What does derate mean?
To derate something is to deliberately lower its officially assigned rating — most often the maximum power output or load a machine may safely handle. The term belongs mainly to technical fields: engineers derate engines, generators, transformers, and cables so they operate well within their limits, extending lifespan and guarding against overheating or failure in demanding conditions such as high altitude or extreme heat. In aviation, derated takeoff thrust is a standard fuel- and wear-saving practice. A secondary, chiefly British sense applies the verb to taxation, where to derate property was to reduce its assessed value for rating purposes. Derived from the prefix "de-" plus "rate", the word carries a neutral, procedural connotation rather than any judgment about quality — a derated machine is not inferior, merely operated conservatively. Its opposite, uprating, means raising a rating instead.
To reduce the officially rated power, capacity, or maximum permissible load of a machine, engine, or electrical device, typically for safety or longevity.
"The turbine was derated by fifteen percent after repeated overheating incidents."
Commonly used transitively; also found as the noun "derating" referring to the practice or the reduction itself.
(Chiefly British, historical) To lower the rateable value of a property, thereby reducing the local taxes assessed on it.
"Charities petitioned Parliament to have their premises derated entirely."
Airlines deliberately derate their jet engines on takeoff — running them below full power — because gentler operation makes the engines last dramatically longer.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of derate
"Derate" is formed transparently from the prefix "de-", indicating reduction or reversal, combined with "rate", which traces back through Old French to the Latin "rata" (fixed amount), itself from "reri", to reckon or calculate. The verb emerged in technical and administrative English, likely around the late 19th or early 20th century, as engineering and tax-rating systems matured. It shares its root with words such as "ratio", "ration", and "prorate". In older engineering literature it frequently appears hyphenated as "de-rate", though the solid spelling now dominates.
How derate is actually used
Primarily technical and formal register, used in engineering, aviation, and electrical contexts. The second sense relating to tax assessments is chiefly British and less common today. Often written "de-rate" in older texts.
Easily confused with derate
"Berate" means to scold or criticize someone angrily, while "derate" means to formally reduce the rated capacity of equipment — they sound similar but share nothing in meaning.
A "rebate" is a partial refund of payment, whereas to "derate" is to lower an engineered or official rating.