You can save X watts at some cost you would never recoup. Fundamentally, I did not want an L3426, I wanted a fire breathing server but it was the only Xeon I could find in Akiba. It cost about 31000, for that I could have got the 3450, which has a rated TDP of 95W, (as opposed to what, 45W?) but in all likelihood would never get near that. I believe the idle power is the same.
The voltage range on the Xeon X3450
is 0.70 to 1.4. The bottom of the L3426 is 0.65, but that 0.05 Volts probably doesn't make much difference.
I believe the "low tdp" versions of processors (e.g. L3426) are generally just cherry-picked versions of their faster, higher-tdp brothers. And depending on marketing/sales forecasts, you could quite easily buy something like the X3450 that literally could
have been binned as an L3426. People (particularly on this forum) do that all the time: buy the cheaper, mainstream part, then use their enthusiast motherboard to undervolt and/or underclock the chip to get virtually the same power performance as the more expensive version of the chip.
The only catch is that the true server-grade motherboards (like the ones we've been discussing here) generally don't have voltage controls like the enthusiast boards. So it would be interesting to see if there's any difference in idle power consumption between the X3450 and L3426. I'd wager there's a slight difference, as the 3426 has a lower minimum v core, and a lower clock speed. However, I wouldn't bet on a substantial difference! The hardware review sites usually do these kinds of comparisons, but, sadly, not with expensive server parts! (So that means someone on this forum with deep pockets has to buy all these parts and do a the testing for us!
So, ask yourself, how much electricity would the extra 10000y (or 100$ or whatever) pay for and at 50W, how long would your money last? In fact, lets do that sum. For the sake of arguments, lets assume that you save 50W continuously by buying the lower power cpu, this is unrealistic of course, since the higher power cpu is unlikely to be burning all those Watts.
Indeed. In my area (Chicago, Illinois), I calculated (roughly) that each watt saved equals about $1/year. So, saving 50 watts over the course of a year is worth $50.
Anyway, as I mentioned before, I really only started this thread to "think out loud". I like planning out builds, even if I never intend to actually buy the parts. For me, it's kind of like going to a car enthusiast forum, and talking about Ferrari and Bentley, even though I'll never buy one.
On the other hand, a "real" recommendation for a NAS w/ECC is the Biostar A760G-M2+
plus a cheap AMD CPU. It's "green" (even if not marketed as such) and
light on the wallet.