A series hydraulic hybrid system for a vehicle and a method of operating the same is described. The series hydraulic hybrid system has a hydraulic circuit, high and low pressure hydraulic accumulators, and a control unit. The hydraulic circuit has first and second hydraulic displacement units in fluid communication. The first hydraulic displacement unit is drivingly engaged with an internal combustion engine. The high pressure hydraulic accumulator and the low pressure hydraulic accumulator are fluidly connected to the hydraulic circuit through at least one accumulator valve. The control unit is adapted to receive an input from an operator, compute a requested torque and a target system pressure based on the input, compare an accumulator pressure to the target system pressure, and control at least one of a speed of the internal combustion engine and a valve state of the accumulator valve based on the outcome of the comparison.