Model-based evaluation has been extensively used to estimate performance and reliability metrics of computer systems, especially critical systems, for which experimental approaches are not always applicable. A significant challenge is constructing and maintaining the models for large-scale and possibly evolving systems. In a recent work we defined the Template Models Description Language (TMDL) framework, an approach to improve reuse in the specification of performability models. The approach is based on the concept of libraries of model templates that interact using well-defined interfaces. To apply the framework, some assumptions must be satisfied. In particular, a template-level version of the formalism that will be used for the analysis needs to be defined. A template-level formalism is essentially a parameterized abstracted version of a certain modeling specific formalism, from which concrete instances can be automatically derived. In this paper we give the formal definition of Stochastic Activity Networks Templates (SAN-T), a formalism based on SANs with the addition of variability aspects that can be used to define model templates. We then discuss its concrete application to a simple example in the telecom domain.