Assuming power is free. Even small wattage differences add up quickly for a server running 24/7 and those older CPUs can be very inefficient.

5 year old CPUs today (i.e., 2019 era chips) are generally pretty efficient.

The cost of your sanity also adds up quickly :)

or even better - old laptop

Or a Raspberry Pi you have kicking around:

https://www.youtube.com/watch?v=l30sADfDiM8

Yep, optimised for power consumption, and comes with a built in UPS

Just be careful leaving an old battery permanently on charge:

https://youtu.be/2Z9190ruyIM