I thought this video was rather interesting, because at 12:27, the presenter crunches the numbers to find out how many years it would take for a new computer purchase to be more environmentally friendly (in regards to total CO2 expended) compared to using a less efficient used model.
Depending on the specific use case, it could take as little as 3 years to breakeven in terms of CO2 if both systems were at max power draw forever, and as long as 30 if the systems are mostly at idle.
Older desktops can have a somewhat hefty idle power draw due to the overall system consumption contributing more than expected, such as the southbridge. According to this old review of the i7-2600k, the system idles at 74w, which at $0.12 per KWh, would cost you roughly $77 per year. Though you might want to confirm that with a Kill-a-watt meter if you can (libraries sometimes lend them out), since I’m pretty sure that total system power chart includes a discrete GPU, so the real number for a GPU-less system is probably around 40 or 50w at idle.
If that is accurate, you could potentially replace your i7-2600 with a used Dell Wyse 5070 thin client from ebay for about $40 (in the US), and that idles at 5w, which would only cost you $5 a year at the same rate.
Older thin clients and laptops tend to have much better idle power draws compared to desktops. For other people reading this, if you’re using a desktop for a low-power use case, it’s probably worth finding out what its idle power consumption is and doing the calculation to determine if it’d be worth replacing it with a more efficient used thin-client or office mini-pc.
In Germany consumer power is something like 0.4 EUR/kWh, so economics of running power-hungry hardware might be different. Solar PV might change the equation once again.
Then you’ll need to factor in the solar initial cost as well!
DIY few kWp solar patback is about 2 years.