I'm planning to add a 1200W PSU, just mainly to future proof it, I don't think I would need that much, probably use only about 700w.
I do plan on having 2x 5870 radeon cards in crossfire
sound card
5-1 memory card readers
etc etc
but I'm worried about the power bill.. Will having a 1200w power supply but only using 700w give me a huge bill?
or it wont be any different to a lets say 850w Psu?
Thanks guys
I would argue that if you can afford all that crap in the first place, it seems trivial to worry about the electric bill.
As with all things PSU related, the brand of the PSU speaks to whether or not it will deliver its rated capacity, or if it's just a "fairy tale" from the marketing department.
Other than that, jobeard is correct that the line draw would profile the cumulative draw of your components, save for the fact you would need to add another calculation to compensate for the efficiency of the PSU itself.
Thus, if you have 300 watts of computer hardware, the PSU would draw perhaps 400 watts from the line, at the average efficiency of 75%.
There is the fact that even if you have components requiring say 500 watts at 100% activity, the average draw would be much lower, perhaps even idling on the desktop at only 200 watts.
Usually, it is considered "nominal" to load the PSU to about 70% of its total claimed output. Here again, a 300 watt system draw would call for a PSU of about 400 watts