Energy policy aims to reduce emissions at least long-run cost while ensuring reliability. Its effecacy depends on the cost of emissions reduced. Britain introduced an additional carbon tax (the Carbon Price Support, CPS) for fuels used to generate electricity that by 2015 added 18 pound/t CO2, dramatically reducing the coal share from 41% in 2013 to 6% in 2018. Policies have both short and long-run impacts. Both need to be estimated to measure carbon savings. The paper shows how to measure the Marginal Displacement Factor (MDF, tonnes CO2/MWh) for wind. The short-rim (SR) MDF is estimated econometrically while the long-run (LR) MDF is calculated from a unit commitment model of the GB system in 2015. We examine counter-factual fuel and carbon price scenarios. The CPS lowered the SR-MDF by 7% in 2015 but raised the LR-MDF (for a 25% increase in wind capacity) by 18%. We discuss reasons for the modest differences in the SR- and LR-MDFs.