One of the most important type of electrochromic devices (smart windows) is essentially a transparent rechargeable thin film battery (rocking-chair type battery) consisting of a pair of complementary intercalation layers, separated by an ion-conducting polymer electrolyte and contacted by transparent conducting oxide electrodes on glass. At least one of that intercalation electrodes must show colouration by anodic oxidation or cathodic reduction. The similar features of electrochromics and rechargeable thin film batteries concern the material properties, structure of the layers, kinetics of switching (charge/discharge cycles), energy and power density and electrical parameters but also differences between them will be comprehensively discussed. Additionally, we have studied electrochromic devices containing WO3 as cathodic and Prussian white (PW) as anodic electrochromic layers (glass/FTO/WO3/K--Polymer/PW/FTO/glass). Such device configurations reaching high colouration efficiencies, especially in the near IR-range, with a long-term cycle stability >10(5) cycles. The optical response proper-ties in relation to the current-time curves of the charge/discharge processes are discussed.