Understanding your derate factor and margin
What the derate factor actually represents
The derate factor is a single multiplier that captures how much your string's voltage grows as temperature drops below the 25C standard test condition. A factor of 1.10 means your string voltage sits 10% above its STC rating. This number stays constant no matter how many panels are in the string, since every panel in series experiences the same temperature and the same percentage increase. The broader temperature derate calculator covers this same underlying temperature effect from the production side rather than the voltage-limit side.
Why installers build in extra margin beyond the theoretical maximum
The calculated cold Voc figure is a theoretical maximum based on datasheet specs, but real-world panels have some manufacturing tolerance, and voltage measurement itself carries some uncertainty. A 3% to 5% margin below the inverter's absolute maximum voltage absorbs these small variations without requiring a perfect match between theoretical and actual performance. Skipping this buffer works fine most of the time, but leaves no room for error on the coldest morning of an unusually harsh winter. The string length voltage calculator shows the full voltage picture across cold, STC, and hot conditions together.
Cold climate regions need real local data
Sites in International Falls, Minnesota or Yellowknife, Northwest Territories see far larger derate factors than sites in Miami or San Diego, since record lows can differ by 40 degrees Celsius or more between these locations. Using a generic national average temperature instead of your site's actual record low is one of the most common string sizing mistakes, and it can understate cold weather voltage risk by a wide margin. Roof pitch is a separate site-specific variable that also affects real-world output, which the roof pitch output adjustment calculator covers on its own. The Department of Energy's PV system design basics cover temperature-driven design fundamentals in more general terms.
How the derate factor interacts with panel count
Because the derate factor applies uniformly across the whole string, a longer string amplifies the absolute voltage impact of the same percentage derate. A 5% cold weather increase on a 6-panel string adds roughly 15V, while the same 5% increase on a 12-panel string adds roughly 30V. This is part of why max panel counts drop meaningfully in colder climates even when using the exact same panel model. The min panels per string calculator handles the opposite hot-weather constraint on string length. The roof pitch output adjustment calculator turns this figure into an array size you can quote.
What to do once you know your margin
A comfortable margin above 5% needs no changes. A tight margin between 0% and 3% is worth revisiting, either by dropping a panel from the string or choosing an inverter with more input voltage headroom. A negative margin means the string exceeds the inverter's rated maximum at your site's record low, which needs a redesign before installation using the string sizing tool for a complete safe range, and NREL's PVWatts calculator can help confirm the production outcome once the redesign is finalized.