Solid-state sodium ion batteries, a relatively safe and potentially cost-effective technology for electrical grid-scale energy storage, has attracted significant scientific attention in recent times. However, identifying solid electrolytes with high ionic conductivity at room temperature is a requirement in order to commercialize solid state sodium ion batteries. Amorphous sulfide glasses are considered as promising candidates with relatively high ionic conductivity. However, the properties of these electrolytes are highly sensitive to the chemical composition. We present a multiscale approach, which employs atomistic simulations coupled with mesoscale modeling, to analyze and evaluate transport properties of solid glassy electrolytes. As a model system, we studied sodium sulfide – silicon sulfide [xNa₂S – (1-x)SiS₂] based electrolytes with varying compositions. We determined the ionic conductivities and activation energies for sodium ion hops using ab initio and classical molecular dynamics (MD) approach. These simulations provide fundamental insights into ion conduction mechanisms as well as help to correlate ionic conductivity with glass structure and composition. We further developed a Monte Carlo model to scale up our calculations to a much larger length-scales that are inaccessible by molecular dynamics simulations. Crucial inputs such as Na⁺ ions distribution and migration energies obtained from classical and ab initio MD served as key inputs to the Monte Carlo model. This model accurately predicts the ionic conductivity of the glasses in agreement with experimental values. Overall, the multiscale model shows excellent promise in identifying optimal glassy electrolytes for sodium ion batteries.