This long-form resource helps facility managers and electricians calculate battery runtime and load sizing for exit signs and emergency lights—so you can pass UL 924 and avoid surprises on annual 90-minute tests. It doubles as a practical battery runtime calculator and emergency lighting load calculator for real-world projects, with examples you can reuse as a battery duration calculator for extended runtimes. Written in plain English for Facilities and EHS teams. — Quick links: Emergency Lights · Exit Sign & Light Combos · Remote-Capable Emergency Lights
Why Runtime & Load Sizing Matters
When the power fails, your egress lighting must remain bright enough, long enough, for people to exit safely. Under-sized batteries lead to early dimming and potential non-compliance; over-sizing wastes budget you could spend elsewhere. The sweet spot is a battery sized to your actual load and conditions—fixture wattage, number of heads, required runtime, and ambient temperature—calculated with a simple emergency lighting load calculator instead of guesswork.
Most units leave the factory pre-sized for their built-in lamps. But any change—adding remote heads, choosing a 120/180-minute duration, or installing in cold environments—changes the math. That’s why a quick, transparent calculation is invaluable before you buy or reconfigure, and why a repeatable battery runtime calculator or battery duration calculator becomes part of your standard design toolbox.
UL 924 & the 90-Minute Rule
UL 924 certifies that an emergency lighting product can deliver compliant illumination for at least 90 minutes during a power outage. That 90-minute baseline appears across U.S. life-safety codes (e.g., NFPA 101) and is the default expectation for most occupancies. Some owners or AHJs require longer (120 or 180 minutes) in certain scenarios. Your battery duration calculator or spreadsheet should therefore start with your required duration and end with a simple question: Do we still meet 90 minutes under worst-case conditions?
Inputs You’ll Need
- Fixture load (W): Either the total fixture wattage, or derive from heads × watts/head; add exit-letter LED load if a combo unit. This is the starting point for any emergency lighting load calculator.
- Number of heads & watts/head: Halogen/Incandescent heads can be 7–12 W; LEDs often 1–5 W per head.
- Required runtime: 90 min (standard), optionally 120 or 180 min—these inputs drive the “duration” side of a battery duration calculator.
- Ambient temperature: Especially important for unconditioned spaces (stairs, warehouses, outdoors).
- Battery voltage: Typical unit equipment is 6 V or 12 V (also 3.6/7.2/9.6 V packs on some designs).
The Math (Wh & Ah, made simple)
Step 1: Total Load (W) = Fixture wattage, or (# heads × W/head) + sign-face LED watts (for combos). This is the core of any emergency lighting load calculator.
Step 2: Energy (Wh) = Total Load (W) × Runtime (hours). Example: 12 W × 1.5 h = 18 Wh.
Step 3: Temperature factor (optional) — if cold, add margin (e.g., 30% at ~0 °C; 60% at ~−20 °C). Adjusted Wh = Wh × factor.
Step 4: Convert to Ah = Adjusted Wh ÷ Battery Voltage. Example: 18 Wh ÷ 6 V = 3.0 Ah.
Step 5: Pass/Fail vs UL 924 — capacity must sustain your load for the target duration. If not, increase battery size, reduce load, or select a higher-capacity/remote-capable unit. The on-page battery runtime calculator automates these steps for you.
Temperature Derating (Cold & Heat)
Batteries are sensitive to temperature. In cold environments, available capacity drops—sometimes dramatically—shortening runtime. A simple planning rule is: add ~30% capacity for around 0 °C and ~60% for deep-cold (≈−20 °C). Units with battery heaters or cold-rated chemistries reduce this penalty; confirm with the spec sheet, and plug the adjusted values into your emergency lighting battery calculator to see the real effect on duration.
Remote Heads & Capacity
“Remote-capable” units can power additional heads. Manufacturer specs will list a maximum remote load (e.g., “50 W remote capacity for 90 minutes”). If you add remotes, your total load becomes unit heads + sign LED + remote heads. Re-run the math and verify compliance; exceeding the rating will cut runtime below 90 minutes. Treat the remote load as just another line in your emergency lighting load calculator so it doesn’t get overlooked.
Step-by-Step Sizing
- List loads: Heads × W/head, + sign face LED (combos), + any remote heads.
- Pick runtime: 90/120/180 minutes per AHJ or owner requirements.
- Check temperature: If cold, add a derate factor or select a cold-rated solution.
- Choose voltage: 6 V or 12 V (common); convert Wh → Ah to compare against available batteries.
- Validate: Expected runtime ≥ required runtime? If not, upsize battery or choose a higher-capacity unit. This is the same process your battery duration calculator follows behind the scenes.
Interactive Battery Runtime & Load Calculator
Enter what you know (fixture watts or heads × watts, your required runtime, and the ambient temperature). The calculator functions as an emergency lighting battery calculator for exit signs and emergency lights: it returns total load (W), required capacity (Wh/Ah), an estimated runtime, a UL 924 pass/fail flag, and a quick unit-type recommendation. In other words, it’s a simple, on-page battery runtime calculator and battery duration calculator built specifically for life-safety systems.
Load Inputs
Runtime & Environment
Optional: Known Battery Size
Total Load
Required Capacity
Estimated Runtime
UL 924 Check
Enter your values and click Calculate to run the battery runtime calculator.
Checklists & Templates (Downloads)
Emergency Lighting Test Log (CSV)
Track monthly quick tests and the annual 90-minute discharge in one place—fixture ID, location, pass/fail, notes. Use it alongside the on-page battery runtime calculator so test results match your design assumptions.
Battery Sizing Worksheet (XLSX)
Quick worksheet version of the on-page calculator—handy for walk-throughs when Wi-Fi is spotty. It functions as an offline battery duration calculator and emergency lighting load calculator you can carry on a tablet or laptop.
Frequently Asked Questions
What if my runtime comes up short?
Either reduce load (fewer/low-watt heads), raise battery capacity (bigger Ah), or select a remote-capable/high-capacity unit. Re-run the battery runtime calculator to verify.
Do I need to include the exit-face LED load on combo units?
Yes. It’s small (often ~2–3 W) but should be counted, especially for extended runtimes or cold locations.
How much cold derate should I use?
As planning guidance: ~30% extra capacity near 0 °C; ~60% in deep-cold (~−20 °C). Use manufacturer data where available.
Can I power remote heads from any emergency light?
No. Only “remote-capable” units are designed for it. Check the spec for maximum remote watts at 90 minutes and plug that into your emergency lighting load calculator.
Does battery voltage change the energy I need?
No—the energy requirement is in Wh (W × h). Voltage only changes the Ah number (Ah = Wh ÷ V) and which battery form factor fits.
Is this the same as a generic battery duration calculator?
General-purpose tools estimate battery life for all kinds of devices. This guide and on-page battery duration calculator are tuned to exit signs and emergency lights, including UL 924’s 90-minute rule, remote heads, temperature derating, and egress-specific load assumptions.