The Carnot Ceiling: Why Heat Pump Performance Collapses on the Coldest Days

Heat pump efficiency (COP) is crucial to building heating costs and carbon emissions, but it's not constant—it depends on the temperature differential between outdoor air and indoor space. On the coldest days when heating demand peaks, this gap widens dramatically and COP plummets, making it essential to understand this thermodynamic relationship for realistic energy budgets and climate impact reduction.