This article presents a validated numerical model of composite latent heat storage (CLHS) used for designing cooling systems for power electronics (PE). Successfully implementing CLHS depends on many factors. Testing all of them on a test rig is expensive and time consuming. A CLHS model allows system designers to test CLHS in complex applications by varying dimensions and operating conditions. The model is written in the equation-based modelling language Modelica and represents the melting process of three-dimensional discretised CLHS. In order to validate the model a test rig is built and validation results are shown. The validated CLHS model is used in two different cooling systems. The first system is an air cooling system for power electronics with a high dynamic behaviour and lower power output. The second cooling system is a more complex liquid cooling system with a significantly increased power output. The article shows and discusses the results of both system simulations.