Flash-sintering is invariably accompanied by a highly non-linear rise in the specimen's conductivity. Thus the specimen temperature rises above the furnace temperature. It is shown that flash-sintering is a transient phenomenon, where the power dissipation rises quickly at first, but then declines towards a steady state, as the power supply switches from voltage to current control. The area under the power spike, which is equal to the Joules expended in the sample during the transient, is absorbed by the heat capacity of the specimen. Therefore, the specimen temperature rises gradually towards this steady state through the transient. Whereas the power spike can exceed a peak value of 1000mWmm−3, the dissipation during the current controlled regime is in the 100–400mWmm−3 range. The extrapolation of sintering time from a few hours, as in conventional sintering, to a few seconds, using the activation energy for diffusion, predicts sample temperatures that are far in excess of the measured specimen temperature during flash sintering.