@vladh: I'm also interested in the measurement piece. There's so much context. If I use power-hungry software, but my electricity is coming from a solar panel, the carbon use situation is very different than if I was pulling electrons from a coal-fired plant.

I think there's something to explore around writing software that works well on old computers. This discourages buying new hardware in order to keep running the software folx need, which seems like big carbon savings.