, even if I just have average power requirements (less than 200W DC say) I'm better off to go with a 500W or 600W highly efficient PSU, rather than PSU with more suitable power output, but with a lower efficiency? Is that right?
Here's a graph I made using the (adjusted) efficiency figures from the S12-600
and the NeoHE 430
Very nice, and yes, you're right. BUT, again, you want to think about the significance of the differences.
Up to ~150W, it's about 1%. Looking at it from the heat generation opint of view, at 150W, the PSUs are converting 20% and 21% of the AC input power into heat. Going back and looking at the actual numbers, it turns out to be 34.4W and just under 38W. Call it 3.5W. At 100W, you're looking at maybe 2.3W. Between 50W DC demand at idle and 150W at full load is about typical for a lot of systems.
Is that worth the price difference between these models? Is it worth the $ saved in energy consumption? Will it make a difference in the PSU or system cooling?
As you go to higher output, the difference gets a bit bigger: ~6W at 200W and maybe a bit over 8W at 250W. Depending on how you've configured your system and how effective your airflow is, these could make a small diffference in both temperatures and noise. BUt if you optimized the system w/the neoHE, you could minimize the differences.
Remember that a lot of factors in a PSU are interrelated. Efficiency, for example, depends partly on how much voltage drop there is in the output, and what temperature the core components are running at. If you can drop the higher load temps by 10C, you can be assured that efficiency will actually increase, perhaps not by much... like 1 or 2%. Or perhaps more, depending on how adequate the cooling was in the first place.
So with a NeoHE, if you had a fresh air intake for it, you'd be letting it run cooler, and esp. as the power demand increased, this would translate to improved efficiency. (Of course you could easily argue that optimizing the S12s will give you even higher efficiency...)
The flip side is that if you want absolutely minimal noise, the best approach is to go with the lowest possible steady-state (non-thermally controlled) airflow in the highest efficiency PSU w/ a cool air intake. You'd be leveraging the high efficiency of the PSUs to give you extra headroom to compensate for the inadequate cooling airflow at higher loads.
Hope all that makes sense, not sure if the coffee has actually percolated into my brain yet.