<p>Plant microbial fuel cell (PMFC) development, still in early stages, faces challenges like drought susceptibility and low power output, hindering practical applications. Biochar, known for enhancing soil water retention and microbial activity, lacks research on its particle size effects on soil–plant-microbe-atmosphere interactions. This study investigates how biochar particle size influences the relationship between bioelectricity generation and soil water characteristics. The experimental design involved a series of PMFCs with biochar of varying particle sizes: (i) &lt; 0.3 mm, (ii) 0.3–1.18 mm, and (iii) &gt; 1.18 mm. The results show that smaller biochar particles improved soil water retention by 25% compared to the control soil. Water retention also diminished over time, particularly with larger particles. The study observed slight fluctuations in bioelectric current over time, with a significant reduction after the air-entry value (AEV). Additionally, the findings indicate a direct relationship between biochar size and the electric current in PMFCs. Bioelectricity production increased by two to three times with substantial biochar particles, achieving a maximum power density of 13.8 µW/cm². These findings underscore the potential of biochar to enhance PMFC performance, contributing to sustainable energy systems.</p> Graphical Abstract <p></p>