Well good to know, thank you!
For anyone that's interested. With my setup running as previously described:
i5-2400
some HDD, I think a WD Blue 500+GB?
intel PRO/1000 dual NIC
Consumes about 40W on average with not enough fluctuation to mention.
$0.12/kWh: $42/yr (US avg)
$0.25/kWh: $88/yr (in-betweener)
$0.37/kWh: $130/yr (HI avg)
To put that in contrast, an SG-1000 runs at 2.5W idle, so if for simplicity we say it runs an average of 4W in use…. it's using about 10% of the power of my dusty old PC build.
My build would cost you about $100-130 to buy if you don't have anything sitting around, vs the SG-1000 at $150 (includes support I think).
Granted, the SG-1000 does not have even a fraction of the horsepower and probably wouldn't serve even my very modest VPN uses. But it's still a good bracket for real world cost to operate vs a dusty old PC.
Ultimately, long term it will certainly pay to buy or put something together that will serve your needs for hopefully many years while remaining low power.
Once I can justify it I'll probably switch to a newer low power fanless pentium or celeron or maybe even an atom if it's at a cheap enough price/perf point, combined with an i340 or 350 and a SATA DOM.
I'd like to get down to about 15W without paying a ton up front and retaining some processing power.