How accurate are smart plug energy monitors?
Reasonably good for comparing and tracking, and not good enough for billing disputes — which is the distinction that determines what you should use one for.
How they measure. A smart plug with energy monitoring measures voltage and current, multiplies them to obtain power, and integrates over time to give energy consumption in kilowatt-hours.
Where inaccuracy comes from:
Cheap measurement components. The accuracy of the current sensing depends on component quality, and low-cost plugs use inexpensive parts with meaningful tolerances.
Low loads. Accuracy is generally worst at very low power draw, which is precisely where people want to measure standby consumption. A device drawing a couple of watts may be reported very imprecisely or as zero.
Power factor. Devices with motors, transformers or switched-mode supplies draw current out of phase with voltage. Real power and apparent power then differ, and a plug that does not measure power factor correctly will misreport — usually overstating. Fridges, power tools and some lighting are affected.
The plug's own consumption, which is small but continuous.
What they are genuinely useful for:
Finding what uses most, by comparison rather than absolute figures.
Discovering standby loads — devices drawing power when apparently off, which people are consistently surprised by.
Measuring cycling appliances over time, where a single instantaneous reading is meaningless: a fridge draws nothing between compressor cycles, so only cumulative measurement over days tells you anything.
Verifying whether a change made a difference.
What they cannot do:
Measure high-power circuits. Electric showers, cookers and immersion heaters are hard-wired, not plugged in, and these are frequently the largest consumers in a house. Whole-house monitoring via a clamp on the incoming supply, or a smart meter, is required.
Serve as evidence against a supplier's meter, which is a calibrated, legally regulated instrument.
The practical point: the largest consumers in most homes are heating, hot water and anything producing heat. Chasing standby loads on plugs is a small effect, and the monitoring is most valuable when it demonstrates that.