We present the principle and implementation of a new type of fast response evaporative calorimeter designed to work at cryogenic temperatures and above-ambient pressures. It is capable of measuring input energy from an electric pulse and the thermal output energy by measuring the evaporation of liquid nitrogen through a mass flow meter. This system may be used to measure either the steady heat output from the system submersed under the cryogen or the heat output that results from a fast square-wave profile electrical pulse of duration from 10 μs or longer. The energy output of metal capillary-wire composite systems has been measured calorimetrically. A four-wire measurement was used to monitor the input electric energy with an uncertainty less than 5% for a typical pulse. Mass flow meters and pressure regulation systems were used to monitor the rate of evaporation of liquid nitrogen with a typical precision of 2 std.-ml/min. For a typical pulse, the integrated mass flow of nitrogen could be determined with an uncertainty less than 3%. The pressure controllers and ballast compliance volumes allow the system to return to a steady state of mass flow in less than 2 min following an electric pulse. The system is capable of housing and measuring four separate wire-capillary systems in a single Dewar. On average, a calibration resulted in 3.9 std. ml evaporated per joule of input energy. This corresponds to a 97% efficiency for this calorimeter.