<p>Biomethane (CH₄) is a renewable energy source that can be generated from organic waste through environmentally sustainable methods utilising anaerobic digestion (AD) technology. This study uses the automatic methane potential test system (AMPTS-III) to examine the biogas potential of Napier grass (NG, <i>Pennisetum purpureum</i>) harvested at various growth stages (1, 2 and 3 months). The range of NG Total solids (TS) and Volatile solids (VS) during the various harvest periods was 0.9406 to 0.9540&#xa0;g g^− 1 of substrate/inoculum and 0.9235 to 0.9848&#xa0;g g^− 1 of TS, respectively. Profiles of biogas production revealed that the 2-month-old NG achieved the highest cumulative volume up to 3083.20 ± 160.30 NmL. The biomethane potential (BMP) of NG were 204.30 ± 25.26 NmL g^− 1 VS, 412.70 ± 45.94&#xa0;N mL g^− 1 VS and 354.43 ± 31.25 NmL g^− 1 VS for 1, 2, 3-month period, respectively. The timing of the harvest significantly influenced CH₄ yield, with the 2-month harvest exhibiting the highest levels of digestibility and organic content. The 2-month-old NG demonstrated a 50.49% increase in BMP as compared to the 1-month samples and a 14.11% increase over the 3-month samples, highlighting the importance of harvest timing in optimising energy recovery for sustainable biogas production.</p> Graphical abstract <p></p>