Integration of solar thermal energy into a coal-fired power station is a promising technology for many coal-dependent countries. This work investigated the off-design performance of such a dual heat source boiler power generation from a system-level modelling approach. As an example study, heat from a solar power tower (SPT) was integrated into a 660 MW supercritical coal-fired power unit, and two integration schemes were considered. A system level analytical model was established that coupled the transient process of heliostat field with one-tank thermocline thermal energy storage. The off-design performance of such a hybrid system in one typical year was analyzed accordingly. The results revealed the importance of the seasonal variation of direct normal insolation (DNI), thermal energy storage scheme and integration methodology. Both the quality of sunshine and the amount of sun flux could influence the solar power efficiency; while an increase in the storage volume could decrease the discharging efficiency. Under the maximum capacity of DNI, increasing the storage capacity by 1 h could improve the efficiency by 0.5-0.8%. For either integration scheme, the coal consumption could be economized at least 9 x 10(3) ton per year. The maximum of solar efficiency for Scheme I, where solar energy was used to heat the superheat steam, could reach 20.42%, which also came with a penalty of reduced efficiency of thermal receiver. Under the minimum storage capacity, the solar efficiency for Scheme I was changed from 16.7% to 19.6%, while for Scheme II the change was from 14.7% to 17.3%. (C) 2016 Elsevier Ltd. All rights reserved.