Renewable power has seen a dramatic expansion in recent years thanks to sharply falling costs. But this growth has raised a new challenge for power-system operators and regulators: wind and solar PV have sometimes been deployed before the power system, including both policies and infrastructure, was ready to handle such variable supplies. In China, 15% of the electricity that could have been produced by wind turbines last year had to be curtailed – by taking otherwise productive wind turbines offline – because the system was unable to integrate the supplied power. In Jilin province, a third of the wind power that could have been produced was curtailed over the course of 2015. This loss was not paid for, highlighting an investment risk for any company wishing to build additional variable renewables. A new IEA publication, Next Generation Wind and Solar: From Cost to Value, describes the root of this challenge and highlights possible solutions. It’s a challenge that depends largely on two things: first, how much of a contribution variable renewables are already making to the power sector and second, the overall flexibility of the power system. Integrating the first few percentage points of variable renewables into generation poses few problems for most power systems. Beyond these levels however, power systems must be adapted and upgraded to take variable renewables into account.