The exact extract asks which option is not a feature of the residential PV+ESS on-grid scenario. The correct answer is C. Unstable mains or frequent power outages. A residential PV+ESS on-grid scenario is defined by grid connection. The system is connected to the utility grid and normally uses PV generation, energy storage, and grid power together to support household electricity consumption. Therefore, option A is a direct feature: it is a grid-connected residential PV+ESS system. Option B is also correct as a feature because the system is composed of PV and ESS components. Option C describes a different scenario: unstable mains or frequent power outages are usually the reason for backup or off-grid/hybrid operation, where a Backup Box or smart guard is required to switch the inverter between grid-tied and off-grid states. Huawei’s Backup Box documentation positions that device for grid-tied/off-grid control, especially when backup power is needed. Therefore, option C is not an on-grid scenario feature. References: Huawei Residential Smart PV & ESS Solution; Huawei BackupBox Quick Guide.