“Residual mix” refers to the mix of generation supplying the electrical grid minus the generation from specific generators that are exclusively claimed by individual retail consumer as supplying their electricity. The mix of generation after these exclusive claims are removed is referred to as a residual. A residual mix average emission factor can be calculated based on the assigned generation.
The practice of utilizing RECs or GOs to estimate Scope 2 emissions for an entity, even when done in combination with a “residual mix” grid emission factors, is a practice of shifting allocation of emissions among entities (i.e., reallocating the indirect emissions from fossil fuel-fired generation on the grid to other entities). This reallocation misrepresents the actual upstream indirect emissions associated with an entity’s physical consumption of electricity, and thereby undermines credibility and purpose of attributional GHG emission inventories.
Note, even the available residual mix emission factors in the United States, such as those published by Green-e®, only factor out Green-e® registered RECs, and therefore does not account for all other renewable energy purchasing claims by consumers on the grid.
 Brander, M., Gillenwater, M., and Ascui, F. (2018). Creative accounting: A critical perspective on the market-based method for reporting purchased electricity (scope 2) emissions. Energy Policy.