I think you can swap "old desktop" for some smaller new power efficient mini desktop, which at idle (for basic home needs lets face it it's pretty close to idle) it's likely to draw way under it's max... but if you are so carbon conscious consider the total environmental cost, buying new shit (new mini desktop or consumer routers) that constantly needs replacing incurs a carbon cost through consumption that people rarely try to quantify because it's not so easy - but it's often still very big. Saving an old PC from the rubbish is free of this cost, it not only saves manufacturing carbon cost but environmental pollution.

On Ebay you can find Dell R210 ii with Ivy Bridge CPU for 150 USD. These idles at 25watt, and are super quiet and small. I have not tested the R220 with Haswell, but I guess idle watt usage is less than 20watt as Haswell had much improved power efficiency when idling.

1 Watt continuous is $1 per year, as a typical USA rule of thumb (at $0.12 per kWh). Places like California are a lot more expensive.

So yeah, it's pretty expensive on an ongoing basis to convert an old desktop to a router.

I must confess, I run an OpenBSD firewall on an old Dell server. Fortunately the Intel CPU doesn't need to do much. So I'm only drawing about 70 W continuous. I keep meaning to replace it with something more power efficient but haven't.