Emergency Lighting Duration Calculator

Work out emergency lighting duration instantly with clear inputs, formula shown and shareable results.

About Emergency Lighting Duration Calculator

Emergency lighting duration is usable stored energy divided by connected load. Nameplate ampere hours are derated because deep discharge damages the battery and capacity falls with age, so eighty percent usable is a realistic planning figure. Codes require one hour where evacuation is immediate and three hours where the building may be reoccupied or sleeping risk exists.

Formula

Duration

Hours = battery Ah x voltage x usable fraction / load in watts

Worked example

With the default values below, Emergency Lighting Duration Calculator returns Duration achievable of 2.80. Change any input above and every figure here is recalculated in your browser.

Inputs used

Battery capacityminimum 0.1 Ah: 7 Ah
System voltageminimum 1 V: 12 V
Connected emergency loadminimum 0.1 W: 24 W
Usable battery capacityaccepts 20–100 %: 80 %

Results

Duration achievable: 2.80 h
Duration in minutes: 168 min
Usable stored energy: 67.20 Wh
Compliance: Meets a one hour requirement but not three hours

Step by step

1Values used
2Duration
3Duration achievable= 2.80 h
4Duration in minutes= 168 min
5Usable stored energy= 67.20 Wh
6Compliance= Meets a one hour requirement but not three hours

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax or compounding frequency into account.
Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
Preliminary estimate only. Emergency lighting design, duration and testing regime must comply with the applicable standard.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Preliminary estimate only. Emergency lighting design, duration and testing regime must comply with the applicable standard.

Frequently Asked Questions

How is the Emergency Lighting Duration figure worked out?

Hours = battery Ah x voltage x usable fraction / load in watts. Emergency lighting duration is usable stored energy divided by connected load.

Why does the duty test matter more than the calculation?

Battery capacity degrades unpredictably. The annual full duration discharge test is the only reliable evidence that the system still achieves its rated duration.

What values do I need to enter?

This calculator takes 4 inputs: Battery capacity, System voltage, Connected emergency load, Usable battery capacity. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.