OK, this is a really dumb question, but if I buy 1kW power supply, it won't draw that all the time will it? Will it only draw what it needs or is there a minimum level like 50% of the max or something?
I'm just thinking I might as well buy a 1kW PSU instead of a 600W PSU as there isn't much difference in price. BUT I don't want to get it if I will get a huge electricity bill as it always draws 1kW.
The rating on a PSU is the maximum power it can supply to components in the computer, it will only draw as much power as it needs to.
If there isnt much of a price difference between a 1kW unit and a 600W unit then you are looking at a POS 1kW unit. Ignore the wattage, pay attention to the combined current rating for the 12V rails, a good 550 has ~40A, a good 750 has ~60A, i would expect atleast 75A from a 1kW unit. The wattage rating on the unit doesnt actually tell you anything, because some rate their PSUs as the max peak power from all rails at 25C, while others rate their unit based on max power from the 12V rails at 40C or 50C, big difference in quality there.
Get a unit with atleast a 5 year warranty, and atleast 80+ basic certification, from a company that isnt know for making firecrackers(raidmax). PC Power & Cooling, Seasonic, Corsair, and Antec(not basiq line) have consistently good units especially for the higher powered units.