This paper describes a new test method for determining the annual energy consumption of a domestic solar water heater incorporating an in-series gas booster on the outlet of the storage vessel to provide auxiliary heating. The test method accounts for the variation in performance of the gas booster as its incoming water temperature and flow rate varies. Systems incorporating both standard and condensing gas instantaneous water heaters are evaluated. An experimental method was developed to characterize the steady-state performance of the gas booster and the resulting performance equation incorporated in a TRNSYS model of the solar water heater to assess gas consumption. Using this method, annual gas consumption is found to be 15% higher than calculated by a current standard where gas booster efficiency is held constant regardless of water flow rate or temperature. The condensing gas booster tested was found to maintain a significant performance advantage over the non-condensing model over a range of solar water heaters. (C) 2011 Elsevier Ltd. All rights reserved.