Assuming all thermal power plants currently planned for construction and replacement are put into operation, coal-fired and gas-fired power plants need to operate below 56% and 43%, respectively, of their capacity factor in order to achieve Japan’s electric power sector’s voluntary emissions intensity targets for 2030. However, the voluntary framework together with supporting policy measures pursuant to the Energy Conservation Act and the Law Concerning Sophisticated Methods in their current form cannot guarantee attainment of these voluntary targets. Potential mitigation measures such as carbon capture and storage (CCS) and carbon offsets cannot be fully utilised to reduce all CO2 emissions from these plants due to technical and institutional constraints. Once plants go online they also “lock-in” high-level carbon emissions for decades to come. A high degree of caution therefore needs to be exercised, not only in light of the 2030 target but also the 2050 80% reduction target. Following the Paris Agreement, preparation in earnest towards a low-carbon transition in electric power is now required. Further, if the 2030 target becomes difficult to achieve by voluntary action, then the government should adopt stricter policy measures—such as emission caps for the electricity sector and an emission intensity/capacity factor-target policy mix.