In this paper, methods of separation of primary work E-P, which is needed to provide the genuine work of production equipment, from the total amount of primary energy E are proposed. Direct estimates of primary work of production equipment E-P on the base of available data for the US economy for the 20th century are compared with alternative evaluations of the same quantity calculated from time series for consumption of labour and primary energy. The relationship among primary energy E, primary work of production equipment E-P, and genuine work of production equipment P (productive energy) is considered. The results allow one to estimate coefficient of efficiency of primary work of production equipment. (c) 2006 Elsevier Ltd. All rights reserved.