The ability of powerful incumbent actors to resist technological or institutional changes that threaten their vested interests is a core part of the literature on sustainable energy transitions, but more often asserted in general than tested in specific cases. This article presents analysis of a detailed study of the introduction of a Capacity Market(CM) for electricity in Great Britain in the period 2010–2011, using a process tracing approach. The study finds evidence to support the view that large electricity generators have a significant degree of structural power in relation to decision makers, that such companies did lobby the government through the CM policy process, and that the ideas deployed had an effect on the shape of the CM. However, there were also divisions amongst the large generating companies on whether they wanted a CM or not, and what its design should be. We also show that the institutional circumstances of the CM policy process we requite specific, giving opportunities for lobbying that may not be present in other cases. We conclude it is essential to have an analysis of incumbent power that is contingent on institutional context, the specific nature of interests and the deployment of ideas. To counter incumbent power and structural dependency during sustainable energy transformation, an independent but legitimate body is needed to the direction of policy.