This study develops a load-adjustment model for a coal-fired power plant using a PID control approach, where proportional, integral, and derivative components are combined to represent unit inertia and delay, and grid dispatch commands are introduced as step inputs to enable dynamic load regulation. Building on this, a joint operating framework for a photovoltaic (PV)–coal hybrid system is proposed, using the coal unit as a flexible regulating source to improve renewable-energy utilization. Environmental benefit indicators are employed to quantify carbon-emission reduction performance, and a decision model is introduced to support the replacement of aging generation units. Empirical results suggest that substituting a larger share of coal capacity with PV can yield higher overall corporate value. Across three environmental benefit indicators, the gap between ideal and negative-ideal values for the evaluated city remains within 0.05, indicating sensitivity of the hybrid load-regulation strategy. Under the proposed method, increasing the PV installation share by 13.92% leads to improved environmental benefit outcomes.