A methodology is presented for the determination of the subfilter energy in large-eddy simulations (LES). The model assumes universal-equilibrium turbulence in the subgrid scales and is based on the integration of semi-empirical functions for energy spectra of homogeneous isotropic turbulence. The model depends on the Reynolds number, the integral length scale and its ratio to the flow containment, as well as the LES resolution. On the one hand, the formalism is suitable for an a priori estimation of the subgrid-energy level, which is useful for preparatory LES set-up. Charts are drawn which connect the Reynolds number and the LES resolution to the expected level of subgrid energy. Furthermore, the difference is evaluated between the present model and a model based on the integration of an idealized k −5/3 spectrum, which is related to the high-Reynolds-number asymptotic behaviour of the energy spectrum. On the other hand, the presented methodology can be used as a model for the subgrid energy in an actual LES, as part of a subgrid-scale stress model. The formulation of the model, based on equations which directly use resolved-scale LES properties, is briefly outlined.