WATSUN 2009 is a program that models the performance of commercial and residential solar water heating systems including those with or without thermal storage. The simulation normally uses hourly time steps but smaller steps may be used when the controller turns the system on or off. The storage tank may be modelled as mixed or stratified and the load may be defined in considerable detail. The program predicts the total energy used by the load, the solar energy collected, the fraction of the load met by solar and several related energy quantities.

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