This work attempts to investigate the melting behavioral pattern and heat transfer performance of phase change materials (PCM) in an oriented rectangular geometry filled with paraffin wax (P-52). The geometry mimics a solar thermal energy storage unit, which can store thermal energy during the daytime. The top wall of the unit is heated with a constant heat flux (600 w/m2) and all other walls are insulated. The total area of the heated surface is kept constant so that the volume of the PCM remains the same for the different cases. The evolved transport equations are solved numerically using the finite volume-based solver and for validation of the numerical model, the melting behavior is compared with the in-house experimental results for the 0º inclination position of the geometry and the comparison displays an outstanding matching among the two outcomes. Then the validated numerical model is utilized to extensively investigate the impact of various inclination angles of the geometry (ϕ = 0º, 30º, and 60º). The outcomes show that the melting performance of the PCM is conduction-dominated initially and contributes to the major portion of the heat transfer. With the progress in time, the buoyancy effect overcomes the viscous force and natural convection takes control of the heat transfer. Results also demonstrate that the initial liquid fraction for a ϕ = 60º inclination is greater than ϕ = 0º or 30º inclination. Furthermore, the case with ϕ = 0º takes less time to melt, and 60º inclination takes the longest time. It implies that, as the inclination rises from 0º to 60º, total melting time increases.