Carrier confinement in III-N semiconductor heterostructures is often studied in the framework of modified continuum-based models utilizing a single-band effective mass approximation. Alloy fluctuations that commonly occur in ternary or quaternary alloys, however, are of a clearly atomistic nature. We have designed a theoretical framework that establishes a connection between atomistic tight-binding theory and continuum-based electronic structure models, here a single-band effective mass approximation, thus facilitating a comparison between atomistic and continuum models for the electronic structure of (In,Ga)N quantum wells. In our approach, the effective masses are the only adjustable parameters since the confinement energy landscape is directly obtained from tight-binding theory. We find that the electronic structure calculated within effective mass approximation and the tight-binding model differ noticeably. However, at least in terms of energy eigenvalues, an improved agreement between the two methods can be achieved by adjusting the band offsets in the continuum model, thus providing a recipe for constructing a modified continuum model that gives a reasonable approximation of the tight-binding single-particle energies. Carrier localization characteristics for energetically low lying, strongly localized states differ, however, significantly from those obtained using the tight-binding model. For energetically higher lying, more delocalized states, good agreement is observed.