Electrochromic windows can reduce cooling, heating, and lighting demand [1], yet their deployment is constrained by control strategies that respond to temperature or irradiance thresholds rather than total building energy [2]. We present an absolute-zero-energy (AZE) paradigm that links device optical properties to energy-optimal operation. Ideal AZE (IAZE) identifies climate-responsive transmittance targets, while practical AZE (PAZE) enumerates the measured states of real non-dual-band and dual-band devices to select the minimum-energy state at each hour. Across eight representative cities, temperature-rule control produced net energy penalties in four cases because lighting increases offset HVAC savings. PAZE avoided these penalties, reduced annual building energy by 2.83-10.35% relative to normal windows, increased the energy-saving rate by 2.06-11.41% over rule-based control, and lowered annual CO₂ emissions by 0.69-8.41 kg m^-2 of floor area. A data-driven screening method reduced 12- and 16-state devices to six representative states while retaining converged energy performance. The optimized states can be implemented as precomputed schedules or lookup tables, separating computationally intensive design from simple building operation. These results provide actionable criteria for device selection, optical design, and control-system integration, helping translate electrochromic windows from materials demonstrations into scalable low-carbon building products.

Electrochromic windows,energy-saving strategy,Energy-saving Behavior,Building simulation