SPCR

I'm looking at buying a new case that comes with a 430W PSU (Antec NSK6580) - the PSU is an EarthWatts, so it has an 80 PLUS rating.

I calculated (using Antec's PSU calculator) that the most I'm every likely to require is about 300-340W depending on how many HDDs I might use in future - given the efficiency of the PSU, am I going to be using more electricity than I might be if I used a smaller PSU (e.g. 380W)?

Just not sure as to how efficient PSUs are when they're only running at say 50%.

Typically PSU's are most efficient at around 50% load.

http://www.80plus.org/manu/psu/psu_repo ... Report.pdf

That's definitely not oversized.

Anything over 20% of the PSUs capacity is fine. Your PSU would be oversized only if you spent most of your time below that 20%, so if it idles at over 86W you have nothing to worry about.

My own PC idles at just under 40W, but oh the difficulty of finding a decent 200W PSU... I'm considering running the monitor off the PCIe power cable instead of its own power brick.

Moon GT wrote:My own PC idles at just under 40W, but oh the difficulty of finding a decent 200W PSU...

As you are in the UK there is always LinITX.com, see

http://linitx.com/viewproduct.php?prodid=10916

for example.