Within the scope of the EU-ALTENER project ＇＇Guaranteed Yield of Solar Water Heaters＇＇, a procedure has been proposed for in situ check of collector array performance. The goal of the procedure is prediction of collector array long-term performance, yearly energy yield in particular. The prediction is carried out using the array parameters identified by short-term monitoring of array performance under non-stationary measurement conditions i.e. transient weather and the system operating conditions. The variability of the influencing variables determines accuracy of the identified parameters for the particular array. The necessary variability may be determined by the detailed performance simulation of a particular array, using the components design parameters of the solar system and the long-term, site-related weather data. The procedure is checked out on a solar water heater with 60 m(2) collector array. Satisfactory accuracy and repeatability of the significant identified array parameters are achieved. The prediction of the array yearly energy yield is accurate within +/-5%.