This study investigated the impact of declining electricity prices on the profitability and optimal sizing of self-consumption photovoltaic (PV) systems in agro-industries with different consumption patterns. A computational approach was employed to analyze over half a million scenarios, considering variations in electricity prices (0.05 EUR/kWh to 0.30 EUR/kWh), consumption seasonality (uniform vs. seasonal), geographic location, solar panel tilt, and installed peak power. The analysis replicated real industrial electricity billing structures using a validated energy balance model that integrates quarter-hourly demand data, PV generation simulations, and economic assessments. The results demonstrate that falling electricity prices significantly reduce the economic benefits of PV investments. Many systems installed in previous years may now be oversized, leading to lower savings or even increased total costs in highly seasonal industries. Under current low-price conditions, new PV installations face limited profitability, with extended payback periods ranging from 7 to 11 years for optimally sized systems. In industries with uniform energy demands, the cost savings peaked at 19%, while seasonal industries achieved only 4% at best. Although surplus energy injection into the grid can improve financial returns, it does not fully offset the profitability decline. These findings highlight the need for adaptive PV sizing strategies and regulatory frameworks, such as dynamic tariff structures and incentive mechanisms, to sustain PV investment viability in agro-industrial contexts with fluctuating energy prices. Unlike previous research, which often assumes stable or rising energy prices, this study uniquely captures the financial risks posed by sudden price drops, particularly for industries with high seasonal consumption variability.