@UP - you have your idea of "efficiency" wrong.
PSUs will run most efficiently around 50~70% of their rated wattage, depending on the quality of the PSU. Getting closer to the maximum rated wattage will generally result in a much lower efficiency.
Two things you have to see here. The first is that PSUs have a maximum rating.
A 1000W PSU doesnt use 1000W, it uses whatever wattage you draw from it. 1k is just the maximum it can handle.
Second is efficiency. If a PSU was 100% efficient, when you draw 500W from the PSU, the PSU draws 500W from the wall.
This doesn't happen; Generally most modern, high end PSU will "burn" around 10% of the energy they take from the wall in the form of heat, thus they are rated "90% efficient".
example;
a 90% efficiency PSU attached to a PC, the pc draws 500W from the PSU, the PSU draws 550W from the wall.
Your PC gets 500W, the extra 50W go in the air as heat.
Now, the efficiency rating of a PSU depends on the load you put on it.
A normal 700W PSU might have for example, 90% efficiency at 500W, but only 80% at 650W.
This means that when you push a PSU near the rated maximum, you will spend more on electricity as it needs to draw more from the wall to get to the wattage your PC is asking.
Also, more heat -> less life from the components.
One final note, PSUs do have a minimal wattage too - if you try to use a 1000W PSU with a 100w PC, you will spend more. Typical PSUs perform their best around 50~70% of their rated W, with both much less and much more being less efficient.