You made the right call on the drive, low end hard disk prices hit a wall pretty hard which leaves them looking a bit silly.

You're vastly overestimating the amount of power required to run the disk arrays, even at peak spin-up loads. The only way you could fit enough hardware into that chassis to come near using that PSU is multiple high-end GPUs.

If you're going to build a home server it's almost always worth investing in an AC power meter if you don't already have one.

Nearly every PSU operates most efficiently somewhere in the middle of it's power range, operating a high power PSU at 20% capacity will use notably more power than a correctly sized one. Take a look at any PSU review over at silentpcreview.com, they do good efficiency testing.