How to compute electric bill for a server netbook
Get a $20 Kill-a-watt and measure the actual wattage during load and idle. Then determine the average amount of time you'll be in each mode and from that you can get a good estimate as to how much power it'll use. 

At the upper end, you know that it can't use more than 19V*2.1A ~= 40Watts (ignoring powerbrick inefficiencies).

40 Watts * 24 hours * 30 days = 28.8 kW*hrs per month. Lets say electricity costs 0.15 cents per kWHr: the monthly maximum charge from using the netbook is $4.32. If it's idle most of the time, it'll likely be a lot less than this.