A method for operating a series hybrid vehicle is provided, in which an engine generates secondary power that is either stored or used as direct input energy by a secondary power source to provide motive power to the vehicle. Regenerative braking is used to convert kinetic energy of the vehicle into secondary energy, which is also stored. When the vehicle driver makes a power demand, the secondary power source is powered by secondary energy from an energy storage device, direct input energy generated by the engine, or both, depending on the amount of stored secondary energy in combination with vehicle speed. When in use, the power level at which the engine is operated is also determined on the basis of available stored energy and vehicle speed. At higher vehicle speeds, the amount of stored energy is allowed to be depleted in order to increase the available storage capacity for regenerative braking.