An original device was made using complementary electrochromic materials. The active working electrode was tungsten trioxide, deposited by r.f. sputtering onto 'K-glass'. The storage counter electrode was nickel oxide lithium doped, electrochemically deposited. The solid ion conductor was a poly (ethylene glycol methacrylate) (PEGMA), containing appropriate dissolved salts. Using these kind of devices (dimension 144 cm(2)), it was assembled as a large optical electrochromic (EC) shutter, arranged as a matrix (8 x 8 devices). The smart window was installed in a large test cell, were the optical and energy performance will be measured. This test permits quantification of the advantages for energy saving during summer and winter season by darkening windows and to evaluate the visual comfort obtainable by transmittance variation.