“And what do we do when the sun doesn’t shine, and the wind doesn’t blow?” This has been the recurring, main concern, of our colleagues at energy ministries and electricity utilities around the world, when we discuss the fact that solar and wind generators can provide the cheapest kWh in most power systems.This fundamental question has been holding back the expansion of clean energy. As a result, too many countries are still exposed to vulnerabilities associated with fossil fuels for electricity generation. But thermal ‘dispatchable’ power generation often results in very expensive electricity, locking many of the world’s poorest countries into a vicious cycle of power sector deficits that prevent investments, and heavy subsidies that lead to fiscal stress and further indebtedness.With this report, the World Bank begins to address the anxieties of ‘intermittent’ solar and wind. We introduce a complete framework that outlines how modern battery energy storage systems can be effectively deployed and alleviate the variability of renewables. Moreover, the paper is accompanied by a Power Purchase Agreement template that can serve as a practical tool to bring private sector investments in the power sector–going beyond solar, or wind parks, with variable output.