Energy sector of developing nations is faced with myriads of problems ranging from insufficient generation to poor grid infrastructure. Introducing variable renewable energy sources (VREs) has been identified as a way to meeting the rapidly increasing energy demand of these nations. Intermittency is one of the major shortfalls of VREs which has a direct influence on the voltage stability and the overall power system security. Hence, a cost-effective investment plan which considers the effect of PV output intermittency on the overall grid security at different level of PV power penetration is essential. In this study, a review of the approach for large-scale solar PV injection into weak grids and its effects on voltage stability is presented. A multi-objective optimal technoeconomic assessment, for three different levels of PV power penetration on the Nigerian 28 bus grid, was investigated using the particle swarm optimization algorithm. The net present cost per hour of system components and the loss of supply probability are the two objectives considered. Cost of PV intermittency and grid enhancement is introduced as a function of the average power injection by the PV system using the site's capacity factor approach. PV penetration level of about 35% of the total load demand is considered safe for the investigated Nigerian power system. The designed approach can be adequately adopted for power systems of other developing nations.