Guide

Smart Plug with Energy Monitoring, Explained: What It Measures and What to Look For

How a smart plug with energy monitoring works: what watts, kWh and cost readings mean, how accurate they are, what it can't measure and what to look for.

By Wattly · Updated 2 November 2026 · 6 min read

A smart plug with energy monitoring is a plug-in adapter that measures how much electricity the appliance plugged into it uses, and shows it in an app: live power in watts, energy over time in kWh, and usually an estimated cost. It also lets you switch that appliance on and off remotely or on a schedule. It is a per-appliance tool: it cannot see hard-wired appliances or your whole home, and its readings are for understanding and comparison, not for billing.

What it measures

Most energy-monitoring smart plugs report some or all of these:

Reading	Unit	What it tells you	Typical use
Live power	Watts (W)	How hard the appliance is drawing right now	Spotting standby draw, checking what "off" really means
Energy used	Kilowatt-hours (kWh)	Total electricity over a period	Working out what an appliance actually costs per day, week or year
Cost estimate	£ or €	kWh multiplied by a price	Quick sense of scale; check it matches your unit rate
History charts	kWh per day, week, month or year	How use changes over time	Seeing patterns, before-and-after comparisons
Voltage and current (some plugs)	Volts (V), amps (A)	Supply and load details	Mostly for enthusiasts; not needed for saving money

The key link: kWh = watts × hours ÷ 1,000, and cost = kWh × unit rate. A plug that shows 50 W for 10 hours has measured 0.5 kWh. At the Ofgem price cap average for October to December 2026 (26.32p per kWh, Direct Debit, England, Scotland and Wales), that is about 13p. At the EU household average of around €0.28 per kWh in the first half of 2026 (Eurostat, as reported by the Brussels Times), it is about €0.14.

How it works

Inside the plug, a small metering circuit measures the current flowing to the appliance and the supply voltage, and calculates the real power from them many times a second. It adds those readings up over time to get kWh. A Wi-Fi radio sends the data to the manufacturer's cloud service, and the app displays it. A relay (an electrically operated switch) inside the plug turns the socket on or off when you tap the app, run a schedule or press the button.

Because the measurement happens inside the plug, it can only see the current going through that socket. Anything on a different socket, or wired directly into the mains, is invisible to it.

Live watts vs history: reading the numbers

Live watts answer "what is it doing now?" Use them to check standby: switch your TV off with the remote and see what remains. Read our guide to standby power for what is normal.

History answers "what did it cost?" Use it for anything that cycles. A fridge's live reading jumps between its compressor running and resting; only the daily kWh tells you its real use. Leave the plug in place for a week before drawing conclusions.

Cost estimates are only as good as the price behind them. Make sure any cost figure reflects your actual unit rate; if in doubt, take the kWh figure and multiply it by the rate on your bill yourself. The standing charge is a separate daily fee and will never appear in a single appliance's cost.

How accurate is it?

Smart plugs are not certified billing meters, so do not expect them to match your electricity meter to the penny. In practice:

They are most useful for comparison. Which appliance uses more, whether a change halved the use, whether something draws power when it should be off.
Very low readings are the least reliable. A reading of 0.3 W versus 0.8 W may not be meaningful on a consumer plug. Treat sub-watt figures as "very little".
Electronics and motors can be tricky. Devices with switch-mode power supplies or motors draw current in uneven ways, which cheaper meters may handle less well.
You can sanity-check. Plug in a kettle and compare the live reading with the wattage on its base. Small differences are normal; a reading wildly off suggests a problem.

If you need billing-grade numbers, use your electricity meter. If you want to know what to change, a smart plug is the more practical tool.

What it can't do

Measure hard-wired appliances. Electric showers, ovens, hobs, immersion heaters, most built-in boilers and ceiling lights are wired in. A smart plug cannot measure or control them.
Show your whole-home use. For that, use your meter or a smart meter in-home display. The two tools work well together; see smart plug vs smart meter.
Save money by itself. It shows you where electricity goes. Savings come from what you do next. We cover this honestly in do smart plugs save money?
Make an unsafe load safe. It must never carry more than its rating, and it is no substitute for a proper circuit for a high-power appliance.

What to look for: a buyer's checklist

The right plug standard for your country. UK Type G (BS 1363) or EU Type F (Schuko), not a travel adapter in between.
A current rating above your appliance's maximum power. Check the appliance's rating plate. For heaters, dryers and kettles, leave a margin.
Overload protection. The plug should switch off if the load goes beyond its rating.
Energy history, not just live watts. Daily, weekly and monthly charts are what make the data useful.
Behaviour when the internet drops. Do schedules run locally? Does the physical button still work?
Power-outage memory. After a power cut, does it return to its previous state, or switch on or off by default? This matters for fridges and freezers.
Its own standby draw. A smart plug always uses a little power to stay connected. Look for a stated figure.
The app and ecosystem. Which app does it use? Does it work with the voice assistant you have? Be clear about what is and is not supported: for example, Alexa and Google Home support is common, while native Apple HomeKit or Matter support is not universal.
Wi-Fi or Zigbee. Wi-Fi plugs need no hub; Zigbee plugs need one but can be more efficient on a busy network. See Wi-Fi vs Zigbee smart plugs.
Physical safety features. A fire-resistant housing and child safety shutters are worth having, and a child lock helps if small hands find the button.

Getting useful data in your first week

Set the plug up and name it after the appliance, not the room, so the history makes sense later.
Pick one question to answer, such as "what does the fridge-freezer cost a year?" or "what does the TV corner use overnight?"
Leave it for a full week without changing your habits.
Read the weekly kWh, multiply by 52, then by your unit rate.
Make one change (a schedule, a setting, a lower temperature) and measure another week.
Move the plug to the next suspect.

Where Wattly fits (and where it doesn't)

The Wattly smart plug is a Wi-Fi plug with real-time energy monitoring in the Tuya Smart app: live watts, cost per hour, day, week and month, cost estimates, and usage history in daily, weekly, monthly and yearly charts. It does not need a hub. Beyond monitoring, it offers schedules, countdown timers, scenes and automations, remote on/off, a child lock, power-outage memory, overload protection, family sharing and a physical button. It works with Amazon Alexa and Google Home through the Tuya skill or action, and with Siri via Siri Shortcuts; it does not support native Apple HomeKit or Matter.

Versions: UK Type G rated 13 A, and EU Type F (Schuko) in 16 A (3,680 W max) or 20 A (4,600 W max). A Zigbee version is listed as coming soon. Its own standby use is under 1 W. Like any smart plug, it only measures what is plugged into it. Setup steps are in our Tuya and Smart Life setup guide, and current versions are on the product page.

Frequently asked questions

What does a smart plug with energy monitoring do?

It sits between a wall socket and an appliance, measures the electricity flowing through it, and sends the readings to an app. You see live power in watts, energy used over time in kWh, and usually a cost estimate, and you can switch the appliance on or off remotely or on a schedule.

How accurate are smart plug energy readings?

Good enough to compare appliances and spot waste, but not billing-grade. Accuracy tends to be weakest at very low loads, so a reading of a fraction of a watt should be treated as approximate. Sanity-check readings against an appliance with a known rating, such as a kettle.

Can a smart plug monitor my whole house?

No. It only measures the appliance or extension lead plugged into it. For whole-home use, look at your electricity meter or smart meter in-home display.

Does energy monitoring work without internet?

On the Wattly plug, live monitoring and remote control in the app need internet. Schedules already stored on the plug keep running offline and the physical button still works.

What size smart plug do I need for a heater?

One whose rating is above the heater's maximum power. In the UK, plugs are rated up to 13 A. In the EU, a 16 A Schuko plug handles up to 3,680 W and a 20 A version up to 4,600 W, but the wall socket and circuit still set the real limit.