A household freezer uses more that the average pc,and switching your pc off and on constantly can cause more harm than leaving it on,as long as their is plenty of ventilation and the cooling fans are doing there job, leaving your pc on for 30hours or so is no problem.
I have come to the conclusion that I can leave my system on for extended periods of time without many problems. It might be a good idea, however, to switch off the Cable Modem (Telewest B.B).
I guess that this question can only be acuratley answered by connecting a 'widget' to measure power consumption? But if I assume that my system will not have any power saving options enabled (I once heard that a TV on 'standby' uses 80% of the power than when its on!)maybe someone might have a go at the Maths?
I fitted that PCI card 2 week's ago & the best thing to a shut down my computer has seen is a quick reboot, I obviously calculated cost's & am looking at one of those micro pc's just for the cctv equipment!!!! I reckon with the right PC I could shave the cost's down to 15 pence or there abouts.