Solar integral collector storage (ICS) devices are a potentially low cost means of displacing a portion of the energy required for domestic water heating. However, since ICS systems are rarely used as a stand-alone system and are more typically utilized as a preheater for conventional water heaters, it is imperative to analyze the overall water heating system in order to determine the advantage of any improvements in the thermal performance of the ICS component. In particular, this paper analyzes the performance of a solar ICS heater, in divided and undivided storage configurations, in series with a conventional electric resistance water heater (ERWH) for a range of ICS storage volumes, heat exchanger NTU, initial ICS temperature, and ERWH storage volumes. The undivided storage configuration corresponds to the typical UPICS system whereas the divided storage configuration corresponds to a recently proposed concept for improving the thermal performance of the ICS device. The results show that the ICS preheater does provide significant increases in solar fraction when adequately sized. Although comparison of the divided to undivided storage concept, with the same total ICS storage volume, shows only modest gains of 5-10% in solar fraction, the ICS storage volume necessary to attain the same solar fraction is much less for the divided storage concept. The smaller required storage volume would, in turn, enable faster charging times and potentially higher initial temperatures thereby leading to even further improvements in overall system performance.