We present a methodology for assessing the economic impact of power storage technologies. The methodology is founded on classical approaches to the optimal stopping of stochastic processes but involves an innovation that circumvents the need to, ex ante, identify the form of the driving process; the methodology works directly on observed data and so avoids model risks. Power storage is regarded as a complement to the intermittent output of renewable energy generators, and is therefore important in contributing to the reduction of carbon intensive power generation. Our aim is to present a methodology suitable for use by policy makers that is simple to maintain, adaptable to different technologies and is easy to interpret. The methodology is shown to have benefits over current techniques and is able to value, by identifying a viable optimal operational strategy, an idealised storage facility based on compressed air technology operating in the UK.