Synthetic jets are driven by a periodic electrical signal to generate pulsated airflow that can provide cooling to a hot surface. The working principle of synthetic jets involves conversion of electrical energy into mechanical and fluid energy. Piezoelectric actuators, comprising of a thin metal substrate bonded to a piezoelectric disk are induced to undergo vibration motion in bending mode by an AC sine-wave voltage with zero bias. Synthetic jets, which consist of two piezoelectric actuators separated by a compliant ring at the outer periphery of the actuators, undergo a bellow-like action due to the periodic motion of the actuators, thereby ingesting air and pushing air at high velocities through the orifice. In this paper, we seek to understand and quantify the efficiency of synthetic jets with a view towards optimizing their design. In this study, energy efficiency of synthetic jet is defined based on thermodynamics principles. Analytical equations for calculating consumed electric power and airflow power are derived. Using the derived equations, energy efficiency of synthetic jets is experimentally investigated. Air velocity at the jet orifice is measured using constant temperature hotwire anemometry. Voltage signal and resultant current waveform are recorded to calculate electric power. In order to understand the structural behavior, laser vibrometer is used to measure the center out-of-plane deflection of the piezoelectric synthetic jet. Electrical power input is varied by changing signal frequency and voltage amplitude. Synthetic jets with two different orifice sizes are tested, and the efficiency of energy conversion is determined. The effects of jet design and operation conditions on energy efficiency are discussed.