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A plug-in energy meter (also called an electricity usage monitor, socket power meter or wattmeter) sits between the wall socket and the appliance. It measures voltage and current and uses them to calculate the current power in watts and the accumulated consumption in kilowatt hours. If you enter your electricity price, the display also shows the cost since the measurement started.

The big advantage over smart plugs: you need no app, no Wi-Fi and no account. The device is ready to use immediately, which also makes it ideal for lending to neighbours or family. Many German municipal utilities (Stadtwerke) and consumer advice centres lend out such devices as well.

Practically all models handle the basics – watts, kWh, measuring time and cost. The differences lie in operation and convenience. A large, backlit display with a rotatable readout helps when the socket is behind a cupboard or close to the floor. A memory function keeps the readings if you briefly unplug the device.

Appliances with steady consumption – routers, lamps, a TV while in use – can be assessed within a few minutes: the wattage multiplied by the operating hours per year gives the annual consumption. For appliances that cycle on and off, such as fridges and freezers, measure for at least 24 hours, preferably a week, and then extrapolate to a year. Measure washing machines and dishwashers over one complete programme cycle.

Note the maximum load rating, usually 16 A or 3,680 W. Plug adapters are unsuitable for permanently wired appliances such as cookers or instantaneous water heaters – there, only a sub-meter installed by a qualified electrician will help.

Basic models only show watts and kilowatt hours. More convenient devices keep readings when unplugged, calculate with two tariffs and forecast annual costs. For the occasional hunt for power hogs the basic version is enough; if you measure many appliances over weeks, you'll benefit from the extra features.