10w is ± 87kwh/year. Depending on your electricity cost, it would take 1 to 5 years to gain anything from switching to a picopsu, that’s it if you even manage to gain 10w, which is not a certainty.
If you really care about those 10w watts, selling the optiplex and getting a second G3 would be a better option I think.
Syncthing is also an option.