We estimate with a branching process model the propagation of load shed and the probability distribution of load shed in simulated blackouts of an electric power system. The average propagation of the simulated load shed data is estimated and then the initial load shed is discretized and propagated with a Galton-Watson branching process model of cascading failure to estimate the probability distribution of total load shed. We initially test the estimated distribution of total load shed using load shed data generated by the OPA simulation of cascading transmission line outages on the 300 bus IEEE test system. We discuss the effectiveness of the estimator in terms of how many cascades need to be simulated to predict the distribution of load shed accurately.