I have looked at the ROI for getting more efficient kit and ended up discovering that going for something like a low-idle-power-draw system like a NUC or thin client and a disk enclosure has a return period on the order of multiple years.
Based on that information, I've instead put that money towards lower hanging fruit in the form of upgrading older inefficient appliances and adding multi-zone temperature control for power savings.
The energy savings I've been able to make based on long-term energy use data collected via Home Assistant has more than offset all of the electricity I've ever used to power the system itself.