The plug says your fridge used 0.8 kWh yesterday and you have no idea whether that is right. Energy monitoring smart plugs are genuinely useful, but they measure in a way that makes them good at some questions and unreliable at others, and knowing the difference is what makes the data worth having.
The short answer
Treat the readings as good for comparisons and rough for absolute figures. A smart plug will reliably tell you that one appliance uses four times more than another, and whether something is drawing power in standby. It is less reliable for the exact cost of a specific device, particularly for anything with a motor or a heating element that cycles.
Why the numbers drift
- Power factor. Cheap plugs measure current and assume voltage, then multiply. For devices with motors and transformers, the current and voltage are out of step, and that simple multiplication overstates consumption. Fridges, pumps and older power supplies are all affected.
- Low-power accuracy. Most plugs are poor below about five watts. Standby loads are exactly in that range, so standby figures should be treated as indicative rather than precise.
- Sampling rate. A plug that samples every few seconds misses short spikes, which matters for anything that cycles on and off like a kettle or a fridge compressor.
- Calibration. There is no calibration standard for consumer smart plugs, and two identical models can differ by several percent.
- Heat. Measurement circuits drift when warm, and a plug carrying a heavy load runs warm.
None of this makes them useless. It means they answer “which of these two things uses more?” well and “exactly what does this cost me?” less well.
The most valuable thing an energy plug tells you is what is drawing power when it should not be. Leave one on a device for a week and look at the overnight figures. A television, games console or set-top box quietly drawing power around the clock is a real and easily fixed cost, and the plug is accurate enough to spot it even if the exact number is approximate.
Sanity check it against something known
Plug in something with a stated wattage, such as a heater or an old filament bulb, and compare. If the reading is within a few percent, you can trust the plug for relative comparisons.
Measure over days, not minutes
An instantaneous reading tells you very little about anything that cycles. Leave the plug in place for a week and use the daily or weekly total.
Compare devices rather than trusting absolutes
Move one plug between appliances over successive weeks. The same plug comparing two devices is far more reliable than two different plugs, because any calibration error is consistent.
Check standby draw specifically
Look at consumption when the device is off but plugged in. This is where the surprises are, and it is the figure that most often justifies buying the plug.
Set your electricity rate in the app
Most apps will convert to cost if you tell them your tariff. Update it when your rate changes, or the cost figures become meaningless while continuing to look precise.
Do not plug in anything over the rated load
This matters more than accuracy. Check the plug’s maximum wattage against what you are connecting. Heaters, kettles and tumble dryers exceed many smart plugs’ ratings.
Use a whole-house monitor for the real picture
If you want to understand your overall consumption, a clamp meter on the main supply or a smart meter display is the right tool. Smart plugs are for investigating individual devices.
Export the data if the app allows
Several apps can export a CSV, which makes it much easier to compare devices over time than squinting at a graph in an app.
Never plug a high-power appliance into a smart plug that is not rated for it. Electric heaters, kettles, tumble dryers and washing machines draw far more than many smart plugs can carry, and an overloaded plug is a fire risk rather than merely a failed measurement. Check the rating printed on the plug, not the socket it goes into.
What is actually worth measuring
- Anything with a standby light. Televisions, consoles, set-top boxes, audio equipment. Individually small, collectively meaningful over a year.
- Old fridges and freezers. An ageing appliance can use several times what a modern one does, and this is where the plug can justify itself in a week.
- Heated devices. Towel rails, aquarium heaters, electric radiators. High consumption, often left on longer than intended.
- Computers left running. A desktop left on around the clock is one of the larger single loads in many homes.
- Anything you suspect. Measuring settles arguments, which is a legitimate use.
Not worth measuring: phone chargers, which use a negligible amount when idle despite persistent myths; and LED lighting, which is small enough that the plug’s own error exceeds the figure.
Are smart plug energy readings accurate?
Good for comparing devices, rough for absolute figures. Accuracy falls at low power and for anything with a motor, where power factor causes overstatement.
Why does my fridge reading seem too high?
Power factor. Motors draw current out of step with voltage, and cheap plugs multiply the two as though they were aligned, which overstates consumption.
Can I put a kettle or heater on a smart plug?
Only if the plug is rated for it, and many are not. Check the maximum wattage printed on the plug. Overloading one is a genuine fire risk.
Is standby power worth worrying about?
Individually small, collectively worth knowing. A handful of devices drawing a few watts continuously adds up over a year, and it is the easiest saving to act on.
Should I use a smart plug or a whole-house monitor?
A whole-house monitor for your overall picture, smart plugs for investigating individual devices. They answer different questions and both are useful.
The bottom line
Use energy plugs for comparisons and for finding standby draw, which is what they do well. Sanity check yours against a known load, measure over a week rather than a moment, and move the same plug between devices rather than trusting two different plugs against each other. And check the wattage rating before plugging anything with a heating element into one.
