A photovoltaic thermal (PVT) system is a renewable cogeneration system that produces low temperature heat and electricity simultaneously from solar radiation. In water based PVT systems, as the thermal load is served, storage tank is replenished immediately with cold makeup water. However, it is possible to determine an optimal water replenishment profile to optimize the overall configuration of PVT system. In this paper, effects of water replenishment on PVT system sizing are studied. At first, the problem is modeled as a mixed integer non-linear programming problem to analyze the impact of water replenishment on PVT system sizing. Subsequently, the design space approach is used to analyze the significance of water replenishment strategy on the PVT system configuration. Finally, based on analytical derivation, an approximate water replenishment profile is determined and its practical implementation strategies are discussed. Applicability of the proposed methodologies is demonstrated through sizing PVT systems for residential as well as industrial hot water usage patterns. Significant reductions in both collector area (10% for residential example and 7% for industrial example) and storage volume (66% for residential example and 23% for industrial example) are observed. (C) 2016 Elsevier Ltd. All rights reserved.