Dielectric materials with high power and energy densities are desirable for potential applications in advanced pulsed capacitors. Computational material designs based on first-principles calculations provide a bottom-up method to design novel materials. Here, we present a first-principles effective Hamiltonian simulation of perovskite ferroelectrics, Ba1-xSrxTiO3, for energy storage applications. The effects of different chemical compositions, temperatures, and external electric fields on the ferroelectric hysteresis and energy storage density of Ba1-xSrxTiO3 were investigated. The Curie temperature was tuned from 400 to 100 K by doping Sr in the BaTiO3 lattice. At a constant temperature, the ferroelectric hysteresis became slimmer as the Sr content increased, and the energy storage efficiency increased. For the same chemical composition, the energy storage density increased as the temperature increased. For the composition x = 0.4, a discharged energy density of similar to 2.8 J/cm(3) with a 95% efficiency was obtained in an external electric field of 350 kV/cm, and a discharged energy density of 30 J/cm(3) with a 92% efficiency was obtained in an external electric field of 2750 kV/cm. The energy storage property predictions and new material designs have potential to create experimental and industrial products with higher energy storage densities.