<p>Pro-natalist policies introduced in response to declining fertility rates may have unintended consequences for household energy consumption and environmental sustainability. This study examines how fertility policy relaxation affects household energy use, taking China’s universal two-child policy as a representative reform. Using micro-level household survey data and a difference-in-differences approach, we find that the policy significantly increases per capita household energy consumption, contrary to the conventional expectation that population growth improves energy efficiency through economies of scale. Additional analysis reveals that both behavioral and structural factors drive this observed increase in energy consumption. Behaviorally, households spend more time at home due to childcare responsibilities. Structurally, the policy has shifted the distribution of births toward urban and higher-income families, which typically exhibit higher energy consumption. Furthermore, the policy’s impact is more pronounced in electricity use, particularly in regions with centralized heating and among households with lower environmental awareness. These findings offer new insights for low-fertility countries seeking to design population policies that align with sustainable energy and climate objectives.</p>