Law is an important tool for national governments to promote the transition to a low carbon energy sector. However, law is the servant of national politics and policies and is embedded in a wider institutional environment, both nationally and internationally. This comparative analysis of the United Kingdom (UK) and China shows that, although national law plays a role in supporting the transition to a low-carbon energy system, the nature of the law and the role it plays depends greatly on the wider institutional environment. The UK is a parliamentary democracy and a liberal market economy. Therefore, the laws passed by the legislature form a critical part of the framework for the low-carbon transition. In contrast, the authority of the executive in China allows the executive to govern the energy sector through decrees and regulations, and diminishes the role of law passed by the legislature. Energy law and policy in both the UK and China are formulated in the light of the prevailing energy policy paradigm: the market-oriented regulatory state paradigm in the UK and a state-centered paradigm in China. Together, these differences explain the contrasting experiences in promoting the low-carbon transition, in general, and the deployment of renewable energy in particular.