It's best to buy two, then you can use one to measure the other ;)

I measured it with a Steffen ammeter: In standby (i.e. ready for use, but with the switch off) 1.05 watts, with the switch activated 1.63 watts. So, well, that's a bit of an exaggeration. For information: 1 watt in continuous operation is about 2 francs a year, depending on the electricity tariff.

The myStrom Switch consumes less than 1 W - but switches devices with up to 2300 W. This is what it says in the description - I would calculate with 1W...

Couldn't try it out yet, as it doesn't work with my WLAN (Sunrise)