Under transient climatic conditions previous research has reported that evacuated tube solar water heaters (ETSWHs) with heat-pipe absorbers are the most effective solution for collection of solar energy. The cost of such systems is greater than the mass produced "water in glass" evacuated tube solar water heater mainly manufactured in China. Previous studies have reported that the costs of solar water heating can be reduced through the adoption of thermosyphon fluid circulation. Well designed thermosyphon systems are as effective as pumped systems but with lower capital and running costs. To investigate if costs could be reduced and performance levels maintained, outdoor testing of three thermosyphon heat-pipe ETSWHs primarily designed for pumped fluid circulation was carried out under a northern maritime climate. Experimental data from a year's side by side monitoring of two thermosyphon ETSWHs (both with the same area of 2 m(2)) was collected and used to validate a correlation based on a modified version of the f-chart design tool between the observed and expected performance for both systems. The R-2 value between measured and predicted monthly solar fractions was greater than 0.99 for both systems. The R-2 value between measured and predicted diurnal solar fractions was calculated as greater than 0.95 for both systems. The only difference between the two was that one utilised internal heat-pipe condensers whilst the other used external ones. The system with internal condensers was found to be 17% more efficient. A simplifying assumption of a constant temperature rise across the collectors reduced the calculations required to predict the performance of thermosyphon heat-pipe ETSWHs and was also statistically significant. To determine if the assumption was valid for other thermosyphon heat-pipe ETSWHs with different collector parameters a third system with internal condensers an area of 3 m(2), a heat removal factor (F-R) of 0.816 based on the absorber area and a collector loss coefficient (FRUL) of 2.25 W m(-2) K-1 was assembled and its performance monitored, when the same assumption was applied the R-2 value between the measured and predicted daily solar fractions was calculated as 0.96 experimentally demonstrating that this relationship was still statistically significant for another heat-pipe thermosyphon ETSWH with different collector parameters. (C) 2011 Elsevier Ltd. All rights reserved.