An ultracapacitor model with automatic order selection of complexity and automatic scaling of capacity is created in Virtual Test Bed platform for complex system simulation and prototyping. The order selection is based upon the variable simulation time step for appropriate level of details of the model, and automatically executed by the model itself, therefore it applies to simulation for both high and low frequencies, or for both fast and slow transients, or even for multi-time scale resolutions. A simple trapezoidal algorithm, rather than the general stiff algorithm, which is complicated in implementation, is used for numerical integration. For a given time step, the truncation error is controlled to be negligible, so that numerical stability and accuracy are ensured for the given order of model complexity. The model is validated by comparing the simulation results to the experimental ones. The application example of the ultracapacitor model in complex system simulation is also presented.