A one-dimensional transient numerical model for flat-plate solar thermal devices is here presented. The model permits the analysis of different configurations and components such as multiple-glazing, transparent insulation, air-gaps, surface coatings, opaque insulation and energy accumulation in water or PCM internal stores. In order to obtain information of practical interest, the solar thermal devices are modelled following virtual testing procedures in accordance to the experimental test methods described by European and International standards. This paper describes the basis of the model and shows some comparison of numerical and experimental data as an example of the validation process that has been carried out in order to assess the credibility of the numerical model. For simplicity, the explanation is restricted to standard multiple-glazed flat-plate collectors. The use of the model in other more complicated configurations as in transparently insulated covers or integrated collector storage devices with phase change materials will be presented in other separate papers. (C) 2009 Elsevier Ltd. All rights reserved.