Nations worldwide are committing to net-zero emission targets, accelerating the integration of distributed energy resources (DERs) into power grids. However, improper planning of DER deployment may result in adverse impacts, including increased losses and voltage violations. This paper presents an optimal placement and sizing framework for solar PV and battery energy storage systems (BESS) in distribution systems (DS), comparing short-term (1 year) and long-term (10 years) planning strategies. The problem formulation simultaneously minimizes three objective functions: total power loss, total voltage deviation, and total system cost. The optimization is solved using Multi-Objective Particle Swarm Optimization (MOPSO), and the Technique for Order of Preference by Similarity to Ideal Solution (TOPSIS) is applied for decision-making among Pareto-optimal solutions. The proposed model is validated on the IEEE 33-bus system under three cases: base case, short-term planning (1 year), and long-term planning (10 years). The selected cases evaluate how different planning horizons, accounting for overall load growth, influence the optimal placement and sizing outcomes. Results show that long-term planning reduces power loss by 50.89%, voltage deviation by 40.81%, and cost by 26.48% compared to the base case. Additionally, renewable penetration improves by 14.3% relative to short-term planning, confirming the benefits of coordinated PV–BESS integration for sustainable grids.