1. Depends what u define as much electricity. A idling (gaming) comp with a good graphics takes around 150w idling and 300W-400W if u play a game.
2. Assuming it has 150w and it is running 24 hours/day u are consuming 1314 kwh/year. 1 kwh costs in Germany 0.2 €, in canada it is cheaper i guess, so u would pay here 260 Euro for the computer without monitors and real usage
3. Depends on the electricity cost. But i would do it all the time if u don't download sth. etc, why i should pay the eleictricity company during sleeping? and of course that the components don't run 24 h/day! So i would do it all the time.
THe power supply is important because cheap power supplies don't have a high effiency so wasting more elictricity. But Dell has good power supplies (80plus, at least in Germany), so u don't need to worry