Does anyone know of any sources that attempt to look at the marginal energy source rather than the average energy source? For example, a region might have 50%

Does anyone know of any sources that attempt to look at the marginal energy source rather than the average energy source? For example, a region might have 50% of its power from hydro and 50% from gas, but 0% of new power plants are hydro and 100% are gas. In such a world, adding a new electricity consumer to the grid causes 100% gas emissions, even though "50% of power is clean hydro." The proper attribution really ought to be the causal attribution, which typically means looking at the marginal source of power rather than the average. I know this is a complicated topic. Entire PhD theses have been written on modeling the emissions impact of EVs, and there's a healthy debate of how to best calculate the 'causal' impact.