Consistency of performance indicators and simulation results in solar water heating studies: A comparison of RETScreen, TSOL and TRNSYS

Solar water heating simulations are widely used for feasibility assessment, system design, and optimisation. However, the reproducibility of such simulations and the consistency of reported performance indicators have received limited attention. This study examines how key performance indicators, particularly solar fraction and system efficiency, are defined in the literature and evaluates how modelling choices influence results for thermosiphon systems. A critical review of 11 studies from a recent meta-analysis identified inconsistencies in indicator definitions and reporting practices. Three simulation tools (RETScreen, TSOL, and TRNSYS) were compared for a reference thermosiphon system across 13 climatically diverse locations. The analysis isolated the influence of meteorological databases, modelling approaches, and tool-specific definitions. The literature review revealed frequent ambiguities in indicator definitions, incomplete system boundaries, and metric misinterpretation. Simulation comparisons showed that meteorological database choice alone introduced global horizontal irradiation differences up to ±12%, leading to net present value variations exceeding 100%. When weather data were harmonised, residual differences between RETScreen and TSOL reached 16% for energy savings and increased with solar resource. Tool-specific definitions also caused discrepancies; TSOL systematically reported 100% solar fraction even when load coverage was below 35%. Comparisons between dynamic simulation tools showed DHW load differences up to 85%, mainly due to differences in the embedded cold water temperature models, with particular impact in high-ambient-temperature climates. The results demonstrate that simulation outputs are strongly influenced by modelling assumptions, meteorological data, and indicator definitions. Improving reporting transparency is therefore essential. A minimum reporting checklist is proposed to improve reproducibility and comparability.