The problem of climate change requires a transition to carbon-neutral energy. Given the variability of key renewable energy sources such as solar photovoltaics (PV) and wind, fully renewable electricity systems require some form of energy storage. But as described in this study, the economics of electrical energy storage are complex. Many reports on cost of electricity storage confuse the cost of charge/discharge power in kW with the cost of energy storage potential in kWh. These parameters vary greatly and create fundamentally different cost structures for different storage technologies. A case study from the island-nation of Mauritius demonstrates that simulation of a complete electricity system is needed to minimize cost of energy storage, and finds that reservoir type storage such as pumped hydroelectric (PHES) is less expensive than using batteries for the storage requirements modeled. Large reductions in battery prices make them more competitive, but do not greatly reduce total costs. PHES is a fully mature technology, and is more widely available than may be assumed. Policy makers can thus be confident that commitments to deploy renewable energy do not depend on development of new storage technology or reduction of battery prices.