What is a smart plug? Relay, metering, limits

A smart plug is an adapter that goes between a wall outlet and an appliance, and switches the appliance’s power on and off when told to. The command can come from a phone app, a voice assistant, a schedule stored in the plug, or an automation running on a hub. It is usually the first smart home device people buy, because it needs no wiring and works with almost anything that has a plug. This guide explains what is inside one, how far to trust its energy readings, what it cannot do, and which radio to pick. It belongs with the smart home guides, next to what a smart home is and the Zigbee vs Z-Wave vs Wi-Fi comparison.

What a smart plug does

A smart plug does one thing: it connects or disconnects the mains supply to whatever is plugged into it. Everything else is a way of deciding when to do that.

• Remote control. Switch a lamp or a fan from a phone, from the next room or, if the plug or hub is reachable from outside, from anywhere.
• Schedules. Turn the hallway lamp on at dusk and off at 11 p.m., or run a dehumidifier for two hours each morning.
• Automations. Switch something because something else happened: a motion sensor saw movement, the last person left the house, your utility’s time-of-use rate changed.
• Voice control, through whichever assistant the plug supports.
• Energy monitoring, on models that include a metering chip: how much power the appliance draws now and how much energy it has used over time.

Because the plug only switches power, the appliance has to cooperate. A lamp with its own switch left on, a kettle with a mechanical switch, a fan with a rotary knob: these all come back on when power returns. More on the ones that do not below.

What is inside a smart plug

Open one and there are four parts:

1. A relay, the switch that carries the appliance’s current.
2. A small power supply that turns mains voltage into the few volts the electronics need.
3. A microcontroller with a radio, often a single chip, that joins the network, keeps the clock and drives the relay. Many Wi-Fi plugs use an Espressif ESP8266 or ESP32, the chips the open-source Tasmota firmware is written for, and the same family covered in the microcontroller guide.
4. On metering models, a sensing circuit: a low-value shunt resistor or a current transformer in the live path, a voltage divider, and an energy metering chip that turns the two signals into power and energy figures.

How it switches, and what the rating means

Most smart plugs switch with an electromechanical relay: a coil pulls a contact closed, and you can usually hear the click.

Every plug has a maximum load rating on its label or spec sheet. For plugs made for standard US 120 V outlets the rating is given in amps, often with a second, lower rating for motors, which draw a surge of current when they start. TP-Link’s published spec for its Kasa HS103, as one example, reads “15 Amps Resistive @ 120VAC, 1/6 HP Motor”, and its product page describes it as for light-duty devices under 15 amps. Ratings differ by model and even by hardware version, so read the one for the plug in your hand, not the product title or a listing. The rating is the most the maker says the plug can carry. Treat it as a ceiling to stay under. The fire-safety advice for the largest everyday plug-in load, a space heater, is under what a smart plug cannot do below.

Energy monitoring and how accurate it is

A metering plug measures the voltage across the load and the current through it many times per cycle, multiplies them to get active power in watts, and adds that up over time to get energy in kilowatt-hours. The chip doing the work is a purpose-built energy metering IC, and its accuracy is only part of the story. The plug’s reading also depends on how well the shunt, the voltage divider and the firmware’s conversion factors match the real parts. The open-source Tasmota firmware, which runs on many plugs, says in its power monitoring calibration guide that each device needs calibrating because of hardware and timing differences. It tells you to calibrate against a known resistive load, such as an incandescent or halogen bulb, and never against LED lamps, computers or motors.

What that means in practice:

• Good enough for comparing. A metering plug will tell you which appliance uses more, whether the freezer is running more often than it did, and roughly what a device costs to run. That is what most people want from it.
• Weakest at small loads. Readings of a few watts are the least reliable part of the range. Tasmota’s documentation notes that two common metering chips, the HLW8032 and CSE7766, can report invalid values below 5 W. This matters for the standby question below.
• Some loads are harder to read. Tasmota’s advice to calibrate only against a resistive load, and never against LED lamps, computers or motors, is a hint about where readings are least trustworthy. Treat figures for chargers, LED lighting, computers and motors as approximate.
• It is not a billing meter. The meter your utility reads is certified to an accuracy class (ANSI C12.20 in the US). A smart plug is not, so expect its monthly total to differ from your bill for the same appliance.

If you want energy readings to show up in more than the maker’s own app, check for Matter energy reporting. The Connectivity Standards Alliance added it in Matter 1.3 in May 2024, through the Electrical Power Measurement and Electrical Energy Measurement clusters, so a Matter controller can read a plug’s watts and kilowatt-hours without a vendor app. Older Matter plugs can switch but may not report energy at all.

Schedules, and where they run

A schedule has to run somewhere, and where it runs decides what happens when things fail.

• On the plug. The plug stores the schedule and keeps time itself, usually by setting its clock from an internet time server when it starts. This keeps working if the internet drops after the clock is set. Sunrise and sunset schedules need your location, which the app sends to the plug or works out in the cloud.
• On a local hub or controller. The schedule lives on a box in the house and the controller sends the plug a command on time. It keeps working without the internet as long as the hub does.
• In the vendor’s cloud. The schedule is held on the maker’s server, which sends the command over the internet. If the internet or the vendor’s service is down, nothing happens at the scheduled time.

The other thing to check is what the plug does after a power cut. Most let you choose: always off, always on, or the state it was in before. In Zigbee and Matter this is a standard setting, the StartUpOnOff attribute of the On/Off cluster, defined in the Zigbee Cluster Library and in Matter’s On/Off cluster definition, so a hub can set it the same way on plugs from different makers. Pick “previous” for a freezer, “off” for anything you would not want to come back on by itself at 3 a.m.

What a smart plug cannot do

• Dim a lamp. A plug is on or off. Plug-in dimmers exist as a separate product, and only work with dimmable bulbs.
• Turn on devices with a soft power button. Many TVs, games consoles, coffee machines, air purifiers and fans with touch controls go back to standby, or off, when power returns. Cutting power turns them off; restoring it does not turn them back on. Check how the appliance behaves before you plan around it.
• Shut a computer down properly. Cutting power to a PC, a network storage box or a router mid-write risks the files on it. Use the device’s own shutdown, or a UPS that signals it.
• Run a space heater. The U.S. Fire Administration tells people to plug space heaters directly into wall outlets and never into an extension cord or power strip. A smart plug is a plug-in adapter, so it sits on the wrong side of that advice. For anything else with a motor or compressor, such as a window air conditioner, check the plug’s separate motor rating first.
• Control a hardwired light or fan. A ceiling light has no plug. That needs a smart switch or a smart bulb.
• Protect against surges. Surge protective devices are tested to their own standard, UL 1449. Do not assume a smart plug protects against surges unless its maker says it is a surge protector and lists it as one.
• Go outdoors, unless it is sold as an outdoor plug for that use. The Kasa HS103 page above, for example, says indoor use only.

The standby draw question

There are two separate questions, and people often mix them up.

How much does the smart plug itself use? Its radio, microcontroller and power supply stay powered all the time, including when the relay is off. That is the price of being able to switch it on remotely. Not every maker publishes the figure, and you should not assume a metering plug includes its own draw in its readings. The way to find out is a separate plug-in power meter, measuring the smart plug with nothing plugged into it. Standby figures for appliances are measured under IEC 62301, the international method for household standby power, if you see one quoted.

The arithmetic is simple. A device that draws 1 W continuously uses 8.76 kWh a year (1 W multiplied by 8,760 hours). Multiply that by the plug’s actual draw and by the number of plugs you own to see what the plugs themselves cost.

Does switching off at the plug save the appliance’s standby power? Yes, the appliance gets no power at all while the relay is open. Whether you come out ahead depends on the sum: the appliance’s standby draw, for the hours it is switched off, against the plug’s own draw, which is constant. A set of speakers and a TV box that idle at a few watts each and spend most of the day unused can be worth it. If the plug’s own draw is higher than the appliance’s standby draw, the plug uses more than it saves.

Using a metering plug to measure the standby draw of the device behind it runs into the accuracy problem above: small loads are where the readings are weakest. Treat a reading of one or two watts as a rough figure.

Which protocol choices matter

The radio decides whether you need a hub, how the plug behaves on your network and whether it helps or loads the rest of the home. The Zigbee vs Z-Wave vs Wi-Fi comparison covers the radios in detail; this is what they mean for a plug.

The points that matter most:

• Mesh plugs strengthen the network. A plug is always powered, so it can keep its radio on and relay messages for battery sensors nearby. In Zigbee, mains-powered devices such as plugs act as routers and battery devices do not, as Homey’s Zigbee mesh guide explains. In Thread, a full Thread device always has its radio on and can forward packets for others. In Z-Wave, mains-powered devices repeat commands for the rest of the mesh. A Wi-Fi plug adds nothing to a mesh and adds one more client to your router.
• Many Wi-Fi plugs need 2.4 GHz. The Kasa HS103, for one, works only on a secured 2.4 GHz network, so check the band and the security modes on the spec sheet before you buy. The 2.4 vs 5 GHz guide and the WPA3 vs WPA2 guide cover the setup.
• Matter makes the brand matter less. A Matter plug can be added to Apple Home, Google Home, Amazon Alexa, SmartThings or Home Assistant, and the Connectivity Standards Alliance describes Matter devices as controllable from whichever ecosystem is most convenient, including more than one at once. Matter uses Bluetooth Low Energy for setup, which is why the table lists Bluetooth that way. Matter over Thread needs a border router; the Thread border router guide explains which devices already include one. See how Matter works for the setup step.
• Local control matters most for schedules. A plug that can be controlled without the vendor cloud keeps its schedule and automations when the internet or the vendor fails. The local-only smart home guide shows how to check.

Before you buy a smart plug

• Check the rating against the load, including any separate rating for motors, and stay under it.
• Look for a safety certification mark on the plug itself, such as UL, ETL or CSA. These marks come from testing labs recognized under OSHA’s Nationally Recognized Testing Laboratory program, which lets a lab authorize its mark on a product after certifying it.
• Decide on the radio first. If you already have a Zigbee or Thread network, a plug on that network extends it. If you have nothing, a Matter plug keeps the most options open.
• If you want energy data, check how it is reported: in the maker’s app only, or through Matter or a local controller you can read yourself.
• Check the power-on behavior setting and whether schedules are stored on the plug.
• Check for physical access: a button on the plug so it can be switched by hand when the app or the network is down.
• Put it on a separate network if it is a Wi-Fi plug. The guide to IoT devices on a separate VLAN and IoT device security basics apply to plugs as to any connected device.

For the bigger picture, read what a smart home is, and the rest of the smart home guides.