The need for domestic hot water in regions with cool climate represents a significant proportion of domestic energy consumption. The steady increase in the electricity costs and environmental concern from use of fossil-based fuel raises the interest in the search for alternative energy sources. In the case of domestic hot water provision, many governments have initiated a gradual switch to more environmentally friendly systems powered by renewable energy such as solar. Solar water heating (SWH) is a mature technology and is gaining popularity in many countries with increasing number of affluent population in society. The increasing adoption of these systems and technologies is a welcome development; however the robust methods of assessment of their thermal performances are required. This paper presents a comparative study of the methods of evaluation of SWH systems' thermal performance in three countries with increasing hot water systems penetration: Australia, Taiwan and Japan. The aim of this comparative study is to discuss merits and weaknesses of each approach and to explore possible common approach that will improve the existing methodologies.