My old clunker at home uses about 150 watts per hour (w/o monitor, router etc). Multiply this with 24 x 365 and you get the power consumption per year. Devide this value by 1,000 and you get kWh (kilo watt hours). This kWh value gets multiplied with the price per kwh your utility is charging and you have the amount per year.
If I were running the above machine on a 24/7 basis, I would run up with about 240 EURO in engery cost alone. Shared hosting is a lot less expensive.