The power+cooling load-to-heat ratio is defined as the effective utilization factor (EUF) for a cogeneration system as a new performance parameter. The optimum value of overall EUF of a solar cogeneration system is investigated. For optimum operation, primary performance parameters of the system are discussed. The results may provide some theoretical bases for the optimal design of solar cogeneration systems. (C) 2001 Elsevier Science Ltd. All rights reserved.