Most energy advice asks you to estimate and assume. A plug-in energy monitor replaces the guesswork with measurement. It is a cheap gadget that sits between an appliance and the socket and tells you exactly what that appliance draws, both when running and when idle, so you can spend your effort on the things that actually cost money rather than the things you imagine do.
What it is and how it works
A plug-in monitor is a small adaptor: you plug it into the wall, plug the appliance into it, and a display shows the power being drawn in watts, and usually the energy used in kilowatt-hours over time. Better ones let you enter your price per unit so they show running cost directly in money. They cost only a few pounds, and unlike a whole-house display they tell you about one specific device at a time, which is exactly what you need to hunt down a culprit.
Reading running cost
The instantaneous watts tell you how hungry something is at that moment, but the useful figure is the energy used over real use. Leave the monitor in place for a representative day or week on a fridge, a games console or a router, and it accumulates the kilowatt-hours, which you multiply by your price per unit, or read straight off if you entered the price. This is how you turn a vague worry into a number. Feed the same figures into the running cost calculator to project the annual cost, and the priorities sort themselves: the device costing fifty pounds a year earns attention, the one costing fifty pence does not.
Catching the standby offenders
The monitor is at its most revealing on standby power. Plug it into the cluster of devices around the television, or an old set-top box, or a desktop computer left in sleep, and read what they draw while apparently off. As the standby power guide explains, the offenders are wildly uneven: some devices genuinely sip nothing, while others quietly pull several watts around the clock. The monitor names and shames them in seconds, so you can put the real culprits on a switched socket and leave the innocent ones alone instead of unplugging everything on principle.
What is worth measuring
Spend your measuring time on the unknowns and the suspected hogs. The fridge and freezer, since they run constantly and the figure surprises people. The tumble dryer and washing machine on a real cycle. The cluster of always-on electronics. An old appliance you suspect is inefficient, to decide whether replacing it is worth it. Mystery devices whose label you cannot find. There is less point monitoring things whose cost you can already work out from a clear wattage label and obvious usage, like a lamp or a kettle.
From reading to saving
A monitor saves nothing by itself; it tells you where to act. Once it has found a hungry always-on device, you can switch it off properly, replace an inefficient old appliance whose running cost justifies the change, or simply use a thirsty one less or more cleverly. The value is in cutting through assumption: people routinely fret over trivial loads while a genuine drain runs unnoticed, and a few pounds of monitor, used on a handful of devices, pays for itself many times over by pointing your effort at the right target. It is the most useful few pounds in this whole list of quick wins.