Security of electricity supply has been an important topic of growing concern in the United Kingdom as the margin of supply over demand tightens and the risk to security of supply widens. The risk profile to security of supply is presented, in the United Kingdom, in terms of the conventional metrics capacity margin and loss of load expectation. However, increasing levels of disruption in the power sector, arising from a combination of policy, technological and customer change, challenge this traditional approach to characterise the risk to security of supply. The paper presents a framework to evaluate the risk profile to security of supply through a series of probabilistic metrics that establish the expected magnitude (power and energy), likelihood, frequency and duration of encountering supply shortfalls. This detailed characterisation of the risk profile enables risks to be fully identified, assessed and prioritised facilitating effective risk management.