Methods, systems, and apparatus, including for back-up power sources. In one aspect, a method includes providing a plurality of first battery devices, each first battery device respectively electrically coupled to a respective server rack in a plurality of server racks and having a respective capacity to provide power to the respective rack for a power anomaly for up to a first duration. Providing a second battery device electrically coupled to the plurality of server racks and having a capacity to provide power to the plurality of respective server racks for a power anomaly for up to a second duration, wherein the second duration is longer than the first duration. A power anomaly is a deviation of mains power from one or more of a nominal supply voltage and frequency.