Two small Synology units (DS120J and DS218+ with attached usb drives for backup), five-port gigabit switch and a modem router, in addition to a Proxmox host (HP 800 G3 mini) runs at about 40W with spun down hard drives, and somewhere around 50W when these are being accessed.
The Synology units automatically shut down at night, at which point the power draw drops to 24W.
It all comes down to about 0.85-0.9 kWh per day.
Assuming a price of 30c/kWh means that even this comparatively small power use comes up to 10€/month or so.
Somewhere between 40 and 50W during the day, with the Synologys active depending on drive sleep status, and 23-24 at night with those shut off
On average about 0.9 kWh per day.