Fundamental properties of all-solid-state electrochromic windows to control the solar energy have been investigated. This system comprises of multilayer represented as Glass/ ITO/NiO/Inorganic Electrolyte (Ta2O5, etc.)/ WO3/ITO/ Adhesive Film/Glass. Of the various electrochromic systems examined so far, the most important features are their environmental stability and the possibility of large area applications. Our system can control the visible transmittance between 72.6% and 17.6% and has a cyclic life over 100000 cycles at 60 degrees C. Based on the accelerated weathering tests, the stability of the system is estimated to be over five years for outdoor applications. For the problem of scaling up, some technical aspect is given and the prototype window of size 40 x 60 cm is exemplified. The present system could be more suitable for architectural and automobile applications in the near future by developing production technology.