A generalised quasi-steady-state collector model is derived which includes the effects of pipe losses, one or more circulation pumps, and either an internal or an external collector-store heat exchanger. This leads to expressions for the increased effective collector loss coefficient, and the increased de Winter heat exchanger penalty factor. These increased loss factors are shown to be coupled, strongly, to the quality of the pipe insulation. This coupling is not indicated in the standard textbook descriptions. Proposed European solar water heating test standards make use of the international standard LSO 9459-5 for complete system testing using a dynamic system test. The links are derived herein which show the relationships between the physical system parameters and the ＇non-physical＇ model parameters obtained by the ISO 9459-5 system tests. It is seen from the theory that separating the pipe losses from the collector losses is not possible without substantial additional control of the test conditions, namely the ambient temperature surrounding the pipes.