http://www.abyssunderground.co.uk/ca...nningcosts.php might be a nice link for you.
The average notebook takes about 30 watts average, so use that as a starting figure. The average unit cost is about 12-14p so you can base it on a higher figure for a worst case scenario.
So for 30watts, for 7 hours a day at 14p/kWh...
£0.03 per 7 hours
£0.21 per week for 7 hours a day
£0.82 per month (28 days) for 7 hours a day
£2.47 per quarter (3 months) for 7 hours a day
£10.7 per year for 7 hours a day
Cheap as chips compared to a similarly speced desktop PC.
I do need to update that script slightly for better wording...