Why is 90 minutes the requirement?

NFPA 101 Section 7.9 and the IBC require emergency illumination of the means of egress to last at least 90 minutes after normal power fails. That window covers occupant evacuation and initial emergency response, so any unit that cannot sustain its load for 90 minutes at end of life fails.

How do I convert amp-hours to runtime?

Multiply amp-hours by the battery voltage to get watt-hours of energy, then divide by the load in watts and multiply by 60 for minutes. A 5 Ah, 12 V battery holds 60 Wh; driving an 18 W load it gives about 200 minutes before derates, comfortably over 90.

What is the end-of-life derate for?

Sealed batteries lose usable capacity as they age, yet the code requires 90 minutes at end of life, not when new. The derate (commonly 20 to 25 percent) reduces the rated energy so the runtime reflects a worn battery near replacement, giving a conservative, code-aligned result.

Why does the tool also show 50% load runtime?

NFPA 101 allows illumination to decline over the 90 minutes provided at least 60 percent of the initial level remains. Many self-contained units step down output to extend runtime, so the 50% figure illustrates how much longer the battery lasts at reduced output as a design sanity check.

Does meeting the runtime satisfy all requirements?

No. Emergency lighting must also provide adequate light levels along the egress path (an average of 1 footcandle initially per NFPA 101) and pass functional testing — a 30-second test monthly and a full 90-minute test annually. This calculator covers only the battery duration portion.

What is the Emergency Lighting Battery Runtime Calculator?

Takes emergency fixture load and battery capacity in Wh or Ah×V with an end-of-life derate, then computes available runtime at full and 50% load, flagging fixtures below the 90-minute NFPA 101 / IBC emergency illumination requirement. It runs free in your browser on Gera Tools, with nothing uploaded.

Emergency Lighting Battery Runtime Calculator

Name: Emergency Lighting Battery Runtime Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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Making sure the lights stay on for 90 minutes

When normal power fails, emergency and exit lighting must keep the means of egress illuminated long enough for everyone to get out. NFPA 101 and the IBC set that minimum at 90 minutes. A battery that is undersized, or simply aged, can fall short — and a failed annual test means a re-inspection. This calculator computes how long a unit’s battery actually lasts and checks it against the 90-minute rule.

How it works

The battery’s stored energy is computed in watt-hours. If you enter amp-hours and voltage, the tool multiplies them; watt-hours can be entered directly. It then applies two reductions: an end-of-life capacity derate, because the code requires 90 minutes from a worn battery rather than a new one, and a driver or inverter efficiency factor for conversion losses. Runtime follows directly:

effective_Wh  = rated_Wh × (1 − derate%) × efficiency
runtime (min) = effective_Wh / load (W) × 60

The result is shown at full load — the value that must clear 90 minutes — and at 50% load, illustrating the extra duration available where a unit steps output down while still meeting the 60% minimum illumination rule.

Worked example

A self-contained emergency unit has a 6 Ah sealed lead-acid battery at 12 V:

rated Wh      = 6 × 12 = 72 Wh
after 20% EOL derate: 72 × 0.80 = 57.6 Wh
after 90% efficiency: 57.6 × 0.90 = 51.8 Wh
load = 18 W (LED emergency driver)
runtime = 51.8 / 18 × 60 ≈ 173 minutes

That unit comfortably clears 90 minutes. Now consider a unit with a smaller 4 Ah battery at 12 V (48 Wh rated), the same load, and a 25% derate:

48 × 0.75 × 0.90 = 32.4 Wh
runtime = 32.4 / 18 × 60 ≈ 108 minutes

Still above 90, but the margin is smaller. If the load increased to 22 W or the derate worsened to 30%, this unit would fail the check. Running this before specifying or replacing units surfaces borderline cases that would otherwise only appear during the annual test.

The end-of-life derate: what NFPA 101 actually requires

NFPA 101 requires that emergency lighting be capable of operating for 90 minutes at the design load — meaning the system must pass this test not when new but throughout the life of the batteries. For sealed lead-acid batteries, capacity drops as the battery ages and approaches the replacement threshold. A 20–25% derate is a common engineering allowance for this end-of-life degradation; some designers use 25% for a more conservative check. The derate is not a code- specified number but a design margin to ensure code compliance through the battery’s replacement cycle.

Monthly and annual testing requirements

Battery runtime is only one compliance element. NFPA 101 also requires:

Monthly 30-second test — confirms the unit transfers to battery and produces light, but does not verify capacity.
Annual 90-minute test — a full discharge to confirm the battery actually sustains load for the required duration. This is the test that will catch undersized or degraded batteries.

Units that fail the annual test must be repaired or replaced before the next occupancy period. Documenting the test results per NFPA 101 is also required; this calculator’s output can support that documentation by providing the expected runtime at design parameters.