This study evaluates how plantation age structure and alternative replanting strategies affect long-term oil-palm empty fruit bunch (EFB) supply and the techno-economic viability of 3, 5 and 10 MW biomass power plants in Southwest Aceh, Indonesia. A regional pre-feasibility model integrates 2016 plantation age cohorts, owner-specific age-yield curves, a reconciled inventory of 15 active palm oil mills, district throughput constraints, spatial supply screening and a 25-year boiler-steam-turbine cash-flow model for 2027–2051. Four renewal scenarios were assessed using deterministic analysis and 100,000-iteration Monte Carlo simulations. The 2027 net power envelope was 15.80 MW for Southwest Aceh and 10.87 MW for Nagan Raya. Without renewal, the Nagan Raya envelope fell below 5 MW in 2036 and reached zero in 2042. Managed renewal at 4% and accelerated renewal at 6% maintained deterministic fuel coverage for 5 MW, but not for 10 MW. The 5 MW option had a base levelised cost of electricity of USD 123.8 MWh−1, a net present value of USD -6.57 million and the strongest joint probability of all-year feedstock adequacy and positive net present value, at 10.1–11.6% under managed renewal. The model links plantation renewal to annual residue availability, electricity generation and financial performance. The findings identify 5 MW as a concept for field validation while showing that static residue estimates can misrepresent lifetime feasibility when feedstock adequacy is not tested annually.