not much, if you are charging off a modern switch mode power supply the current draw on the mains side is going to be very low, maybe about 20-40 watts at the most I would have thought(unless you are running at its max current draw), electricity is measured in Kw hours,so you can draw 1Kw(1000 watts) for 1 hour and you will be charged 1 unit of electricity for this (not sure how much it is per unit now) but if you know then you can roughly work it out now
