I have two boxes at home, a 10 year old desktop, and a 4 year old SFF (small form factor). The SFF idles at about 12 watts, with 3 drives in it (2.5", RAID 5). The desktop idles at 120w with 2 drives in it. With more drives it idles at 200w.
I use the desktop as a duplicate of the SFF, but it only runs for 1 hour overnight for sync to occur (I also have cloud backup via storj.io).
The desktop ran with 2 drives for years, my electricity cost was about $1 per day. That pretty much doubles with the new multi drive setup (which is why it only runs once a day).
Power efficiency is a real concern, so paying for a newer computer is likely worth it. My long term plan is to build a new system periodically, both for power but also because hardware eventually dies.