In order to facilitate detailed solar system simulations and to make such simulations readily available to solar engineers in their daily practice of system dimensioning and performance calculations, user friendly software for personal computers has been developed as a system specific simulation program for a frequently installed group of water based solar system types. The program package has been developed as part of the OPSYS R&D-program of the Commission of the European Communities and has been named EURSOL. The set of system configurations considered includes different types of thermal solar systems for space heating and hot water production. The simulations are based on validated physical system models. The program offers complete freedom with regard to all parameters describing the systems and includes on-line help, economic evaluations and graphical output. This book describes the system considered in EURSOL, the mathematical algorithm, the physical model of each component used, the processing of solar radiation data and the economic evaluation criteria. Although the book is mainly a manual for the use of the simulation program, it is also a guide for the development of simplified simulation models for solar systems.