Aluminium-based battery technologies have been widely regarded as one of the most attractive options to drastically improve, and possibly replace, existing battery systems—mainly due to the possibility of achieving very high energy density with low cost. Many reports have demonstrated primary or rechargeable Al-based battery chemistries in both aqueous and non-aqueous electrolytes. However, the practical realization of these battery chemistries has been difficult over a long period of time (170 years). In fact, no Al-based battery has been shown with the required stability or touted energy density. Typically, the performance of Al-based batteries is overstated in the literature due to imprecise considerations that do not fairly evaluate practically achievable energy densities. Here we provide accurate calculations of the practically achievable cell-level capacity and energy density for Al-based cells (focusing on recent literature showing ‘high’ performance) and use the results to critically assess their future deployment.