The effect of using external vertical fins in Phase Change Materials (PCM) modules to improve the natural convection coefficient in water is studied in this paper. The use of PCM in a water tank working with a solar system is able to store a lot of energy, but it is necessary to transfer this energy to the water during demand. Optimization of the natural convection to the water is of great importance for the heat transfer. External fins increase the heat transfer surface and the heat transfer coefficient changes. Experimental work was carried out to determine natural convection heat transfer coefficients for PCM modules with two different external vertical fin geometries. Results were presented as a temperature variation over time of the PCM and the water, and the heat transfer coefficient as a function of the temperature difference for different fins. These results were used to validate a simulation code to determine the behaviour of the system using finned PCM modules. The results prove the technical potential of external fins for heat storage systems using PCM. The experimental results were used to determine different correlations for Nusselt number as a function of different dimensionless numbers.