The classical equal area criterion in the power-angle domain, which is typically used for power systems planning/stability studies, is modified to the time domain for out-of-step protection. Wide-area measurements are necessary to obtain the power-angle information if the traditional equal area criterion is applied for out-of-step detection. However, its time-domain modification requires only the information local to the out-of-step relay. The proposed method evaluates the transient energy, which is the area under the power-time curve, and by comparing the areas, differentiation between stable and out-of-step swings is made. The proposed method can be directly applied to multimachine power systems without network reduction, unlike the conventional equal area criterion in which the equivalent two area of the multimachine system needs to be obtained. The proposed out-of-step detection method has been tested on a Single Machine Infinite Bus System and a 17-Bus Multimachine system.