To address the issues of DC bus voltage drop, source-load power imbalance, and battery overcharge/overdischarge in off-grid PV-storage-charging systems under photovoltaic power fluctuations and load transients, a control strategy combining energy storage-side control and system energy management is proposed. First, linear active disturbance rejection control (LADRC) is introduced into the DC bus voltage regulation of the Buck-Boost converter. A linear extended state observer is employed to estimate and compensate for the total system disturbance, thereby improving the dynamic response delay and poor parameter adaptability of traditional PI control. Second, a virtual capacitor model is introduced, and a three-loop competitive control strategy consisting of bus voltage regulation, battery constant-voltage charging, and battery constant-voltage discharging is designed to achieve DC bus power balance and battery overcharge/overdischarge protection. Furthermore, by integrating the MPPT/derating switching control of the PV Boost converter and the EV-side load derating control, a load-priority energy management strategy is established. Experimental results demonstrate that the proposed method can reduce the DC bus voltage drop during load switching, achieve coordinated operation among PV generation, energy storage, and loads, and improve the stability and reliability of the off-grid PV-storage-charging system.