In the market of High Intensity Discharge (HID) lamps there is a gradual shift from traditional high-pressure plasma light sources based on single-metal emitters (e.g. high pressure sodium and high pressure mercury lamps) towards high-pressure plasma light sources with multiple plasma components. In the last group especially metal-halide lamps with improved energy efficiency and light-technical properties are becoming increasingly important. Since these plasmas are more complex in terms of the plasma chemistry and physics, an improved understanding is needed. In this article a model will be presented for the energy balance of metal-halide lamps. The model includes several submodels, including radiation models for optically thick and optically thin radiation, 2-D radiation transport, a model for the chemical equilibrium reactions between the different components in the plasma, heat conductivity and electrical conductivity, and a 1-D numerical solution of the Elenbaas-Heller equation. As an example model calculations in comparison to experimental results will be shown for high pressure sodium-iodide lamps.