The hybridization of middle-temperature solar heat with traditional coal-fired power plants has profound and realistic implications for China. Most solar hybrid power plants, which differ from solar-only power plants, are not designed to include a thermal storage system to maintain system performance at nominal conditions during off-design solar radiation periods. Thus, a proper solar field size is an important design parameter. An excessively large field is partially useless under high solar irradiance, whereas a small field mainly reduces the work output and leads to the poor utilization of invested capital. This paper presents an economic optimization of the solar multiple (SM) for typical solar-coal parabolic trough plants with different unit scales (200-600 MW). The thermal performance for each solar hybrid power plant demonstrates that the methods to calculate the annual solar electricity produced with SM varies. Once the annual electric energy generation is identified, the levelized cost of energy (LCOE) and payback period (PP) for each case can be calculated. Thus, the optimized SM values yielding a minimum LCOE value or PP can be obtained. Furthermore, the influences of local solar radiation resources and turbine operation load conditions on the economic optimization of a system are estimated. Results indicate that the optimization of the solar field size is promising for the economic improvement of a solar-coal hybrid system. The results may also provide a theoretical reference for investors and government officials in designing a solar-coal hybrid project. (C) 2016 Elsevier Ltd. All rights reserved.