Coal-fired plant: 0.324 kg CO₂/kWh fuel — highest CO₂ saving per kWh not burned. Heavy fuel oil: 0.267 kg CO₂/kWh. Natural gas: 0.183 kg CO₂/kWh. A coal-fired furnace achieves roughly 1.8× more CO₂ reduction per unit of heat saved versus gas.
From our guide: How Insulation Reduces Scope 1 Emissions — full context, charts and sources there.