The accumulation of impurities in a controlled thermonuclear reactor makes steady-state operation unlikely. The energy output during the burn phase will depend on the ion temperatures and densities. A dynamic model of the burn cycle of a tokamak is used to investigate the ion densities and temperatures as a function of time. The total energy output per cycle is investigated as a function of the ion feed rates, plasma current, and the divertor efficiency. The point-kinetics model of the plasma incorporates ion and energy balance equations and explicitly accounts for the impurity ion buildup. The D-D, D-T, and D-3He reactions are all considered in this model. The energy carried off by the neutrons in the D-D and D-T reactions is lost from the plasma. Impurities enter the plasma as a result of wall interactions with escaping ions and neutrons. The trapped-ion mode is used for calculating the confinement times. An equilibrium state vector was obtained using currently projected operating parameters. The total energy density for a burn cycle was found to be a monotonically increasing function of the source rates and the plasma current. The energy density was not substantially increased until the divertor efficiency was greater than ∼60% when the other parameters were held constant.