How Long Does a House Alarm Battery Last Without Power

A house alarm battery is designed to provide temporary backup power in the event of a mains electricity failure. In most UK alarm systems, the backup battery can keep the alarm operational for between 4 and 24 hours, depending on the system’s age, type, usage, and battery condition. After this period, if mains power is not restored or the battery is faulty, the system may stop working altogether or begin to malfunction.

Understanding how long your house alarm battery lasts without power is important for both home security and insurance purposes. If your alarm fails during a power cut, you could be left vulnerable to intrusions or unable to meet your insurer’s requirements for an active security system.

What Is the Purpose of the Backup Battery?

The backup battery in a house alarm system ensures that your intruder alarm continues to work during short-term power outages. It keeps sensors, control panels, and sirens active so the system can still trigger if a door or window is opened or movement is detected. It also helps prevent tampering, as many alarms will activate if power is cut unexpectedly.

Most systems are designed to switch to battery power automatically and notify the user with a warning beep or control panel message. When mains power is restored, the battery begins recharging to prepare for the next outage.

How Long Will the Battery Last in Practice?

Modern intruder alarm systems with fully charged, well-maintained batteries can last for around 8 to 12 hours on battery power alone. In some systems, particularly higher-end or business-grade units, the battery can last up to 24 hours. Older systems or those with deteriorating batteries may last only 1 to 4 hours before the system shuts down or reports a fault.

Wired alarm systems with many components tend to use more power than wireless or simpler setups, which can influence battery duration. If your system has a backup battery in both the control panel and the external sounder, the siren may continue to operate for longer, even after the control panel goes offline.

What Affects Battery Life During a Power Cut?

The main factors affecting how long the battery will last include the age and condition of the battery, the power draw of the alarm system, and the temperature of the environment. Alarm batteries tend to degrade over time and may lose capacity even if they are regularly recharged. Most manufacturers recommend replacing the battery every 3 to 5 years.

If your alarm is actively triggering during a power cut, the sirens and lights will drain the battery much faster. Even without activation, passive functions like monitoring, sensor polling and status updates still use energy.

How to Know If Your Battery Is Still Reliable

Most modern alarm systems include a battery status indicator or will alert you with a low battery warning. Some systems connected to monitoring services will send an alert to a monitoring centre or your mobile app. Regular servicing can also help identify battery issues before they cause problems.

You can also test the system manually by isolating the mains power and seeing how long the battery sustains the alarm. This should only be done with care and preferably under supervision from your alarm provider or a qualified engineer.

What Happens If the Battery Runs Out?

If the backup battery runs out during a prolonged power cut, your house alarm will stop functioning. This may trigger a tamper alert or siren in some systems, or it may shut down silently depending on the design. Once power is restored, the alarm should reboot and recharge the battery automatically, but some systems may require a manual reset.

If the battery is very old or faulty, it may fail to recharge or hold power. In this case, the alarm may display a persistent fault code or be unable to switch to battery backup in future outages.

Should You Replace the Battery Regularly?

Yes, it is best practice to replace the alarm battery every 3 to 5 years, even if it appears to be working normally. Regular replacement ensures your alarm will continue to protect your property during outages. If your home is often unoccupied or in an area prone to power cuts, battery health is particularly important.

Most alarm providers offer annual servicing, which includes battery testing and replacement if necessary. Keeping a log of the last battery replacement date can also help you stay on top of maintenance.

Conclusion

A house alarm battery will usually last between 4 and 24 hours without mains power, depending on the system type and battery condition. To maintain continuous security during power cuts, it is important to check the battery regularly, replace it every few years, and ensure your alarm is properly serviced. Doing so helps avoid unwanted outages, system faults and potential insurance issues should a break-in occur during a power failure.