Plug in the appliance for a while and it displays how much current it uses, it's cost to run etc.
Also displays the mains voltage and frequency.
Jc.
I bought one from Maplin a while ago when it was on half price offer, just to see how accurate it was verses a true power meter, which cost very much more.
The power reading is very inaccurate for a number of things I tried it on (which would imply things like cost calculations will be wrong too). It's OK for resistive loads, and given it tries to work out the phase angle, it might be OK on inductive and capacitive loads (I didn't check), but it is hopeless on anything with a complex current waveform such as electronic appliances, microwave ovens, etc. On something like a compact fluorescent, the reading is out by a factor of 3.
They had these in Lidl for £4.99 including the lithium cell. Different design (larger LCD) but same functions overall.
As the other poster mentioned, accurate for simple stuff with a motor, or many older switch-mode power supplies.
Screws up badly with anything with power-factor correction.
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