This report describes the design and operation of a new diagnostic instrument for measuring the conversion of laser energy into plasma ions and soft x rays during ICF laser‐target interaction experiments. The instrument is operated as a differential calorimeter, and uses low mass thermistor flakes as both energy transducers and calibration sources. The calorimeter provides its own running energy calibration, in situ, by inducing electrical self‐heating of the thermistor elements. The instrument can be calibrated over a range of energies, either automatically or by manual selection. Calibration and operating principles are discussed, as well as physical and electronic design considerations, measurement and calibration data, and error sources.