My electricity costs just over 8p for 1KWH (1000 watts on for one hour) So, something using 2W will cost 8p to run for 500 hours (1000/2), and so it costs 0.016p per hour (8/500). There are 720 hours in a month, so it will cost 0.016 * 720 to run for a month which works out at approximately nothing ...