Getting my info from HWMonitor as I can't find a better option without buying a hardware solution.
CPU: around 15W DRAM: around 10W GPU: around 60W if I'm reading that bit right...
Total (and adding a bit for other components and PSU efficiency losses): around 115W
Over a (30 day) month that would be 82.8kWh or £26 a month (actually a bit less than this as I'm on an economy 7 tariff which halves the costs per kWh overnight, I went with worst case as the server uses more energy during the weekend days).
Would be interesting to boot up the old server and check use there, willing to bet a dual Xeon X5670 machine with spinning rust for storage would be far higher than the current Xeon E5 2690 v4 with M.2s in there (a quick Google search indicates 174W for a machine with less hardware than my old server, at idle)
Maybe energy is cheap there or OP is rich, I live in the UK and was surprised by the temperatures they quoted.
I would say though that Scandinavian houses are known for being very well insulated and energy efficient.
My heating is on 19C when I'm home, lowered to 10 while I'm out/sleeping because my cat has his own fur coat to keep him warm and I have a 13 tog duvet
That's an E5 v2 xeon iirc?
I picked up a T5810 recently for a similar price and slapped 80GB RAM and a 14c v4 CPU in there for another £60 or so.
Much better than my old dual Xeon X5670 box.
Look up SAMBA setup for the file sharing, you can simply map a network drive on Windows clients and mount the share(s) on your Linux client with SMB client (part of the SAMBA package).