Smart windows were fabricated with different thicknesses of amorphous V2O5, which acts as an ion storage layer. In these devices, V2O5 was deposited by thermal evaporation at a substrate temperature of 200 oC, and an electrochromic layer (WO3) was deposited by electron beam evaporation at a substrate temperature of 250 oC. Both layers were amorphous. V2O5 was found to exhibit direct-forbidden electron transitions, whereas the WO3 layer exhibited indirect-allowed electron transitions. An increase in the thickness of V2O5 from 78 nm to 313 nm reduced the colouration efficiency from 64 to 48 cm2/C, and the time of the transmission variation curve from the coloured state to the bleached state was increased from 82.41 s to 558 s.