Quick definition
Panel wattage ratings come from lab testing under specific standard conditions that real installations rarely match exactly. A derate factor rolls together all the real-world losses, heat, wiring resistance, inverter conversion losses, dust and soiling, and more, into one combined percentage that adjusts the rated output down to a realistic expected number.
It's the difference between what a panel's label says and what a system actually delivers under normal operating conditions.
At a glance: production estimating tools commonly apply a combined derate factor in the 80 to 85% range as a starting point before adjusting for site-specific conditions; NREL's own PVWatts calculator uses a default total system loss of 14%, working out to an 86% derate factor, as its baseline before any site-specific adjustment.
Why it matters
A production estimate that ignores derating would badly overstate what a system actually produces, since it would essentially assume the system runs at its lab-tested peak rating year-round. Real systems never do that. Panels run hotter than their test conditions on a sunny rooftop, wiring has some resistance loss over its run length, inverters aren't perfectly efficient at converting DC to AC, and dust or pollen on the panel surface blocks some sunlight from ever reaching the cells.
Each of these losses is individually small, often just a percentage point or two, but they stack together into a meaningful combined effect. This is exactly why a reputable production estimate, whether from a design tool or an installer's proposal, should be built around a realistic derate factor rather than the panel's raw nameplate rating alone.
Derate factor also matters for comparing production estimates from different sources. If two proposals for the same roof show noticeably different expected annual output, checking what derate factor each one assumed is often where the real explanation for the difference lives, not necessarily a difference in equipment quality. The U.S. Energy Information Administration's solar explainer describes the real-world factors that keep a system's output below its rated capacity.
Where you'll see this
The temperature derate calculator isolates just the temperature component, showing how much heat alone reduces a panel's output compared to its standard test rating.
On the equipment side, the inverter noise and heat derate calculator shows how an inverter's own operating conditions affect its conversion efficiency.
This term comes up in every serious production estimate, in system design software, and any time two solar quotes for the same roof show meaningfully different projected output.
Related terms
Frequently asked
Many production estimates use a combined derate factor somewhere around 80 to 85%, meaning the system is expected to deliver roughly that percentage of its theoretical rated output once all real-world losses are accounted for together.
No, they're different concepts. A derate factor accounts for ongoing real-world losses present from day one, like heat and wiring resistance. Degradation rate is the separate, additional decline in panel output that happens gradually over years as the panel ages.
Temperature is often the largest single contributor, since panel efficiency drops as cell temperature rises above the standard test conditions used for panel rating labels. Hot climates and poorly ventilated roof mounts tend to see a bigger temperature-related derate than cooler climates.
Some factors are within a homeowner's control, like keeping panels clean to reduce soiling loss, or choosing a mounting style with better airflow underneath to reduce heat-related losses. Other factors, like inherent inverter efficiency and wiring losses, are largely set by system design and equipment choice.