What is the energy-only market
In terms of the underlying concepts, an energy-only market is the opposite of a capacity market. An energy-only market only compensates power that has been produced. A capacity market, on the other hand, compensates the mere readiness, or capacity, for power production.
To ensure supply is guaranteed, the energy-only market is supplemented by various flexibility options, such as control reserve markets. Beyond that, there are usually additional reserve models similar to a capacity market aimed at guaranteeing supply.