Luminescent solar concentrators (LSCs) use down-converting luminophores embedded in a waveguide to absorb sunlight and deliver high irradiance, narrowband output light for driving photovoltaic (PV) and other solar energy conversion devices. Achieving a technologically useful level of optical gain requires bright, broadly absorbing, large-Stokes-shift luminophores incorporated into low-loss waveguides, a combination that has long posed a challenge to the development of practical LSCs. We will present a combined theoretical and experimental study of LSCs based on giant effective Stokes shift CuInS2 nanocrystal (NC) phosphors, which demonstrate best-in-class performance as large-area, semitransparent concentrators suitable for use in energy-harvesting window layers and other applications. A new analytical optical model will be discussed that allows accurate determination of each major loss mechanism without the need for time-consuming Monte Carlo ray-tracing simulations. We will show that nanocrystal clustering in polymer composite waveguides leads to light scattering losses that ultimately limit efficiency at large geometric gain. By optimizing NC concentration, we demonstrate optical power efficiencies up to 5.7% under AM1.5 illumination for devices having a geometric gain 6.7×, with limiting achievable efficiencies predicted to exceed 10%.