This paper proposes a reliable approach-based Harris Hawks Optimizer (HHO) to evaluate the optimal parameters of the Proportional–Integral (PI) controller simulating load frequency control (LFC) incorporated in a multi-interconnected system with renewable energy sources (RESs). During the optimization process, the integral time absolute error (ITAE) of the frequency and tie-line power is handled as the objective function. The HHO is selected due to its ease and requirement of less controlling parameters. Two different interconnected power systems are investigated as test systems to demonstrate the robustness of the proposed controller based HHO by comparing to other optimizers and traditional controller. The considered two systems include, one comprises two interconnected area of thermal and photovoltaic (PV) plants while the other system has four plants of PV, wind turbine (WT), and two thermal plants considering governor dead-band (GDB) and generation rate constraint (GRC). Different cases of load disturbance are studied, and the obtained results via the HHO are compared to Sine Cosine Algorithm (SCA), Multi-verse optimizer (MVO), Antlion Optimizer (ALO), and Grey wolf optimizer (GWO) as well as traditional controller. Moreover, sensitivity analysis is performed by changing the system parameters in a range of ± 10% and the performance of the proposed HHO-LFC is evaluated. The obtained results confirmed the reliability and superiority of the proposed approach based HHO in designing LFC for the considered systems.