To improve the reliability and the energy efficiency of data centers, as well as to reduce infrastructure costs and environmental impacts, we experimentally evaluated in-rack powering of servers with a hybrid 12 kW Proton Exchange Membrane Fuel Cell (PEMFC) and battery system. The steady state and the transient performance of the PEMFC and battery in response to dynamic AC loads and real server loads have been evaluated and characterized. The PEMFC system responds quickly and reproducibly to load changes directly from the server rack. Peak efficiency of 55.2% in a single server rack can be achieved. The effect of fuel cell coolant temperature on the hybrid system transient behavior is also captured and evaluated. The observed PEMFC transient responses obtained from the experiments were used to design the size of the energy storage component for the hybrid system. Simulations and analysis of various types of energy storage devices for the hybrid system were carried out: To provide power to meet the most significant transient demand, energy storage capacity greater than 0.3 kWh is required for all battery types, while only 0.053 kWh capacity is required for the ultracapacitor. During charging, the ultracapacitor uses the shortest amount of time to recover to the original SOC, while the charging duration for the lead acid battery is twice as long as that of the ultracapacitor. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.