The increasing integration of photovoltaic (PV) power systems poses challenges for nighttime voltage regulation, as long high-voltage (HV) and ultra-high-voltage (UHV) underground cables generate capacitive reactive power that elevates grid voltage. Conventional compensators, based on passive inductors and capacitors, are bulky, costly, and inflexible, making them unsuitable for existing substations. This study proposes an optimization-based strategy that leverages the existing inverter infrastructure of PV plants to provide nighttime reactive power compensation without additional hardware. A genetic algorithm (GA) determines the optimal number and spatial deployment of inverters to minimize line losses. Field validation at a 120 MW PV plant with 1,292 inverters shows that the strategy reduces reverse reactive power from 0.84 MVAr to 0.00214 MVAr and line losses from 1.8235 kW to 0.386 kW using only 55 inverters, achieving near-zero additional capital expenditure (CAPEX). The method enhances voltage stability and system efficiency while reducing investment and maintenance costs.