Calculate the amount of U-235 in Metric Tonnes consumed in a GWe-yr. Assume the Q-value for a U-235 fission reaction is 200 MeV, and assume an efficiency of 30%. A GWe-yr, or GigaWatt electric year, is the amount of power actually supplied to the grid after efficiency of the power plan have been taken into account. A GWt-yr, or GigaWatt thermal year, is the amount of energy actually generated by fission in the core of the nuclear reactor.