This paper presents a novel review and laboratory test results of four standard test methods for determining the energy consumption of microprocessor controlled gas tankless water heaters. The test standards examined are from Australia, Europe, Japan and North America. The European test standard is found to be technically the most comprehensive in measuring all aspects of energy consumption for these types of appliances. Four water heaters, both conventional and condensing type, ranging in capacity from 28 to 55 kW are tested to each of the test standards. The European and North American tests deliver similar results. Those to the Australian standard yield higher energy use while the Japanese method produces the lowest consumption. The relative difference in energy use between water heater models tested to any single standard is consistent, with the exception that the Australian test appears to favour the smaller water heater. Therefore, while consumers cannot use standard test results to calculate their actual running costs, in general, any of the standards will reasonably predict relative running cost differences between models if used as part of the product selection process. While the correlation between the standards varies according to the capacity of the water heater, a simple approximation is developed to convert from the efficiency measured by one standard to another. (C) 2014 Elsevier Ltd. All rights reserved.