In electromagnetic calorimetry large-size silicon detectors, employing relatively low-resistivity (and, therefore, inexpensive) material, can be applied. The energy scale is defined by calibrating the detector with single non-showering relativistic particles. In this experiment the response of a large-area detector and its associated, especially developed, electronics to the energy-loss of single relativistic particles was tested. The electronics can be calibrated and can work in showering conditions. The standard deviation of the Gaussian noise contribution, which included the effect of the detector leakage current, capacitance, charge collection, and cabling, was 99.3 ± 5.3 keV. The energy resolution performance of the electronics, versus the equivalent detector capacitance (180 to 2000 pF), was found to be good. A silicon sandwiched calorimeter is expected to have good energy resolution compared to the conventional sandwiched calorimeters.