With further maturity and applicability of plasma actuator flow control it becomes increasingly important to classify and quantify the net efficiency of plasma actuators, taking into account their effect on the flow and including a careful analysis and evaluation of the corresponding energy requirements. The present work provides such an evaluation standard with a comprehensive discussion of plasma actuator performance and efficiency. Existing evaluation strategies are reviewed, contradictory definitions presently found in the literature are clarified, and all discussed quantities are ordered into a universal power-flow diagram, serving as a guideline to classify and quantify plasma-actuator performance in flow control applications.