An efficient and simple simulation approach for thermosyphon solar water heaters has been developed and compared with experimental data. This approach, valid for solar-only systems, gives the ability to link the system main design and constructional parameters with the expected energy output through an analytical determination of the coefficients of the characteristic input-output equation of the system. The proposed methodology can be used not only for energy optimization of the system in the design phase but also for evaluation of test results of an existing system in order to improve it further. (C) 2002 Elsevier Science Ltd. All rights reserved.