Electrochromics is the emerging technology for energy conservation and indoor climatic control through smart windows. In this study we are reporting four layer electrochromic device: ITO (400 nm)/commercially procured Nafion (183 μm)/WO3 (44 nmto 200 nm)/ITO (400 nm). The active area (A) of the electrochromic devices are 3 cm2. The tungsten oxide (WO3) and ITO thin films have been deposited at room temperature (300 K) by reactiveDCMagnetron sputtering. The sheet resistance of ITO is 20 Ω/?. The 'as deposited'WO3 films are amorphous and have high optical transmission (75%-85%) in the visible spectrum. The optical band gap decreases with increasing thickness ofWO3 thin films. The coloration efficiency (CE) of the electrochromic device increases with increasing thickness of theWO3 layer. The CE for the device withWO3 thickness 200 nmis 184 cm2 C-1: the highest reported so far for a hybrid electrochromic device. The increase in the CE with thickness has been explained (for the first time) by replacing the surface charge density (Q/A) with the volume charge density (Q/A∗t) in the coloration efficiency formula derived from the Beer Lambert's law. ©2019 IOP Publishing Ltd.