Smart Lock or Traditional Key Lock?

A smart lock and a traditional key lock do the same job, but they fail in different ways. A smart lock replaces your door lock, or part of it, with a digital system that opens by PIN code, fob, fingerprint, phone app or voice, depending on the model¹. A key lock opens with a key and nothing else. The question for most households is not which is more modern, but what happens on the day the power goes, the battery dies or the internet drops.

The reassuring answer is that most smart locks are battery-powered, so a mains power cut does not stop them working: the door stays locked and the motor still works³. Most also come with a manual backup, usually a key, so you can still get in if the battery dies². What you lose in an outage is usually the remote side: unlocking from away, access logs and voice control may stop until the connection returns³.

The practical choice comes down to your circumstances. If you rent, share a home or let a property, PIN codes let you change access without changing the lock². If your broadband is weak, a lock that works offline may suit you better⁴. If anyone in the house finds keys fiddly, a fingerprint lock can help, provided you keep a backup⁵. This page sets out how each type behaves when things go wrong, and how to keep a way in.

Smart lock or key lock: how each behaves in a power cut

A traditional key lock is entirely mechanical. It has no battery, no motor and no connection, so a power cut changes nothing about how it works. A smart lock is an electronic lock that works without physical keys, and it is either hard-wired in or battery-powered². Most are battery-powered, which is why they keep working when the mains supply fails⁸. Verisure states that its smart locks can still function during power outages thanks to built-in battery backup and manual key or thumb-turn options⁵.

The difference shows up in what you can still do. With a battery-powered smart lock in a power cut, the door stays locked and the motor still works, and you can enter with a keypad, fob, fingerprint scanner or a nearby phone over Bluetooth³. What may stop is anything that needs the internet: remote unlocking, access logs and voice control³. A hard-wired lock with no battery backup is the exception. If the power fails, it may stay locked until power returns, so check how yours is powered before you rely on it.

There is a second power question that matters more than most people expect: the lock's own battery. A smart lock that is battery-powered is not affected by a mains outage, but it is affected by a flat battery, and Verisure warns that if there is a power outage or the battery dies, you could find yourself stuck². That is why the manual backup matters, and why the battery warning matters more than the power cut itself.

For a household deciding between the two, the power cut test is simple. A key lock always works, because it has nothing to fail. A smart lock works in a power cut if it is battery-powered and charged, and it works if you have kept the manual backup to hand. If either of those is uncertain, the key lock is the more predictable choice.

What happens when a smart lock's battery dies

A smart lock's battery is the part most likely to let you down, and the good models warn you long before it does. Nuki locks warn when the battery level drops below 20 per cent⁶. The Aqara U600 sends app push notifications when the remaining battery reaches 20 per cent and 10 per cent, and plays a reminder tone each time the door is opened⁷. The Aqara U200 Lite has a low battery alert specifically to avoid being locked out unexpectedly¹⁰.

How long a battery lasts depends on the lock and how it is used. Nuki states that battery life depends heavily on usage, door type, locking modes and other factors, and that its Smart Lock Pro lasts for months on a single charge¹¹. Aqara states that the U200's batteries last over a year in a one-person household¹². Those are the maker's own figures for its own products, and they vary with how often the door is used.

The batteries themselves differ by model, which matters when you come to replace or recharge them:

The safest habit is to act on the first warning rather than the last. SwitchBot's own instruction is to replace batteries upon low battery notifications¹⁴. If you manage several locks, Nuki Web can send battery notifications by email so you do not end up with empty batteries¹⁵. And whatever the warning says, keep the manual key where you can reach it, because a lock with a flat battery and no backup is the one scenario that turns a convenience into a problem.

When Wi-Fi or Bluetooth is down: the features you lose

Most smart locks will function just fine without Wi-Fi, but you may not be able to use their monitoring features or remote access⁴. Verisure puts it plainly: if these connections are down, you might find yourself unable to use some features such as remote access, although that should not affect your ability to lock or unlock the door². The lock itself keeps working; the extras that depend on the network do not.

What you lose depends on how the lock connects. Bluetooth-enabled locks connect when in proximity to your device, often with fewer features and no remote control³. Wi-Fi locks let you see who is entering and exiting from an app and can often provide temporary access to certain people, so those are the features that stop when the connection drops³. Some locks rely on a wireless connection to run at all, whether Bluetooth, Wi-Fi or another network such as Zigbee, so check before you buy⁴.

The lock's own behaviour offline varies. Aqara states that if its lock loses Wi-Fi access and goes offline, local access control remains active while remote access is disabled, and in a total power failure the device remains locked until power is restored¹⁶. Igloohome builds its locks to work offline, with no Wi-Fi or internet connection needed at the lock, which it says gives greater reliability and security against Wi-Fi hacking¹⁷.

If your broadband is weak, the choice of lock matters more than the choice of brand. Verisure advises that homes in more remote areas where a strong broadband connection is not guaranteed may want a lock that does not require a Wi-Fi connection to run⁴. For remote access, some locks have built-in Wi-Fi and need no bridge, while older models need an external bridge or a hub¹⁵. Nuki's Smart Lock Go needs a one-time Remote Access purchase to connect to the internet, and without it all features work only within Bluetooth range⁶.

PIN codes for shared homes and rented properties

For households with multiple occupants or rental properties, smart locks with a PIN code reduce the risks that come with having many copies of a key, because you can change the access combination as often as you wish without replacing the whole lock². Rental property owners often opt for PIN code-operated locks for the same reason: they can change the access code whenever a new guest checks in⁴. An administrator can change the combination or add and remove guest access without changing the whole lock¹.

The mechanics vary. Igloohome locks generate a PIN code at random with the validity duration built into it, and the lock translates the code into the desired action, such as granting access once only¹⁷. You can set up to 199 PIN codes on an igloohome lock, and generated codes do not need to be synced within Bluetooth range to work¹⁷. Codes can be shared by WhatsApp, Facebook Messenger, SMS and more, and guests do not need to download an app to receive access¹⁷. For hosts, an Airbnb listing calendar can be synchronised so PIN codes are created automatically for guests¹⁸.

If you rent rather than let, the position is different. Verisure advises that if you are a renter and want to install a smart lock, you should check with your landlord or property manager first⁴. Some locks are designed to be installed and removed without leaving residue or marks, which makes them more suitable for rented flats¹¹. The underlying cylinder matters too: for rented flats a single-differ lock is often used, where the door cylinder only locks with the corresponding key¹⁹.

Biometric smart locks: fingerprint, voice and face

Biometric locks open with a part of you rather than a key or a code. Some smart lock models allow unlocking via a keypad, a key fob, or biometric access such as fingerprints, palm, face or voice recognition⁸. Depending on the model, the lock could be operated via a PIN code, your biometrics using fingerprints or face recognition, a fob or voice recognition². For locks that work with biometrics, you can even find out who is locking or unlocking the door².

Fingerprint capacity is the specification that decides whether a biometric lock suits a family. The Aqara U600 stores up to 50 fingerprints, which the maker says easily covers multiple household members⁷. The Aqara U200 offers mechanical keys, PIN codes, fingerprint recognition and app integration¹², and the wider Aqara range adds NFC, custom passcodes, Apple Home Key and the Zigbee protocol¹⁶. SwitchBot's Lock Ultra offers fingerprint, auto unlock, NFC, voice or a smartwatch²⁰, and the Lock Ultra Max adds facial recognition, palm vein recognition, passcode, NFC card, voice control and Apple Watch²¹.

Voice and face recognition are the least predictable of the biometrics, because they depend on the model and on the conditions. Some locks work with voice assistants such as Amazon Alexa and Apple Siri¹⁰, and the Aqara range pairs with Alexa, Google Assistant or Siri for voice-activated control¹⁶. That is convenience, not a substitute for a reliable primary way in.

For families, the practical points are these. A fingerprint is quick for a child or for anyone who finds keys fiddly, and the Alzheimer's Society advises that if keys are fiddly or difficult to use, you should consider changing the locks so they are easier to operate⁵. But a finger that is wet, cut or not recognised needs a fallback, so keep a PIN code and a physical key as well. And check the security of the biometric data: the Aqara U200 uses AES encryption and fingerprint module encryption binding to safeguard personal data and access credentials²².

Keeping a way in if the smart lock fails

The single most important habit with any smart lock is to keep a way in that does not depend on the electronics. Both types tend to be equipped with a manual backup so you can still lock and unlock your door in the event of a power outage or if the battery dies². Verisure states that most smart locks, including its LockGuard, come with a manual backup such as a key which lets you lock or unlock the door even if the power is down².

The backup takes different forms. With a Nuki Smart Lock Go, you can still unlock from the outside with a physical key if you use a Nuki Universal Cylinder or a cylinder with an emergency function, or by other means such as Nuki Web, a compatible smartwatch, a keypad code or fingerprint, a fob, or someone with access permission⁶. Nuki recommends using a cylinder with an emergency function so you can always unlock the door with your mechanical key even if a key is inserted on the inside of the cylinder, as is the case when using the lock¹⁵. The Aqara U200 requires a cylinder with an Emergency Function, and fits over existing locks without modifications¹².

The Electrical Safety First charity's advice for smart homes applies directly here: have a plan in case the smart system itself fails, including a manual alternative such as a manual light or a door that can be opened manually²³. That is the principle behind every backup key, and it is worth testing before you need it.

There is a wider security point. The National Cyber Security Centre warns that when updates are no longer provided, smart devices are easier to hack, or may stop working as designed²⁴. Which? advises creating a long, unique password, keeping the software updated, and connecting the lock to other security devices where possible³. A lock that no longer receives updates is a lock you should think about replacing.

Where to get help and what to check before you choose

Before you choose, check the door and the cylinder. Some smart locks replace only the deadbolt part of your lock, while others replace the whole system, including the door handle². A smart lock can replace your whole lock, including the latch and handle, or replace the deadbolt only, depending on the model⁸. If you rent, check with your landlord first⁴. If anyone in the house has dementia or finds keys difficult, the Alzheimer's Society advises considering changing the locks so they are easier to operate⁵.

The physical lock underneath still matters. A smart lock is only as good as the cylinder it turns, and a cylinder with an emergency function is what lets your key work when a key is inserted inside¹⁵. Avocet's ABS locks come with a security key card with the option of registering²⁵, and the range is available in keyed alike, thumbturn, singles and double profile formats²⁵. For the standards behind the cylinder, see our guide to doors and locks and to British Standard door locks.

On security more broadly, Sussex Police advise double-locking the door every single time you leave the house, because simply shutting the door will not prevent burglaries²⁶. Police Scotland advises changing your locks if you want to when moving into a new home²⁷. For the smart lock itself, Which? advises a long, unique password, updated software and, where possible, connection to other security devices³.

If you are locked out, do not force the door. Our guide to being locked out sets out what to do and how to choose a locksmith. If the lock fails in a power cut, our page on smart locks in a power cut goes further, and how long smart lock batteries last covers the battery side. For the wider picture, start with Home Security: A Household Guide and Smart Locks: How They Work and What to Check.