Integral collector storage (ICS) solar domestic water heating systems are an alternative to help meet the hot water energy demands in a household. In order to evaluate the potential benefits and contributions from the system, it is important to be certain that the modeling scheme is as accurate as possible. The overall heat loss coefficient (U loss ) plays an important roll in such a scheme and in the performance prediction methodology of the ICS. This paper presents the results of an investigation of the application of optimal control theory to find how U loss varies with time, for a particular nonconcentrating ICS system. The time-varying U loss obtained was used in a proposed model, and the resulting simulated ICS performance was compared to the real measured performance and the simulated case when U loss was time-invariant. After comparison, it was determined that the use of a time-varying U loss in the modeling scheme significantly improves the ICS performance prediction.