There are low power machines that idle on 8 watts (Dell Optiplex Micro Form Factor), and my whole lab costs around USD$100 a year to run on local power prices.
You're right, it's really important to pick components that are low power.
With Home Assistant automation, you can even start and stop machines so they only draw power when you need, e.g. for a testing pipeline.
I couldn't recommend more the channel "Jim's Garage" on YouTube to help someone get started in this space. Fantastic content!