Energy policy aims to reduce emissions at least long-run cost while ensuring reliability. Their effectiveness 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/t CO2, dramatically reducing the coal share from 41% in 2013 to 8% in 2018. This paper shows how to estimate CO2 reductions, arguing that policies have both short and long-run impacts. Both need to be estimated and combined to measure carbon savings. The paper shows how to measure the Marginal Displacement Factor (MDF, tonnes CO2 /MWh) for wind. The short-run MDF is estimated econometrically while the long-run 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 short-run MDF by 7% in 2015 but raised the long-run MDF (for a 25% increase in wind capacity) by 33%. The CPS raised the 2016 wholesale price by £6.22/MWh with impacts on interconnector trade.