The finished module footprint ranges from 145 to 420 kg CO₂e per m², a span of around 60% driven by supply chain choices. The first two stages dominate. In solar-grade silicon production, over half the impact comes from electricity, with the rest from heat and the embodied metallurgical-grade silicon feedstock. In crystal production via the Czochralski process, electricity and solar-grade silicon are the largest contributors. For the wafer, cell and module stages, the embodied impact of the silicon components carried forward outweighs everything else, which points firmly back to those first two stages as the targets for reduction. Interestingly, switching the metallurgical-grade silicon source alone changes the final footprint by only around 5%, because the energy intensity of purification and crystallisation dwarfs the feedstock’s own footprint. Per kilogram, metallurgical, solar-grade and single-crystal silicon need roughly 10 to 20, 40 to 200, and 120 to 300 kWh respectively. The real lever is how cleanly that purification energy is supplied.