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About this tool
Emergency light runtime from battery voltage, Ah, depth of discharge and LED load, with Peukert derating and recharge time.
This calculator gives the run time of an emergency light as usable energy divided by drawn power: usable Wh = voltage x amp-hours x depth of discharge x state of health, and drawn watts = LED load / driver efficiency + standby load. It also applies the Peukert equation t = H x (C / (I x H))^n, which is why a 12 V 7.2 Ah lead-acid battery running a 9 W lamp lasts closer to three and a half hours than the four the arithmetic alone suggests. Intended for anyone specifying an emergency light, an inverter-backed LED circuit or a battery upgrade.
Open Emergency Light Backup Calculator on AltFTool — it loads instantly in your browser.
Enter the values you already know.
Fine-tune the options to match your scenario.
Read the result and use it in your planning or reporting.
Uses 50% for lead-acid and 80% for lithium instead of pretending the whole battery is usable.
Shows the shorter, realistic lead-acid run time at the actual discharge current.
Adds the charge factor, so you know how long the light is unprotected after a long outage.
Multiply battery voltage by amp-hours to get watt-hours, multiply by the usable depth of discharge, then divide by the watts the load actually pulls from the battery. A 12 V 7.2 Ah lead-acid battery at 50% depth of discharge holds 43 Wh, and a 9 W LED behind an 85% efficient driver pulls about 11 W, giving roughly 3.9 hours before Peukert derating and about 3.4 hours after it.
One hour is the minimum duration for escape lighting under EN 1838, rising to three hours where the building is occupied at night or evacuation will not be immediate; Indian practice under the National Building Code uses the same one-hour and three-hour bands. The duration your local fire authority signs off on is the one that counts, so confirm it before sizing the battery.
Lead-acid capacity is quoted at the slow 20-hour rate, and discharging faster than that returns fewer amp-hours — the Peukert exponent of about 1.15 for VRLA is what quantifies the loss. On top of that, only half a lead-acid battery is usable in cyclic service, the driver wastes 10 to 20 per cent, and an aged battery may hold only 70 to 80 per cent of its printed capacity.
The usual regime is a short function test every month and a full-duration discharge test once a year, with the result written into the log book. A battery that fails to hold its rated duration in the annual test should be replaced rather than topped up, since lead-acid capacity fade is not recoverable by charging.
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