Originally Posted by requiem1
remember psu's only achieve 80% of their stated wattage.
All PSUs are supposed to achieve at least 100% of their output. Sadly, some cheap [CENSORED]s ignore that and make PSUs capable of achieving 80%, or 60% or even 40% of their rated output before they go so far out of spec they shut down. Unfortunately, the more extreme examples of said are so bad they don't even have the protection circuitry to shut down. Those explode violently instead.
But even if a low-mid-range PC actually used around 250W in real life (at peak) you'd still get a 400-500W PSU instead of a 250-300W one to save money. Because the smaller PSU being run so hard would run the system less efficiently and die earlier. The extra power wasted by the cheaper PSU would make it more expensive in the long run. Realistically you want to run a PSU at 50% of its rating all the time as it wastes the least electricity that way, but us gamers have to compromise and run them a bit higher when gaming, owing to the huge difference in power usage between normal computing and gaming. Y'see, running a PSU too lightly (especially under 20% of their rating) won't damage it but can still waste a lot of energy!