Collector storage solar water heater systems that use phase change materials as heat storage media have attracted increasing attention because of their clean, pollution-free properties and high energy storage density. The design and application of these systems require efficient and optimized phase change thermal storage units (TSUs). This study aims to optimize a TSU by using theoretical analysis methods and to fill the research gaps attributed to the lack of theoretical analysis. The TSU in this work comprises a multichannel flat tube and rectangular fins as heat exchange elements. The performance levels of this TSU, including its effectiveness, compactness factor, gravimetric specific power, and fin efficiency, under different structures are compared. The economic viability and energy efficiency of the proposed solar collector phase change thermal storage system are preliminarily evaluated. Results indicate that the structure of the multichannel flat tube TSU is relatively reasonable when the fin aspect ratio (H-fins/x) and thickness are 8.60 and 0.001 m, respectively. The effectiveness of the optimized TSU during charge and discharge processes respectively increases by 52.0% and 16.3% relative to the original multichannel flat tube TSU. The corresponding recommendation for the use of multichannel flat tube TSUs in household baths is four groups/person. (c) 2020 Elsevier Ltd. All rights reserved.