A few years ago, I bought a Kill-A-Watt meter. This device allows you to monitor your electricity usage for a single electric/electronic item through an LCD display. It measures Watts consumed by whatever device is currently plugged into its outlet, as well as total kWh consumption over the number of hours the television/fridge/radio/charger has been been plugged into the Kill-A-Watt meter.
Recently, I plugged in my Raspberry Pi, which was running several Python applications and let it run for 40 hours. The results? Power consumption over 40 hours was 0.09 kWh, or 2.3 Watts per hour.