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.

Emergency Lighting Battery Runtime Calculator

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:

runtime (min) = effective energy (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.

Example and notes

A 60 Wh battery driving an 18 W emergency load, with a 20% end-of-life derate and 90% efficiency, delivers about 144 minutes — well over the minimum. Reduce the battery or raise the load and the runtime drops below 90, failing the check. Remember the code also requires adequate light levels on the egress path and functional testing: a 30-second test monthly and a full 90-minute discharge annually. This tool addresses duration only.