To comprehensively demonstrate the building HVAC energy saving ability, the remarkable optical properties of the Cu-aqueous electrochromic device are imported into EnergyPlus as the emissivity of buildings' sidewalls and roofs to calculate the year-round energy-saving. The calculation is conducted for 15 cities, corresponding to 15 climate zones in the U.S. (Figure ), to demonstrate the widerange applicability of this smart building envelope. In Figure , we plot the operational HVAC energy saving and saving percentage of the building enveloped with the electrochromic device. The result manifests that most buildings located in weather with high variations, including daily and seasonal fluctuation, require dynamic thermal management. The dynamic emissivity tunability of the device helps the building save more energy compared to the constant low-e or high-e building envelopes (Figure ). When the cooling load is larger than the heating load, especially in hot areas 1A and 3B, the low-e building envelopes suppress radiative heat loss to the environment, causing excessive cooling load and more energy consumption. In hot/dry areas like Honolulu, Austin, Los Angles, Phoenix, the low-e building envelope even consumes more energy than the normal values, pointing out the necessity of switchability between cooling and heating modes (high-e and low-e states). In Figures and, the electrochromic device adaptive to the fluctuating weather conditions can achieve higher energy saving, especially in cold areas. The building envelopes in Helena and Fairbanks can save 48.26 MBtu and 57.26