The Chinese government has set long-term carbon neutrality and renewable energy (RE) development goals for the power sector. Despite a precipitous decline in the costs of RE technologies, the external costs of renewable intermittency and the massive investments in new RE capacities needed to achieve carbon neutrality would drive electricity costs up. Here, we develop a power system expansion model to comprehensively evaluate changes in the costs of electricity supply over a 30-year transition to carbon neutrality, including the effects on the costs of various factors. RE supply curves across China, operating security constraints, and characteristics of various generation units are modelled in detail to assess the cost variations as accurately as possible. According to our results, approximately 5.5 TW of wind and solar photovoltaic capacities would be required to achieve carbon neutrality in the power system by 2050. The electricity supply costs would increase 21.5%, or 10.4 CNY¢/kWh (1.51 USD¢/kWh), which equates to a rise in the effective marginal price of carbon to 1082.6 CNY/t (157.0 USD/t) in 2050. The major cost increments would result from substantial investments in the new RE capacities, flexible generation resources, and network expansion that would be required to achieve carbon neutrality.