ASHRAE vs. Record Lows
Low temperatures create high PV voltages, and PV system design needs to account for these temperatures to ensure the array is compatible with the inverter. But how do we predict what the low temperatures will be for the next 30 years?
PV industry standard practice was to use an area’s record low temperature for maximum voltage calculations, but designers are now turning to data published by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE), as recommended by the 2011 National Electrical Code (NEC). The ASHRAE data is based on complex formulas of average and historical temperatures, and tends to be much less conservative than the record low temperature. For example, the record low for Pitkin, Colorado, is -60°F, while the ASHRAE extreme annual mean minimum design dry bulb temperature for a nearby area is -24°F. For the Metzlers’ system, this 36°F difference would have made a big impact in system design, and using the record low temperature would have limited our options for string sizing. As suggested by the NEC, and knowing there is already some wiggle room built into the system design to begin with, we felt confident using the ASHRAE data for this project.