Increasing the efficiency of solar thermal collectors is extremely important as they are essential for many applications, ranging from the UV up to the NIR spectral range, from water heating systems up to micro-electromechanical systems. In this work, a plasmonic multilayer nanocomposite thin-film system that efficiently absorbs solar radiation across an extended spectral range was simulated and experimentally tested. Novel to our approach, copper nanoparticles in an alumina matrix were chosen as the nanocomposite material. Compared to other plasmonic materials such as gold or silver, copper is more abundant and economic. The alumina matrix provides high thermal stability, good optical properties, and corrosion protection. Using a multiscale-modeling approach, we inspect on computational grounds the effect of the nanoparticle filling factor, the angle of incidence, and the thin-film thicknesses on the absorber performance. We found that an optimally designed device absorbs up to 90% light energy from 200 nm to 1800 nm. To validate the simulations, two promising absorber layouts are experimentally realized. Their performance compares very well with simulations.