Depends entirely on the psu. There's one overriding spec to any psu, Quality. Generally there's a certain amount of wattage actually used by any pc, but thermal and efficiency values, plus the unknown like spikes from gpus etc, dictate that @ 50-70% is the best range to be. Assuming a perfect psu, if your pc uses 300w, you'd want a 450w-500w psu. But many psus are far from perfect, really mediocre or bad quality. They have a tendency to lie, flat out fabrications of ability, by adding in the 3.3v and 5v+ rails (which see very little actual use, almost everything is 12v based).
So a quality unit might have right around 450w on the 12v rail, but a cheapo unit might have 300w on the 12v rail, and an extra 300w on the minor rails and claim a 600w psu. 1 small gpu spike, and the pc shuts down after drawing 320w on the 12v rail, if not toasting the psu because of lack of protective circuitry.
So even though you technically only need 300w 12v, you'll see ppl using 800w or bigger psus, because the whole world knows that bigger is obviously better.
Quality and ability go hand in hand, but are not the same thing at all.