Plasmonic nanoparticles have been used for a variety of applications, including light trapping in thin film solar cells. Due to costs associated with the most commonly studied plasmonic metals (Au and Ag), other metals such as Al, In, Mg and Ni have recently emerged as alternative, earth-abundant plasmonic metals. Though the plasmonic properties of these metals have been studied individually (particularly Al) there have been few reports on the plasmonic properties of alloys of earth-abundant plasmonic metals. This is of interest because alloying could offer a degree of plasmonic tunability, improved corrosion resistance and drive down costs. In this report we study the light trapping ability of Al–Cu, Al–Au, Al–Ni, Al–In and Al–Mg alloys in an amorphous silicon solar cell. We use a combination of Mie Theory, Transfer Matrix Modelling and Effective Medium Theory to develop a simple numerical solution to this problem, and benchmark it against known theoretical results. Al–In and Al–Mg alloy nanoparticles emerge as the most suitable (of the candidates studied) for this application. Al (75%) In (25%) (by volume fraction) nanoparticles of radius 15 nm are shown to improve the absorption of amorphous silicon by 18%. We provide maps of the plasmonic behaviour of alloy nanoparticles for a range of radii (1-100 nm) and alloy compositions, enabling materials design. Therefore, we hope that this report will spur further interest into the utility of Al-alloy nanoparticles for plasmonic applications. © 2021 Elsevier B.V.