This study investigates the effects of convective hydrothermal pretreatment (CHTP) compared to microwave pretreatment (MWP) on the anaerobic digestion of hybrid Napier grass for biomethane production. For rapid estimation of methane yield (YCH4), enzymatic hydrolyzability (EH), whose test lasts only 2 days was used as a surrogate parameter instead of the biochemical methane potential (BMP) assay that normally takes 45−60 days. The relationship between EH and BMP was successfully modeled with satisfactory accuracy (R2 = 0.9810). From CHTP results, quadratic regression characterised by p < 0.0001 and R2 = 0.8364 shows that YCH4 increase was clearly sensitive to detention time at all CHTP temperatures. The maximal YCH4 achieved of 301.5 ± 3.0 mL CH4/gVSadd was 53.2% higher than the control. Then, MWP was employed at various power levels and microwave exposure times. Changes in lignocellulosic structure by Fourier-transform infrared spectroscopy (FTIR) and energy balance demonstrate that MWP caused more damage to plant cells, which proved more effective than CHTP. In the best conditions, approximately 50% of energy was needed for MWP to achieve the equivalent improvement in YCH4. However, CHTP is a more suitable option since waste heat, i.e., from a biogas CHP (combined heat and power) unit, could be used, as opposed to the electricity required for MWP.