This study provides the first empirical evidence that a tropical heritage masonry building can achieve over 90% reduction in annual electricity consumption without compromising its historic integrity. A full-year monitoring campaign was conducted at Lawang Sewu in Semarang, Indonesia, combined with calibrated EnergyPlus and WUFI® Pro simulations. ANOVA confirmed significant thermal variation between interior zones, supporting a targeted rather than uniform retrofit strategy. The optimized solution combined reversible roof-mounted CIGS thin-film photovoltaics with selective façade electrochromic glazing, achieving a mean 90.3% energy reduction (95% CI: 87.9–92.4%) while maintaining comfort within ISO 7730. Multicriteria decision analysis verified full visual compatibility and reversibility. The study delivers a transferable workflow for hot–humid heritage contexts, while noting that multisite validation is required to strengthen generalizability.
