I think you’re ignoring the concept of it being marginal, it’s marginal because they’re on the margin of choosing to mine or to turn up the heating. Those other activities aren’t marginal you need to cook, you need to drive, etc.

Cooking, though, is an activity that puts demand on the grid for a short time. Mining BTC to heat your house is a constant and predictable load. There are plenty of houses in canada with electric furnaces - Last I checked Hydro Quebec would help subsidize the cost if you switched from gas to electric heat.

Even a constant and predictable load will cause some extra carbon dioxide output. How much depends on a variety of factors.

For example, during any period that the nuclear and hydro are running at capacity. If you reduced your constant power usage by 1kWh then during those peak periods, 1kWh less gas generation would be used by the network.

If your constant 1kWh of electricity would otherwise be exported to the US if you didn't use it, then it could be that 1kWh less coal generation would be used at some times.

Most people find it difficult to think about how their marginal increase in usage very often will cause extra CO2. Even writing this comment down has made me think about how CO2 production also depends on the future predictions of generators. It is complicated, but worth spending time to think about.

I know - it is complicated. The commercial rate here in Ontario for large consumers is based on the delta between expected load and actual load. If the expected load is much higher than actual, it can actually go into negative pricing (if they have a big nuclear reactor they can’t or don’t want to spin down, they will literally pay you to use the electricity.) I had a friend at a big company here that had some large refrigeration units for medical related stuff, but they had quite a wide range of temperatures they were allowed to be. So my friend wrote a script that scrapes Ontario Hydro’s website to fetch the commercial rate, and when it was high he’d let the temperature climb a little, and when it was low he’d cool the units a little more. Ended up saving seven figures a year.