The short answer
Yes — a properly specified burglar alarm keeps working without wifi or mains electricity, because it has a backup battery and can signal over the mobile network rather than your broadband. Every compliant alarm includes a rechargeable standby battery in the control panel that runs the system through a mains power cut for a number of hours; wireless detectors have their own batteries too. For sending alerts, a monitored system can use mobile (GSM) signalling, so it still reaches the monitoring centre or your phone even if the home broadband is down. The genuine limits are the backup battery's runtime in a prolonged outage and, for app-only systems, whether the alert path depends on broadband. Choosing a system with battery backup and mobile signalling closes these gaps.
Outages are exactly when an alarm matters most, so resilience to power and broadband loss is built into good systems. The sections below explain how, and where the limits are.
Power & connectivity
- Mains power cutStandby battery keeps it running
- Detector powerWireless sensors have own batteries
- Broadband downMobile (GSM) signalling backup
- Backup runtimeHours, varies by system/grade
- Weak pointVery long outages, app-only paths
Surviving a power cut
A burglar alarm is designed to keep working when the mains fails. The control panel contains a rechargeable standby battery that takes over instantly during a power cut and runs the system for a number of hours — the exact runtime depends on the system and its security grade, and the panel reports if the battery is low or failing. In a wireless system, each detector also has its own battery, so the sensors keep working independently of the mains regardless. The siren and strobe continue to function on backup power.
This means a burglar cannot disable an alarm simply by cutting the electricity. When mains power returns, the panel recharges the standby battery automatically. The practical limit is duration: in an unusually long outage the standby battery will eventually deplete, after which the system cannot operate until power or the battery is restored — which is why backup capacity is specified to cover realistic outage lengths.
Working without wifi or broadband
Connectivity is a separate question from power. A bells-only alarm needs no internet at all — it just sounds locally, so broadband is irrelevant. A monitored or smart alarm needs a path to send alerts, and good systems do not rely solely on home wifi. The strongest arrangement uses mobile (GSM) signalling: a SIM in the panel sends alerts over the mobile network, independent of your broadband. Many monitored systems use broadband (IP) as the primary path with mobile as automatic backup, so if the broadband drops or is cut, signalling fails over to mobile.
The vulnerable case is a self-install smart alarm that signals only over home wifi: if the broadband goes down (or is unplugged by an intruder), its app alerts may stop, though its local siren still sounds. The UK's switch from old analogue phone lines to digital makes mobile-backed signalling more important than ever, because alarms can no longer rely on a traditional landline. The table shows what keeps working in each scenario.
| Scenario | Local siren | Off-site alert |
|---|---|---|
| Mains power cut | Yes (battery) | Yes if mobile/IP backup |
| Broadband down | Yes | Yes via mobile (GSM) |
| Wifi-only smart alarm, broadband cut | Yes | No (app path lost) |
| Bells-only alarm | Yes | N/A (no off-site path) |
What continues to function under power and connectivity loss, by system type.
How to make sure your alarm is resilient
To keep an alarm working through outages, choose a system with two features. First, a standby battery sized to cover realistic power cuts (any compliant graded system has one; check its rated runtime). Second, a mobile (GSM) signalling path for any monitored or app-based alerts, either as the primary route or as automatic backup to broadband — this is what keeps alerts flowing when wifi fails. For self-install smart alarms, check whether the kit offers a cellular backup module rather than relying on home wifi alone.
Maintenance matters too: standby and detector batteries degrade over time, so acting on low-battery warnings and following the servicing schedule keeps the backup genuinely ready. With battery backup and mobile signalling in place, neither a power cut nor a broadband outage leaves the home unprotected, which removes one of the simplest ways an intruder might try to defeat a system.
Supervised signalling and the digital switchover in detail
There is an important difference between an alarm that merely has a backup path and one whose path is supervised. In a professionally monitored system the signalling link is itself watched: the panel polls the Alarm Receiving Centre on a schedule, and if those check-ins stop — because a line was cut, a router lost power or a SIM lost signal — the ARC treats the silence as a fault and acts on it. The relevant standards (EN 50136 alongside EN 50131) define how often a path must report and how quickly a failure must be noticed, with higher grades demanding tighter supervision. This is what stops a cut connection from becoming a silent blind spot: a supervised dual-path alarm not only fails over from broadband to mobile, it also raises the alarm if both paths are lost.
The UK's digital phone switchover sharpens why this matters. As the old analogue (PSTN) network is retired in favour of digital voice over broadband, alarms that historically dialled out over a landline can no longer depend on that route, and a power cut now also takes out a broadband router unless it has its own backup. Modern systems answer this by signalling over IP with a mobile backup, or over mobile alone, neither of which relies on a traditional phone line, and by carrying their own standby battery so the panel keeps signalling even when the router is dead. If you have an older monitored alarm, the practical action is to confirm with your installer that its signalling has been migrated away from the analogue line; a self-monitored smart alarm should ideally have a cellular module so its alerts do not vanish the moment the broadband does.
Frequently asked questions
How long does an alarm battery last in a power cut?
The control panel's standby battery is rated to run the system for a number of hours, with the exact runtime depending on the system and its security grade. The panel warns you if the battery is low, and it recharges automatically when mains power returns. Very long outages can eventually exhaust it.
Can a burglar disable my alarm by cutting the power or internet?
Not a well-specified one. Cutting the mains triggers the standby battery, and the local siren still sounds. Cutting the broadband is covered by mobile (GSM) signalling, which sends alerts over the mobile network independently. A wifi-only smart alarm is the exception, as its app alerts depend on the broadband.
Do I still need a phone line for a burglar alarm?
No. The UK is retiring traditional analogue phone lines, and modern alarms signal over broadband (IP) and mobile (GSM) instead. Mobile signalling is now the key backup, because it does not depend on a landline or home wifi to reach a monitoring centre or your phone.
Sources & further reading
- Which? — Burglar alarms buying guide (signalling and backup)
- BSIA — alarm signalling and the digital phone switchover
Figures on this page are typical UK ranges drawn from published sources and depend on your specific property and system. They are guidance, not a quotation.