> using it all day (say 8h) costs between 0.7 and about 14 kg of CO2 in the US,

How do you get to this range? That's quite a spread.

When I last ran the math, my daily usage (efficient and effective productivity, not spamming Gas Town) came to about 0.67 kg of CO2, which is roughly equivalent to my individual emissions from the 1 mile public bus ride home from work.

Data is from https://app.electricitymaps.com/map

The difference is so big because renewables are just that much more efficient than coal and, to a lesser extent, natural gas. You can have 60% coming from renewable sources and still emit 400g/kWh with a coal and gas mix, whereas all hydro is 24g/kWh according to that source. The production component is what makes renewables not completely emission-free