Electrification is a keyword for many industries today, and thermal management is essential. This paper aims to review the sustainability perspective and analyse the need for thermal conductivity. A model for thermal conductivity is developed for as-cast and heat-treated states to be used as a basis for a sustainability impact index to allow quantitative decisions on sustainable alloying for heat transfer solution alloy development and selection. In the analysis, it was necessary to consider microstructural features for a satisfactory description of the thermal conductivity measured from a set of Al-Si-based alloys in the as-cast and heat-treated states. An environmental impact index was developed for the alloying elements to gauge the effectiveness of different alloying elements, including the environmental impact. An effort to include the microstructural effects was made. Due to the low thermal conductivity of the matrix phase in the as-cast state, the eutectic regions provide a positive contribution to the correction factor developed. The heat-treated stat microstructure seems to lack importance in the ranges of Si additions used in the current study.