In the field of solar energy, collector models have been employed to study the effects of system configuration, pump control strategies, fluctuating insolation, collector design, etc. [1–4]. In many studies the investigators feel it is necessary to have a transient collector model if meaningful results are to be obtained. The amount of detail included in the transient models varies from study to study, however. The intent of this paper is to demonstrate the impact of several of the simplifying assumptions commonly employed in transient collector models. To accomplish this, simulated flat-plate and trough concentrator responses based on four types of models are compared.