Energy storage could be a game-changer for renewables. Affordable, efficient storage would enable volatile renewable energies to be better integrated into electricity systems, it would greatly improve the economics of home solar systems, even allowing people to go off-grid, and it could help catalyse a revolution in electric cars as well as helping to solve the problems of the grid going down or grid overload. This report seeks to interrogate what the cost base of an array of storage technologies really means. The key conclusion is that a narrow focus on levelised cost alone can be misleading. Throughout the cost modelling process, the same issues repeatedly emerged, namely the importance of defining the business model under consideration and how the storage plant was being operated.