Overall fuel cell system efficiency must include consideration of the parasitic loads associated with operating the stack. For an air-cooled open-cathode polymer electrolyte fuel cell (PEFC) the air blowers form the largest parasitic load and have a direct influence of the performance of the stack and its temperature; increasing air flow often leading to improved performance at the expense of increasing parasitic load. Here, electro-thermal performance maps are used to characterise stack operation with varying air flow rate (for both cooling and cathode oxygen supply). The non-linear power requirement of air blowers is used to modify stack-level electro-thermal maps to give system-level performance maps capable of identifying the optimum air flow rate to maximise net power output and consequently efficiency at a given operating point. The electro-thermal map concept is invoked in the consideration of dynamics of fuel cell system operation with issues pertaining to speed of response, durability and system self-sustainable operation.