Batteries in conventional fuel cell-based backup power systems carry out secondary tasks to support fuel cells in transient states. In this work, batteries fulfill an additional function: contributing to supply the load in stationary state, thus widening possibly the range of economical backup sizing solutions. Next, the sizing problem for this type of backup hybrid power systems is formulated in a generic way and discussed specifically when energy demand is given by an hourly load profile. A methodology was developed to solve this particular sizing problem by applying optimal control in energy management to reach optimal solutions. From a practical viewpoint, this methodology is used to size a backup power system for a telecommunication system. Several cases, based on realistic data, are studied with different parametric values. Hybrid solutions prove the best in a high number of cases.