A model equation for the magnetohydrodynamic (MHD) residual energy (difference between the turbulent kinetic and magnetic energies) is proposed with the aid of the two-scale direct-interaction approximation (TSDIA) analysis, a statistical theory for the inhomogeneous turbulence. Combined with the equations for the turbulent MHD energy, its dissipation rate, and the cross helicity (velocity--magnetic-field correlation), a multi-equation turbulence
model is developed. The model is applied to the solar wind. Some important characteristics of solar-wind turbulence obtained from spacecraft observations are discussed from the viewpoint of the inhomogeneity effects in the turbulence model.