Smart plugs: what they are genuinely good for
The cheapest way into home automation, most often bought in a four-pack and forgotten. Used deliberately they teach you more about your house than anything else.
Smart plugs are the cheapest way into home automation and the device most often bought in a four-pack, used for a fortnight and forgotten. That is a shame, because a handful of them used deliberately will teach you more about your house than any other purchase.
Here is what they are genuinely good at, what to check before buying, and the uses worth building.
What is actually inside one
A smart plug is a relay, a radio and sometimes a power meter in a plug-shaped housing. That is all, and the simplicity is the point.
The three specifications that matter:
Current rating. Most are rated at 10 or 13 amps, which is roughly 2.3 to 3 kW on European voltages. Check it against what you intend to plug in, because heaters, kettles and washing machines approach that limit and a plug run continuously near its rating is a plug that runs hot. Also check the continuous rating rather than a peak figure.
Energy metering, and whether it is included at all. A plug that reports watts and kilowatt-hours is far more useful than one that only switches, and the price difference is small. Accuracy at very low loads is generally poor across the category — a plug measuring a 1-watt standby draw is not reliable at that resolution — but it is perfectly adequate for anything above about 5 watts.
Physical size. The commonest complaint. A bulky plug blocks the adjacent socket on a double outlet, or will not fit behind furniture. Look for a slim or offset body, and note that plugs with metering are usually larger.
The one thing to check before buying
Does it work without the internet?
A great many cheap Wi-Fi plugs route every command through a manufacturer's server, so a connection failure or a discontinued service leaves you with a plug that is permanently on or off. Others operate entirely locally.
Plugs speaking Zigbee, Z-Wave, Thread or Matter are local by design, work with any compatible hub, and can be adopted by a different controller later. They also relay for other mesh devices when mains-powered, which is a real bonus — a smart plug in the right place fixes a range problem for everything else nearby.
Given the price difference is small, we would buy local protocol plugs and not think about it again.
What they are genuinely excellent at
Measuring things. The most valuable use and the least advertised. Plug one into an appliance for a week and you learn what it actually costs, which is almost always different from what you assumed. Rotate it around the house and you will find the standing loads and the surprises.
Turning off standby loads. A media cabinet, a printer, a games console, a set of chargers. Individually small, collectively meaningful, and they run 24 hours a day.
Shifting flexible loads to cheap hours. A dehumidifier, a heater, a pool pump, a charger. On a time-varying tariff this is the plug paying for itself.
Making dumb appliances scheduled. A fan, a slow cooker, a towel rail, an aquarium light, Christmas lights, a coffee machine.
Noticing when something finishes. A washing machine drawing 400 watts and then 2 watts has finished its cycle. This is one of the best automations in a house and it needs nothing but a metering plug.
Noticing when something fails. A freezer that stops cycling, a pump that never runs, a dehumidifier that has switched itself off. Absence of expected consumption is a fault signal.
Providing a mesh relay in an awkward part of the house.
What not to plug into one
Being clear about this, because it matters.
Nothing whose failure is dangerous. Medical equipment, a fish tank heater keeping something alive, a sump pump preventing a flood. A plug is one more thing that can fail or be switched off by mistake.
Nothing that must never be left unattended. A space heater, an iron, a tumble dryer. Remote switching means switching something you cannot see, and the fire risk is real. If you do use a heater on a plug, insist the heater has its own tip-over and overheat protection and never rely on the plug for safety.
Anything near or above its rating. Kettles, ovens, immersion heaters, tumble dryers, high-power heaters. Check the appliance label against the plug rating and leave margin.
Fridges and freezers, ordinarily. Monitoring is excellent; switching is not, since a compressor restarted immediately after being cut can be damaged, and an accidental off means spoiled food. Use a plug that measures and configure it so it cannot be switched off, or use a separate clamp meter.
Daisy-chained plugs and extension leads, which is how ratings get exceeded quietly.
Buy plugs with proper safety certification for your market — a CE or UKCA mark with a traceable manufacturer, or the relevant national approval. This category is full of very cheap unbranded products, and a mains-connected device that fails badly is a fire in a socket. If a plug runs warm to the touch in normal use, stop using it for that load. Also check the plug is designed for your socket type rather than adapted with a travel adapter, which is a common and poor arrangement.
Automations worth building on day one
Washing finished. Power drops below a threshold for two minutes after having been above it, notify. Endlessly useful and it stops laundry sitting damp for a day.
Dishwasher finished, same pattern, optionally with a reminder to unload in the morning.
Freezer fault. A freezer normally cycles. No consumption for several hours means a problem, and this alert has saved a great many freezer contents.
Standby off at night. Media and office cabinets off between midnight and 6am, with a check that nothing is in use.
Cheap-hour charging. Anything with a battery on a plug that only powers during the cheapest window.
Dehumidifier on humidity. A plug plus a humidity sensor turns a dumb dehumidifier into a controlled one, and it is far cheaper than a smart model.
Left-on reminders. A curling iron, a soldering iron, a heated towel rail still drawing power at 11pm.
Holiday lamp routines, following sunset with variation.
Absence alerts for anything critical you are monitoring, which is the inverse of most automations and often the more useful direction.
Where they stop being the right tool
Plugs are a stepping stone, and it is worth knowing when to move on.
For whole-house energy monitoring, a clamp meter on the incoming supply or in the consumer unit gives you the total that plugs never will, and circuit-level clamps cover hard-wired loads a plug cannot reach.
For lighting, a switch or module is better than a plug in a lamp, because the wall control keeps working.
For anything hard-wired — oven, hob, immersion, shower, heat pump — you need in-line modules or clamps rather than plugs.
For high-power loads, a purpose-made relay module rated for the job.
The sensible progression: use plugs to find out where the energy goes, then invest in permanent monitoring or control for the loads that turn out to matter.
A plan for a four-pack
- Plug one into the media cabinet for a week and read the standby figure.
- Plug one into the washing machine and build the finished-cycle notification.
- Plug one into the freezer for monitoring only, with the switch function disabled.
- Rotate the fourth around the house — a week per appliance — and write down what each one actually uses.
- Then decide what deserves permanent switching, permanent monitoring, or replacing.
- Buy local-protocol plugs so they double as mesh relays.
- Check ratings against everything you plug in.
Step four is the one that changes behaviour. Almost everyone who does it finds at least one appliance costing several times what they assumed, and it is rarely the one they suspected.