $0.02 per watt. Some developers and EPCs rebin their modules on the project site to improve array performance. Most installers expect that rebinning modules—for example, from a 5% to a 1% range—will improve system energy yield 1%–2%. However, the magnitude of these benefits should be confirmed prior to paying a premium for smaller binning tolerances or rebinning modules on-site.

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