My PC only uses at full load 345 watts at the wall, so less watts at 80% efficiency.
I don't really know what you mean when you say "less watts at 80% efficiency".
Power supplies in general are very inefficient electronic devices. And basic, generic power supplies typically have a "bell" shaped efficiency curve. That is, they are most efficient (and usually only about 70% efficient) at one load level (around 50 - 70% load). For electronic devices that only do one thing all the time and thus present a consistent steady load, a bell shaped curve is just fine, as long as that supply is matched so the load and peak efficiency are properly aligned.
But of course, computer loads are constantly varying, from only a few watts at idle, to 100s of watts when heavily tasked. The whole point of the 80 PLUS certification process is two-fold. (1) To create a standard (though volunteer standard) where not only is the efficiency at least 80%, but (2) so the efficiency curve is "flat", that is (ideally), at least 80% from near idle all the way up to 100% load.
The computer (motherboard, RAM, graphics, drives, CPU) will pull from the PSU only what it needs. If that is 300W, then it will pull 300W, regardless the efficiency of the PSU and if the PSU is a 500W supply or a 1000W supply. And the PSU will pull from the wall just 300W, plus a little more due to PSU inefficiency. So if the PSU is 80% efficient, it will pull from the wall 375W (375 x .8 = 300) again, regardless if a 500 or 1000W supply. That extra 75W is wasted in the form of heat.
If those 500 and 1000W PSUs are 90% efficient across all loads, then they will pull just 333W from the wall (333 x .9 = 299.7).
So if by "less watts at 80%" you mean your computer is consuming less than 345W because that is what your PSU is pulling from the wall, then that is true. If 80% is correct, your computer components are consuming only 276W and 69W are being wasted in the form of heat.
Well, give or take a little because the PSU fan consumes some power too.
