Verified answer

it doesn't do any damage to it. Typically they use about the same as a 75 - 100watt light bulb left on all the time. I leave my desktop at home on 24/7 unless we go out of town.

You can monitor your electricity usage with a simple gadget (http://www.thinkgeek.com/gadgets/electronic/7657/)... Then, simply multiply by your local electricity cost.

I think the worst damage is the hard drive. If you leave it on all day, you may have a breakdown within 2-3 years due to normal wear of mechanical components.

It depends on what system you are using here is an idea

let's say you have a big high-end computer with a gaming-level graphics card and CRT monitor, and you leave them on 24/7. That's about 330 watts x 24 hours x 365 days/yr = 2,890,800 watt-hours, or 2891 kilowatt-hours. If you're paying $0.14 per kWh, you're paying $405 a year to run your computer.

A typical laptop psu uses about 1-1.5 amps so would cost about 70p per day if it was in use all the time, but if you stopped using it, it would go into standby (depending on your settings) and use hardly anything Best to turn it off when not in use.

it is impossible to tell you with out the tech specs for your laptop