This paper presents an attempt to address the challenge of modeling complex systems in which people, energy, and the environment meet. This challenge is met by developing a simple domain specific language for building systems models in a federated modeling environment. The language and its support infrastructure are designed for simplicity and ease of use. This language is demonstrated using a thermodynamic model of a biomass cookstove for the developing world as an example, and the use of the tools described in this paper to further extend that cookstove model into an end-to-end design tool for cookstoves and other energy systems for the developing world is discussed.