At what point is it more planet-friendly to manufacture a new energy-efficient underpowered low-end PC, rather than repurpose an old power-hungry low-end PC?
It's a good but difficult question to answer if one wants to break into the details of it.
The process of manufacturing all of the components likely results externalities that are represented in the price. There is also the cost of how the disposed machine is processed.
You can also factor in the user behaviors. My grandmother probably is not going to leave her desktop powered on for longer than the 1 hour or so per day that she might use it. People here, myself included, are probably used to running their machines 24-7.
There's also the perspective that if this machine is going to a 10 year old kid, that it's a machine s/he can break and it won't be a big deal. This can be a significant advantage for a low income home.
I think it really depends on what you mean by 'a low end PC'.. if you mean a full desktop with an extensible (PCIe etc) motherboard.. I believe the answer is... way after you'd ever want to use the old PC[1]. But if you're moving towards the something in the vein of the Raspberry Pi, much sooner.
I don't know about more planet friendly, but just in terms of dollars, if you're paying for your own electricity getting a modern desktop for $200 is more economical after a couple months.
The process of manufacturing all of the components likely results externalities that are represented in the price. There is also the cost of how the disposed machine is processed.
You can also factor in the user behaviors. My grandmother probably is not going to leave her desktop powered on for longer than the 1 hour or so per day that she might use it. People here, myself included, are probably used to running their machines 24-7.
There's also the perspective that if this machine is going to a 10 year old kid, that it's a machine s/he can break and it won't be a big deal. This can be a significant advantage for a low income home.