Telecommunications tower infrastructure providers in Nigeria operate under weak grid and off-grid conditions, where diesel plays a prominent role as an energy source, affecting energy cost and supply reliability decisive for business continuity. This paper presents a comparative techno-economic and lifecycle assessment of two power portfolios for TowerCo sites, namely diesel-only baselines and diesel–solar–battery hybrid systems. The study is anchored on a five-year project horizon and a stringent 99.98% uptime requirement, translating reliability into explicit constraints on allowable downtime and energy that are not served. Using operational available datasets such as fuel logs, genset telematics, site solar potential, and outage history, we develop site models that capture heterogeneity in grid quality, logistics exposure, and load behaviour. For each model, we evaluate candidate portfolios through a reliability-constrained optimisation of ownership (CAPEX, fuel, Operations and Maintenance, replacements, and operational overhead) while meeting autonomous and operational limits. Environmental performance is measured through the lifecycle greenhouse gas intensity, with transparent treatment of component lifetimes and residual value within the five-year boundary. The results include the least cost sizing ranges for solar and storage by model, expected diesel displacement and runtime reduction, delivered-energy cost, and lifecycle emissions trade-offs under sensitivity to diesel price volatility, outage severity, solar yield uncertainty, and battery degradation. The proposed framework provides a scalable, data-driven basis for prioritising hybridisation investments across TowerCo portfolios and for benchmarking fossil-renewable supply strategies for firm, low-carbon energy delivery in emerging market telecommunications infrastructure. Results present minimum-expense PV and storage sizing strings by model, reflecting substantial diesel reduction, and generator run-hour. The hybrids decrease delivered energy cost (per kWh) and yield attractive 5-year NPV and return on investment versus diesel-only baselines, while providing significant lifecycle emission reductions. Sensitivity analysis confirms the economic viability of hybrids, from diesel price volatility to degree of outage, solar yield uncertainty, and different rates for battery degradation. The model provides a scalable, data-informed approach for TowerCos to prioritise investments in hybridisation throughout their fleets and creates a benchmark for transitioning power infrastructure in new and emerging markets to stable, low-carbon sources of energy.