OK, let's assume we're in California and electricity is like $0.50 / kWh.

An extra 10W being consumed around the clock would cost 24 h/d * 365.25 d * 0.50 $/kWh 0.01 kW = $43.83. Indeed, saving 10W would save enough in 10 years to buy a whole new NAS! (Sans the disks.)