Emergency Lighting Duration Calculator

Calculate

Duration Check: does this battery run long enough? Required Capacity: how large must the battery be?

NFPA/NEC basis judges compliance against 90 minutes regardless of entered duration. Custom basis uses entered duration as-is but labels the result as not an NFPA/NEC check.

Used to trigger soft checks. DC unit equipment typically has inverter efficiency 1.0; central AC inverters run 80–90%.

Enter total emergency watts directly, or let the calculator multiply fixture count × watts each.

Total connected emergency-mode watts. Use emergency-mode output — LED drivers often run fixtures at reduced power in emergency mode.

Enter watt-hours directly, or amp-hours with battery voltage (Wh = Ah × V). Note: Ah ratings are often at the 20-hour rate — a 90-minute discharge may yield less.

Nameplate watt-hour capacity of the battery.

Leave blank to use the 90-minute NFPA 101 / NEC 700.12 minimum. Enter a higher value to screen against a stricter AHJ or facility requirement. Entering less than 90 on the code basis flags the entry — compliance is still judged against 90.

Efficiency as a decimal (0 < f ≤ 1). DC unit equipment: 1.0. Central AC inverter: typically 0.80–0.90 per manufacturer. Blank defaults to 1.00.

End-of-life design margin (0 < f ≤ 1). Screening default: 0.80. Replace with manufacturer end-of-life data where available.

Reduce below 1.0 for cold locations per manufacturer capacity-temperature data. Blank defaults to 1.00.

Overview

Use this calculator to check whether an emergency-lighting battery system will run for the required duration, or to size the battery capacity a system needs. Enter the battery capacity in watt-hours or as amp-hours and voltage, the connected emergency load in watts or as fixtures times watts each, and the calculator returns the runtime and screens it against the requirement. In capacity mode, enter the load and the duration, and it returns the required watt-hours and amp-hours after derating.

The benchmark is 90 minutes. NFPA 101 (7.9.2.1) requires emergency illumination for a minimum of one and a half hours after loss of normal power, and NEC 700.12 carries the same period for battery and unit-equipment systems. UL 924 is the listing standard for emergency lighting and power equipment used in this context; listed equipment is evaluated for 90-minute performance, and the specific listing and manufacturer data govern. The calculator judges compliance against the 90-minute minimum: entering a shorter target does not relax the screen, it flags the result as below the code minimum instead.

The runtime estimate applies a derating chain — battery aging, temperature, and inverter efficiency — with stated screening defaults you can replace with manufacturer and site data. It is a design and review aid, not a substitute for the manufacturer's runtime data or a life-safety design.

How to Use This Calculator

  1. Pick the mode — Duration Check takes a battery and a load and tells you how long the system runs. Required Capacity takes a load and a duration and tells you how large the battery must be.

  2. Enter the load as total emergency watts, or as fixture count × emergency-mode watts each. Use the emergency-mode watts: LED drivers often run at reduced output in emergency mode.

  3. For Duration Check, enter battery capacity in watt-hours, or as amp-hours with battery voltage.

  4. Set the required duration or leave it blank for the 90-minute code minimum. On the code basis, entering a shorter duration does not lower the compliance screen — it flags the entry as below the code minimum instead.

  5. Review the derating factors — battery aging defaults to 0.80, temperature to 1.00, inverter efficiency to 1.00 (DC unit equipment). For central AC inverters, enter the manufacturer efficiency (typically 0.80–0.90).

  6. Read the result — the headline shows runtime vs the required duration, or the required capacity, with the status badge and derating chain below.

A passing runtime estimate is a design aid, not a substitute for manufacturer runtime data, UL 924 listing verification, or an engineered life-safety design.

Inputs & Outputs

Inputs

  • •Calculation Mode — Options: Duration Check — does this battery last long enough?, Required Capacity — how large must the battery be?
  • •Standard Basis — Options: NFPA 101 / NEC 700.12 / UL 924 (90-minute code basis), Custom requirement (non-NFPA/NEC check)
  • •System Type — Options: DC Unit Equipment (self-contained battery + driver), Central Inverter AC system, Custom / Other
  • •Load Entry Method — Options: Total emergency watts, Fixture count × emergency watts each
  • •Emergency Load (W)
  • •Number of Fixtures
  • •Emergency Watts per Fixture (W)
  • •Battery Capacity Entry — Options: Watt-hours (Wh), Amp-hours × Voltage (Ah × V)
  • •Battery Capacity (Wh)
  • •Battery Capacity (Ah)
  • •Battery Voltage (V)
  • •Required Duration (min)
  • •Inverter Efficiency
  • •Battery Aging Factor
  • •Temperature Derating Factor
  • •System Voltage (for Ah output) (V)

Outputs

  • •Emergency Load (W)
  • •Usable Battery Energy (Wh)
  • •Effective Load (after inverter efficiency) (W)
  • •Overall Derating Factor
  • •Runtime (min)
  • •Effective Required Duration (min)
  • •Margin above Required (%)
  • •Shortfall (min)
  • •Required Battery Capacity (Wh)
  • •Required Amp-Hours (Ah)
  • •Compliance Status

Formula

Calculator Formula

This calculator uses a two-mode derating-chain model.

Derating chain:

usable_Wh        = capacity_Wh × aging_factor × temperature_factor
effective_load_W = load_W ÷ inverter_efficiency
overall_derating = inverter_efficiency × aging_factor × temperature_factor

Capacity in Ah: capacity_Wh = Ah × battery_voltage

Load as fixtures: load_W = fixture_count × emergency_watts_each

Duration Check:

runtime_min = usable_Wh ÷ effective_load_W × 60
margin %    = (runtime − required) ÷ required × 100

Required Capacity:

required_Wh = (load_W × minutes ÷ 60) ÷ overall_derating
required_Ah = required_Wh ÷ system_voltage

Effective required duration (code basis):

effective_required = max(entered_duration, 90 min)

On the NFPA 101 / NEC 700.12 basis, the effective required duration is the larger of the entered value and 90 minutes. Entering 60 min does not lower the screen — it flags the entry as below the code minimum and still judges compliance against 90 min.

Compliance gate:

NON-COMPLIANT when runtime_min < effective_required − 0.005 (EPS tolerance for floating-point equality).

BELOW-CODE-MINIMUM when basis is NFPA/NEC and entered duration is below 90 min but runtime is ≥ 90 min.

Margin bands (COMPLIANT only):

0 % ≤ margin < 15 %   → MINIMAL MARGIN
15 % ≤ margin < 50 %  → MODERATE MARGIN
margin ≥ 50 %         → AMPLE MARGIN

The requirement is met at exactly 90 minutes. Margin bands describe design headroom, not degrees of code compliance.

Variable Reference

Variable Meaning Units
capacity_Wh Nameplate battery capacity Wh
Ah, battery_voltage Capacity in Ah × V → Wh Ah, V
load_W Emergency-mode load W
aging_factor Battery end-of-life margin 0–1
temperature_factor Cold-temperature derating 0–1
inverter_efficiency Inverter conversion efficiency 0–1
overall_derating Product of all three factors 0–1
effective_required Actual compliance threshold min
runtime_min Computed runtime min
required_Wh Required battery energy after derating Wh

Code Basis vs Custom Duration

The calculator separates two questions: what the code requires, and what you typed. On the code basis, the requirement is the NFPA 101 / NEC 700.12 minimum of 90 minutes, and the effective requirement is the larger of 90 and your entry — so entering 120 minutes raises the bar, while entering 60 does not lower it. A 60-minute entry produces a below-code-minimum flag and the screen still runs against 90, because lowering the target is not a compliance path.

The custom basis exists for the cases a different standard genuinely governs — a specification, a non-US code, a process-safety requirement. There the entered duration is used as-is, and the result carries a label that it is not an NFPA 101 / NEC 700.12 compliance check. Authorities having jurisdiction can require more than 90 minutes; none of the governing documents permits less.

What is Emergency Lighting Duration

Emergency lighting duration is the time a battery-powered emergency lighting system keeps the egress path illuminated after normal power fails. The figure that matters is 90 minutes: NFPA 101 requires emergency illumination for not less than one and a half hours, and NEC 700.12 applies the same period to storage-battery systems and unit equipment. The period exists to give occupants time to evacuate, including people who move slowly or need assistance, before the lighting fades.

The runtime of a battery system is its stored energy divided by the load it feeds. A battery holding 67 usable watt-hours feeding a 36-watt load runs for about 112 minutes. The complication is the word usable. A battery's nameplate capacity is reduced by age, by cold, and by the rate of discharge, and a central inverter consumes part of the energy as conversion loss. That is why the calculation applies a derating chain rather than dividing nameplate numbers, and why the amp-hour rating on a battery label, usually stated at a 20-hour discharge rate, overstates what the battery delivers in a fast 90-minute discharge.

Two further requirements sit alongside the duration and are checked by other means. NEC 700.12 requires the battery to hold at least 87.5 percent of its nominal voltage through the discharge period, which is verified against the manufacturer's discharge curve. And the illumination itself must meet photometric levels: an initial average of 1 footcandle with a 0.1 footcandle minimum along the egress path, permitted to decline to 0.6 and 0.06 at the end of the 90 minutes.

The 90-Minute Requirement

Ninety minutes is the floor across the governing documents: NFPA 101 section 7.9.2.1 for the life-safety requirement, NEC 700.12 for the electrical installation, and UL 924 as the equipment listing standard. An authority having jurisdiction can require more, which is why the required duration is editable upward. It cannot meaningfully be set lower for a code check, so when a shorter duration is entered on the code basis, the calculator judges compliance against 90 minutes and labels the entry as below the code minimum.

Battery Ah Ratings and the 90-Minute Discharge

The amp-hour figure on a battery label is tied to a discharge rate, commonly the 20-hour rate. Discharged over 90 minutes, the same battery delivers noticeably less energy, because capacity falls as discharge current rises. Multiplying nameplate Ah by voltage therefore gives an optimistic watt-hour figure for emergency duty. The aging factor in the derating chain absorbs part of this, but the reliable source is the manufacturer's 90-minute or one-hour rating for the specific battery.

What This Calculator Does Not Prove

A passing runtime estimate is one input to a compliant design, not the whole of it. This calculator does not prove:

  • UL 924 listing — that comes from the listed equipment itself and its documentation
  • The NEC 700.12 requirement that battery voltage stay at or above 87.5 percent of nominal
  • Footcandle levels or uniformity along the egress path
  • Transfer within 10 seconds, charger capacity, or recharge time
  • A passed annual 90-minute test, or acceptance by the authority having jurisdiction

Key Facts

  • NFPA 101 and NEC 700.12 establish the 90-minute (1.5-hour) minimum duration; UL 924 is the equipment listing standard for emergency lighting and power equipment.
  • Runtime is usable watt-hours divided by the effective load: a 67 Wh usable battery on a 36 W load runs about 112 minutes.
  • Battery aging, temperature, and inverter efficiency all reduce runtime; the screening defaults here are 0.80, 1.00, and 1.00.
  • Amp-hour ratings are discharge-rate dependent; a battery delivers less in a 90-minute discharge than its 20-hour-rate label suggests.
  • NEC 700.12 also requires the battery to hold at least 87.5 percent of nominal voltage through the period, verified from the manufacturer's discharge curve.
  • Emergency-mode fixture watts are often lower than normal-mode watts on LED emergency drivers; the emergency figure is the one that counts.
  • NFPA 101 requires a monthly 30-second functional test and an annual 90-minute full-duration test.
  • Transfer to emergency power must occur within 10 seconds of normal power loss.

Applications

  • Checking whether a central inverter or battery unit carries its connected emergency load for 90 minutes
  • Sizing the battery capacity for a new emergency lighting system before selecting equipment
  • Verifying remote-head loading on a unit-equipment host against its battery capacity
  • Checking emergency-mode LED driver load before adding remote heads to a unit
  • Reviewing an existing system before adding fixtures to an emergency circuit
  • Estimating whether an aged battery still covers the load ahead of the annual 90-minute test
  • Comparing battery options in watt-hours and amp-hours at the system voltage
  • Plan review, inspection screening, and exam preparation on emergency lighting requirements

Example Calculation

Example 1 — Runtime check

A 12 V, 7 Ah battery feeding 36 W of emergency lighting, default factors (aging 0.80, temperature 1.00, efficiency 1.00), 90-minute requirement.

Capacity: 12 V × 7 Ah = 84 Wh
Usable:   84 × 0.80 × 1.00 = 67.2 Wh
Runtime:  67.2 ÷ 36 × 60 = 112 min (1:52)
Margin:   (112 − 90) ÷ 90 = 24.4%
Result:   COMPUTED / COMPLIANT — MODERATE MARGIN

Example 2 — Non-compliant load increase

The same battery with the load increased to 50 W.

Runtime:  67.2 ÷ 50 × 60 = 80.6 min — short of 90 by 9.4 min
Result:   COMPUTED / NON-COMPLIANT
Fix:      capacity of at least 50 × 1.5 ÷ 0.80 = 93.75 Wh (7.8 Ah at 12 V)

Example 3 — Required capacity for a central inverter

A 200 W central-inverter load, 90 minutes, inverter efficiency 0.85, aging 0.80.

required_Wh = (200 × 1.5) ÷ (0.85 × 0.80 × 1.0) = 441.2 Wh
required_Ah = 441.2 ÷ 12 = 36.8 Ah at 12 V
Result:   COMPUTED (green)

Select the next manufacturer-rated size above 36.8 Ah and confirm its 90-minute rating.

Example 4 — Entered duration below the code minimum

A 60-minute target entered on the code basis, with the Example 1 battery and load.

Effective required: max(60, 90) = 90 min
Runtime: 112 min ≥ 90 — meets code minimum
Result:  COMPUTED / BELOW-CODE-MINIMUM — the 60-minute entry was not used for compliance

Standards & References

NFPA 110 relates to generator-based emergency power systems and is outside this calculator's scope. Codes are adopted, and sometimes amended, by each jurisdiction, and clause numbering varies by edition; confirm the requirements with the authority having jurisdiction.

Units

Battery capacity is in watt-hours (Wh), or amp-hours (Ah) with a stated voltage — the calculator converts Ah × V to Wh internally. Load is in watts (W) using emergency-mode fixture output. Duration is in minutes (min), displayed alongside h:mm format. Derating factors are dimensionless values between 0 (exclusive) and 1 (inclusive). These electrical units are SI-derived and do not vary with the metric/imperial toggle because there is no length or mass to convert.

Limitations

  • A runtime estimate built on constant emergency-mode load and entered derating; actual runtime depends on battery chemistry, age, temperature, and the discharge curve. Manufacturer runtime data governs.
  • Does not verify the NEC 700.12 requirement that battery voltage stay at or above 87.5 percent of nominal through the period; that comes from the manufacturer's discharge curve.
  • Does not verify UL 924 listing; listed unit equipment is evaluated by its listing rather than by this estimate.
  • Does not compute illumination levels, uniformity, or photometric layout, and does not evaluate emergency driver lumen output.
  • Does not model voltage drop to remote heads, charger capacity or recharge time, transfer time, or test scheduling.
  • Generator and fuel-cell emergency sources are out of scope; this is for battery and inverter systems.
  • Handles one total load or one fixture group; for mixed fixture types, enter the total emergency watts.
  • Amp-hour entries use the nameplate rating; the discharge-rate effect on a 90-minute discharge is flagged but not modeled.

Common Mistakes to Avoid

  • Using normal-mode fixture watts instead of emergency-mode watts. LED emergency drivers often run fixtures at reduced output.
  • Treating nameplate amp-hours as available energy. Ah ratings are usually stated at a 20-hour discharge rate; a 90-minute discharge delivers less.
  • Dividing nameplate watt-hours by the load with no derating — this assumes a new battery at room temperature with a lossless inverter.
  • Leaving inverter efficiency at 1.0 for a central AC inverter, which typically runs at 80 to 90 percent.
  • Entering a required duration under 90 minutes to make a design pass. The code basis judges against 90 regardless.
  • Sizing the battery for today's fixtures with no margin, then adding fixtures to the emergency circuit later.
  • Treating a passing runtime as proof of illumination compliance. Footcandle levels are a separate photometric check.
  • Assuming battery voltage stays flat during discharge. It falls, and the NEC 700.12 voltage-maintenance requirement is checked from the manufacturer's discharge data.
  • Skipping the manufacturer's 90-minute rating when selecting the battery, which is the figure the annual test will actually exercise.

Frequently Asked Questions

How long must emergency lighting last?
A minimum of 90 minutes, or one and a half hours, after loss of normal power. NFPA 101 sets the life-safety requirement and NEC 700.12 applies it to battery and unit-equipment systems; UL 924 is the listing standard for the equipment itself. An authority having jurisdiction can require more.
How do I calculate emergency lighting battery runtime?
Divide the usable battery energy by the effective load and convert to minutes: usable watt-hours are the capacity times the aging and temperature factors, and the effective load is the connected watts divided by the inverter efficiency. A 12 V, 7 Ah battery (84 Wh) at 0.80 aging feeding 36 W runs 67.2 ÷ 36 × 60, about 112 minutes.
How do I size a battery for 90 minutes of emergency lighting?
Multiply the load by 1.5 hours and divide by the overall derating. For 200 W with 0.85 inverter efficiency and 0.80 aging, that is 300 ÷ 0.68, about 441 Wh, or 36.8 Ah at 12 V. Select the next manufacturer-rated size above the figure and confirm its 90-minute rating.
Why is my battery's amp-hour rating not the whole story?
Amp-hour ratings are tied to a discharge rate, commonly 20 hours. Discharged over 90 minutes, the same battery delivers less, because capacity falls as discharge current rises. The manufacturer's 90-minute or one-hour rating is the reliable figure for emergency duty.
What does this calculator not prove?
UL 924 listing, the 87.5 percent voltage-maintenance requirement of NEC 700.12, footcandle levels and uniformity, transfer time, recharge time, and the annual test result. Those come from the equipment listing, the manufacturer's discharge data, and the photometric design; this tool estimates runtime and capacity.
Do I use the fixture's normal wattage or its emergency wattage?
The emergency wattage. LED emergency drivers commonly run fixtures at reduced output in emergency mode, so the emergency-mode watts are what the battery feeds and what the runtime depends on.
Can I check a system against a duration shorter than 90 minutes?
On the code basis, no: compliance is judged against the 90-minute minimum, and a shorter entry is flagged as below the code minimum. A custom basis computes against the entered duration, but the result is labeled as not an NFPA 101 / NEC 700.12 compliance check.
Why is there a battery aging factor?
Because batteries lose capacity over their service life, and the system has to pass the annual 90-minute test near end of life, not just when new. The 0.80 default is a screening margin; replace it with the manufacturer's end-of-life figure where available.

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