Here we propose a new measurement approach to quantify the band edge movement of 2D semiconductor electrodes. Our optical approach involves the following steps: (1) measure potential-dependent absorbance spectra; (2) correlate carrier concentration (𝑛) by matching the phenomenology of a manybody model to its predicted spectra; (3) interpolate the potential-dependent band gap energy based on first-principlesbased effective-mass model calculations of the 2D semiconductor ; and (4) construct an energy level diagram that quantifies BGR as a function of applied potential. The novel aspect of the approach is how it quantifies 𝑛 via an easily accessible absorbance measurement. The Discussion section describes critical assumptions in steps 2-4 that currently limit the method to the model ML-MoS2 system, and pinpoints needed developments in experiment and theory to validate the approach for a wide range of 2D materials.