Efforts to achieve a decarbonized society are being promoted worldwide, and in Japan, the government has been driving the adoption of net-zero energy houses through various policies. This study focuses on the potential of shifting energy from nighttime to daytime to leverage the reduced CO2 emission factors during solar power generation, and evaluates decarbonization methods for detached houses using dynamic CO2 emission factors. The dynamic CO2 emission factors in Tokyo were estimated based on hourly data collected from power plants. Using the BEST-H energy and thermal environment simulation tool, this study calculated the CO2 reductions from thermal insulation upgrades, equipment updates, shifting the operation times of appliances, and installing of photovoltaic systems and battery storage. The results showed that even houses upgraded from low to standard insulation levels achieved greater CO2 reductions when combined with equipment updates and operational time shifts, compared to insulation upgrades to high-performance levels alone. Additionally, shifting the operation time of heat-pump water heaters from nighttime to daytime reduced the annual CO2 emissions from water heating by 15%. The use of dynamic CO2 emission factor enabled a quantitative evaluation of the effect of shifting energy use.