A concept is considered for storing 10–100 MJ of electrical energy in a relatively lightweight and compact device. Energy is stored as electron kinetic energy confined in the vertical betatron field of a toroidal ring. Electrons are injected into a ring at full voltage but low current and low power. The stored electron energy is released on a fast time scale. Synchrotron radiation, wall fields, and instabilities constrain the energy that can be stored in each ring.