Checks the runtime a battery-backed emergency/egress lighting system (unit equipment or a central battery inverter) can deliver at a given load, against the common code-minimum duration — or, in sizing mode, works backwards from a target load and duration to the required battery capacity, applying usable depth-of-discharge, inverter/regulation efficiency, and an end-of-life aging margin.
Standards followed
NFPA 101 (Life Safety Code) §7.9.2.1 — minimum 1.5 hour (90 minute) emergency illumination durationNEC 700.12(F) / UL 924 — unit equipment minimum 90-minute supply requirement
What it can do
- Check mode: battery Wh + load W → runtime in minutes, compared against the 90-minute common minimum.
- Size mode: load W + target duration → required nameplate Wh and Ah, with DoD, efficiency and aging-margin derating.
- Grading band from BELOW MINIMUM to GENEROUS MARGIN relative to the 90-minute reference.
Example problem
Check mode: 60W emergency luminaire load, 108Wh battery, 80% usable DoD, 90% inverter efficiency.
- Usable Wh = 108 × 0.80 × 0.90 = 77.76 Wh
- Runtime = (77.76 / 60) × 60 = 77.76 minutes
≈77.8 minutes — BELOW MINIMUM against the 90-minute reference; battery/load combination needs revisiting.
The 90-minute figure is the widely-used NFPA 101/UL 924 reference duration — some occupancies, jurisdictions, or specific system designs (e.g. certain high-rise or assembly occupancy requirements) may call for a longer duration, so always confirm the actual required duration with the applicable code edition and AHJ before finalizing a design.