ISO 9459-4:2013 specifies a method of evaluating the annual energy performance of solar water heaters using a combination of test results for component performance and a mathematical model to determine an annual load cycle task performance under specified weather and load conditions. The procedure is applicable to solar water heaters with integral backup or preheating into a conventional storage or instantaneous water heater and to integral collector storage water heaters.
System operating requirements specified in ISO 9459-4:2013 are for the purpose of determining an annual performance rating for domestic water heaters. There are no product design or operation requirements in ISO 9459-4:2013.