Prior to the detailed design of a concentrating solar power (CSP) plant, it is necessary to develop a conceptual design of the plant. Conceptual design activities help in screening of various design objectives prior to the detailed design of the overall power plant. Conceptual design of a CSP plant includes type and size of solar field, power generating cycle and the working fluid, sizing of the power block, etc. One of the most important parameters for conceptual design of a CSP plant is to determine the design radiation of the plant. Design radiation for a CSP plant is the direct normal irradiance (DNI) at which the plant produces the rated power output. Due to daily and seasonal variation of radiation, determining appropriate design radiation is extremely important; low design radiation results in excessive unutilized energy and high design radiation results in low capacity factor of the plant. In this paper, a methodology is proposed to determine the thermodynamically and the cost optimum design radiation for CSP plants without hybridization and storage. The proposed methodology accounts for the characteristics of the collector field as well as turbine characteristics. There is no such analytical methodology reported in the literature. Applicability of the proposed methodology is demonstrated through illustrative case studies incorporating parabolic trough collectors (PTC) as well as linear Fresnel reflectors (LFR). Design radiations for ten different places in India are also reported. However, the proposed methodology is not restricted to India only. (C) 2014 Elsevier Ltd. All rights reserved.