This paper considers the scaling principles associated with the power and energy density of batteries and generators as applied to mobile robots and similarly-sized vehicles. We seek to identify, based on present technology, the size range at which a generator inclusive of a direct current electric motor, gearbox, and internal combustion engine can be effectively used to replace modern batteries. Models were derived to scale each component of the generator as a function of power, mass, efficiency, and speed. For a given power, energy, or mass requirement, these models illustrate that generators based on conventional technology are ill-suited for smaller robots. The results indicate that there is an intermediate robot size above which a hybrid generator/battery architecture is desirable. Using these scaling principles with modest extensions of existing battery technologies, it is also possible to infer the near-future performance of robot power technology and thus illustrate whether the generator-versus-battery tradeoff will shift toward or away from a hybrid robot topology for smaller vehicle systems.