How many watts for a FreeNAS server w/ 12 5400RPM HDDs?

I know I've asked a similar question before, but I need some clarification.

Here's what I'm building:

FreeNAS server:
AMD Athlon II 210e Regor 2.6ghz (45W TDP)
Asus ATX Mobo (will fill one PCI-e slot with SATA controller eventually)

I want the power capacity for 12 x 3.5 inch HDDs (5400RPM)

No optical drive, no graphics card, nothing fancy (except a handful of case fans including one 220mm fan).

The calculators out on the web provide me with power numbers between 330W and 420W depending on the site.

Honestly those numbers seem really high to me.

Here's the math that I see:
CPU: 45W max
Memory: 15W?? (2 sticks of DDR3 1333mhz ECC memory)
onboard GPU + other mobo activity: 40W? (will be running headless, so probably much lower than this for wattage, right?)
12 x 5400RPM 3.5" hard drives: 84W (12 x 7W peak)
Sata controller card: 40W?

The total for all that only comes up to 224W and I think I am being very conservative in the estimate. Real power draw should probably be much lower.

Do these calculations seem right to you? Where am I going wrong?

Energy efficiency is important since this server will be on 24/7 so I don't want to buy too big of a PSU and waste energy. PSU will probably only be energy efficient up to around 85% or so.

Thanks.