---
title: "Derate (capacity)"
description: "A derate is reduced operating capacity below the equipment's nameplate, imposed because a limiting condition has been reached. Unlike a forced outage (full shutdown), a derate keeps the unit running at lower throughput while the limit persists."
canonical_url: "https://sylio.co/glossary/derate-capacity"
last_updated: "2026-06-28T02:29:31.718Z"
---

A **derate** is reduced operating capacity below the equipment's nameplate, imposed because a limiting condition has been reached. Unlike a [forced outage](/glossary/forced-outage) (full shutdown), a derate keeps the unit running at lower throughput while the limit persists.

## Fouling-driven derates

- **ID fan capacity limit** — high baghouse [ΔP](/glossary/differential-pressure-baghouse) demands more fan power than available, forcing load reduction
- **Boiler tube-metal temperature limit** — [fouling](/glossary/fouling) reduces heat absorption, raising tube-metal temperature; protective derate engaged
- **Stack opacity limit** — [ESP](/glossary/electrostatic-precipitator) efficiency loss forces load reduction to meet emission limits
- **HRSG approach-temperature limit** — fouling on gas-side surfaces reduces heat recovery; gas-turbine output drops

## Economic impact

Derates are usually less costly per hour than outages but can persist much longer. A 5% derate sustained for a month on a 500 MW unit loses ~9,000 MWh — comparable to a multi-day forced outage but easier to overlook in the maintenance ledger.

## Sonic horns and derate avoidance

[Sonic horns](/glossary/sonic-horn) preserve heat-transfer effectiveness, [ESP](/glossary/electrostatic-precipitator) collection efficiency, [baghouse](/glossary/baghouse) ΔP and [hopper](/glossary/hopper) discharge. Each of these directly defends against the most common fouling-driven derate triggers.

## Related terms

- [Forced outage](/glossary/forced-outage)
- [Heat rate](/glossary/heat-rate)
- [Fouling](/glossary/fouling)
- [Differential pressure (baghouse)](/glossary/differential-pressure-baghouse)