Intruder alarm battery calculator
Work out the standby battery an intruder alarm control panel or power supply needs. Enter the standby and alarm currents, choose the standby period and sounding time, and get the capacity in Ah plus the next common 12V battery size up.
- Standby options for Grade 2 and Grade 3 systems
- Shows the working, so you can check it and note it on your paperwork
- Runs in your browser — nothing you enter is stored
12 h — Grade 1 or 2
For 12V valve-regulated lead-acid batteries. The 25% margin is a working assumption, not a figure from the standard.
How intruder alarm battery capacity is worked out
If mains power fails, the battery has to run the whole system for the standby period, then still have enough left to sound the alarm. The calculator adds those two loads and applies a margin.
Capacity (Ah) = 1.25 × (standby current × standby hours + alarm current × sounding hours), with currents in amps.
Standby time by grade
The grade under BS EN 50131-1 sets how long the system has to run on battery. Power supply manufacturers publish 12 hours for Grade 1 and 2, and 30 or 60 hours for Grade 3, depending on whether a mains failure is reported to the alarm receiving centre. Your specification and the power supply's data sheet have the final word.
Worked example
Standby current 150 mA (0.15 A), Grade 2 standby of 12 hours, alarm current 0.8 A sounding for 15 minutes (0.25 h): 1.25 × (0.15 × 12 + 0.8 × 0.25) = 1.25 × 2.0 = 2.5 Ah. The next common battery size up is 3.2 Ah.
Common battery sizes
The calculator rounds up to 1.2, 2.1, 2.3, 3.2, 4, 5, 7, 7.2, 9, 12, 17, 18, 24 or 38 Ah. Check the power supply's maximum battery size and charging current before fitting a bigger one.
What a calculator can't know
The result is only as good as the currents you enter. Add up every load on the supply: keypads, detectors, expanders, the communicator, and anything powered from the auxiliary output. Battery capacity also falls with age and in the cold, which is why the calculator adds a margin. Treat the answer as a check on your design, not a replacement for it.
Common questions
How do I calculate the battery size for an intruder alarm?
Multiply the standby current by the standby hours, add the alarm current multiplied by the sounding time in hours, then add a safety margin for battery ageing and temperature. This calculator uses a 25% margin and rounds up to the next common 12V battery size. It shows the working so you can check it.
How many hours of standby does an intruder alarm battery need?
The BS EN 50131-1 grade sets it. Power supply manufacturers publish 12 hours for Grade 1 and 2, and 30 hours for Grade 3 when a mains failure is reported to the alarm receiving centre, or 60 hours when it is not. Check the grade and the standby time in your own specification and the power supply's data sheet.
What current do I use?
Use the standby current of the whole system from the manufacturer's data sheets: the control panel, keypads, detectors, expanders, communicator and any power the sounder holds in standby. Use the alarm current with the sounders and strobes sounding. Measured figures are better than data sheet figures where you can take them.
Is the 25% margin from a standard?
No. It is a working assumption for battery ageing and cold, and you can change the result by choosing a larger battery. Follow the power supply manufacturer's limits on battery size and charging current, and your design specification.
Can I use this instead of a design calculation?
No. It is a quick guide for engineers and installers. The design calculation, the manufacturer's current figures and the power supply data sheet take priority.
Does Compliar cover security contractors?
Yes. Compliar is built for security contractors alongside electrical and fire protection, and covers intruder alarm systems. See the intruder alarm grades and maintenance guide for what is checked on a visit.
Intruder alarm maintenance, kept in one place
Compliar keeps your intruder alarm certificates and service records together for security contractors. Book a demo to see it on your own jobs.