Complete electrochromic (EC) devices based on tungsten oxide are characterized by the chronocoulometry technique in a wide range of applied voltage and time. An empirical model is proposed to approximate the recorded charge evolution over extended periods of time required for the full coloration transition. The model is described by an analytic expression based on the logistic population growth of tungsten bronzes that are responsible for the coloration upon ion intercalation into tungsten oxide, and just four parameters characterize the dynamics of the EC device. The model is verified by fitting experimental chronocoulograms recorded from two different commercial EC windows. It is discussed how the model can be used for the description of the coloration dynamics and estimation of the coloration efficiency. (C) 2016 The Electrochemical Society This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. All rights reserved.