🔋 Alarm System Battery Backup Runtime Calculator

Enter your alarm system's backup battery capacity and typical power draw to estimate how long it will run during a power outage.

Battery capacity (Ah)

System draw (Amps)

How the runtime is worked out

A straight capacity-over-current division:

runtime (hours) = battery capacity (Ah) / system draw (A)

7 Ah / 0.5 A = 14.0 hours

That is the textbook figure, and it is optimistic. Sealed lead-acid capacity is quoted at a slow 20-hour discharge rate, you should not pull a panel battery below about 10.5 V, and a cell three or four years old holds well under its nameplate. Multiplying the answer by 0.7 is a fair working allowance, which turns 14 hours into roughly 10.

Finding your real current draw

Add up the standby figures on the panel and device spec sheets, or clamp a meter on the battery lead with mains off. Typical manufacturer standby numbers:

Device	Standby draw
Control panel	0.15 - 0.25 A
Keypad	0.05 - 0.10 A
PIR detector	0.02 - 0.03 A
Cellular communicator, idle	0.03 - 0.06 A
Siren, sounding	0.5 - 1.5 A

Those are the maker's bench figures under ideal conditions, so treat them as a floor. The siren row is the one that ruins the arithmetic: fifteen minutes of bell at 1 A takes 0.25 Ah, and a repeated activation during a long outage eats a 7 Ah battery quickly.

Enter zero draw and the script substitutes 0.01 A, printing 700 hours rather than an error - check the draw field before trusting a very large answer. Standby duration requirements differ by system type and jurisdiction, so confirm what yours has to meet locally.

Frequently asked questions

How long does an alarm run on a 7Ah battery?

About 14 hours at a 0.5 A draw on paper, and closer to 10 once you allow for ageing and the cutoff voltage. Halve the draw to 0.25 A and the nominal figure doubles to 28 hours.

How do I work out my alarm panel's current draw?

Sum the standby current of the panel, every keypad, each detector and the communicator from their data sheets. A clamp meter on the battery lead with the mains isolated gives the real number in one reading.

How often should the backup battery be replaced?

Every three to five years, sooner in a warm loft or cupboard, since heat is what kills sealed lead-acid cells. Many panels flag a low-battery trouble only once capacity has already fallen a long way.