Does the derating factor account for both aging and temperature, or just one of these effects?

The standard 20% derating factor is generally understood to provide margin covering realistic aging-related capacity decline over a battery's typical service life; it isn't necessarily calibrated for extreme temperature conditions specifically. A system installed in an unusually cold, uncontrolled environment may warrant additional engineering judgment beyond the standard derating factor alone.

Do lithium batteries (LiFePO4) need the same 20% derating factor as sealed lead-acid?

Lithium battery chemistries generally have different, often more favorable, aging and capacity-retention characteristics than SLA batteries, which is part of why some FACP manufacturers now support lithium options with different documented service life expectations — but any specific derating approach for a lithium-based system should be based on that specific battery's manufacturer-documented performance data, not simply assumed identical to the traditional SLA-based 20% convention.

What happens if a battery is installed without applying any derating factor?

A battery sized only to the raw (non-derated) calculated requirement may perform adequately when brand new, but as it ages and its real capacity declines through normal sulfation and other effects, it can fall below the actual required standby-plus-alarm capacity well before its next scheduled replacement — a genuine safety and code-compliance gap that the derating requirement exists specifically to prevent.

Engineering·6 min read·August 14, 2026

📉 Why NFPA 72 Requires a Derating Factor on Calculated Battery Capacity

What the NFPA 72 §10.6.7.4 derating factor actually accounts for, why battery capacity declines with age and temperature, and how the derating calculation ensures batteries remain adequate throughout their service life.

By EngineersUniverse Editorial Team

Engineering Editorial & Technical Review Team

Published August 14, 2026

Why the Raw Calculated Capacity Isn't the Final Answer

Once standby and alarm ampere-hour requirements are calculated and summed (C = I_standby × T_standby + I_alarm × T_alarm), NFPA 72 §10.6.7.4 requires dividing that raw figure by (1 − D), where D is a derating fraction — commonly 20% — before arriving at the actual minimum battery capacity to specify. This isn't a safety margin added out of general caution; it accounts for a specific, well-documented, and otherwise-unaddressed physical reality: battery capacity genuinely declines over the battery's service life, and a battery selected to exactly match the raw calculated requirement when brand new would fail to meet that same requirement well before its next scheduled replacement.

Why Sealed Lead-Acid Battery Capacity Actually Declines

Sealed lead-acid (SLA/VRLA) batteries, the most common chemistry used in fire alarm system battery backup, experience a well-understood aging mechanism: sulfation, the gradual formation of lead sulfate crystals on the battery's internal plates during normal charge/discharge cycling, which progressively reduces the plates' effective surface area and therefore the battery's usable capacity. This process is gradual but steady and essentially unavoidable over a battery's normal service life, which is why battery capacity is understood to decline meaningfully — not remain constant — from installation to end of service life.

Why Temperature Also Reduces Effective Capacity

Beyond age-related sulfation, battery capacity is also temperature-dependent — SLA batteries deliver reduced effective capacity at lower ambient temperatures compared to their rated capacity (typically specified at a standard reference temperature, often 25°C/77°F), meaning a battery installed in a cooler equipment room or unconditioned space may deliver less usable capacity than its nameplate rating would suggest, even when relatively new. This temperature effect compounds with age-related capacity decline, both working in the same direction (reducing effective available capacity below the nameplate rating).

What the Derating Calculation Actually Does

Dividing the raw calculated requirement by (1 − D) — for a 20% derating factor, dividing by 0.80 — effectively inflates the specified battery size so that even after accounting for expected end-of-service-life capacity decline (from aging and, implicitly, reasonably expected temperature conditions), the battery still delivers at least the raw calculated capacity when it's most needed. For example, a raw calculated requirement of 40 Ah, derated by 20%, requires specifying a battery rated for at least 40 ÷ 0.80 = 50 Ah — the extra 10 Ah of nameplate capacity is specifically the margin expected to be lost to aging and other capacity-reducing effects by the time the battery is near its scheduled replacement.

Why 20% Is the Commonly Cited Value, But Not Universal

A 20% derating factor is widely cited and commonly used as the standard NFPA 72 default, but the code framework allows for the specific derating factor to be adjusted based on documented battery performance characteristics and expected service conditions — a battery chemistry or specific product with documented superior aging characteristics, or a system with a shorter planned replacement interval, could potentially justify a different derating value, though 20% remains the conventional default absent specific justification otherwise. This site's Battery Backup Calculation Worksheet offers 20% as its default with 10% and 25% as selectable alternatives, reflecting that the appropriate value can vary by specific project circumstances and applicable local code interpretation.

Why the Battery Replacement Interval and Derating Factor Are Linked

The derating factor implicitly assumes the battery will be replaced on a defined schedule before its capacity has degraded beyond what the derating margin was designed to cover — NFPA 72 §10.6.7.6 typically calls for SLA battery replacement around every 4 years (or per manufacturer guidance, or upon failing a load test), and the 20% derating factor is calibrated with this kind of replacement interval in mind. A system that neglects scheduled battery replacement and runs batteries well beyond their intended service life is operating outside the assumption the derating calculation was built on, regardless of how correctly the original sizing calculation was performed.

Topics covered

NFPA 72 derating factorbattery capacity agingsealed lead acid battery degradationbattery derating calculation