The exact extract asks which feature can reduce O&M costs. The correct answer is Automatic SOC calibration. SOC means state of charge, and calibration is important in battery energy storage systems because inaccurate SOC estimation causes inefficient charge/discharge management, maintenance intervention, and possible capacity underutilization. Huawei's Smart String ESS positioning specifically identifies automatic SOC calibration as an O&M cost-reduction feature. Huawei states that automatic SOC calibration minimizes manual interventions and reduces operational costs, while other Huawei ESS material states that automatic SOC calibration eliminates the need for onsite O&M by experts and supports higher comprehensive energy efficiency. The other options do not directly match the ''reduce O&M costs'' wording. On/off-grid switchover is mainly a continuity and power-supply resilience feature. TOU is an energy-cost optimization strategy. Top explosion venting is a safety-pressure-relief design. Therefore, the feature directly associated with reduced O&M workload and lower maintenance cost is automatic SOC calibration. Reference: Huawei Smart String Grid Forming ESS Solution and FusionSolar Smart String ESS material.