The main goal of this work was to study the energy characteristics of deep ultraviolet light-emitting diodes and to establish the physical reasons for the limiting of output optical power and conversion efficiency of such devices. The voltage-current, light-current and spectral characteristics of the AlGaN multiquantum wells flip-chip light-emitting diodes emitting at a wavelength of 270 nm were experimentally studied in a wide range of operating current densities of 0.01–2.5 kA/cm2 and ambient temperatures of 200–350 K. Using the ABC-model, it was found that at a relatively high internal quantum efficiency of radiation of ~70–90% and a quite acceptable value of series resistance of ~1 Ω.  The main factor (key obstacle) limiting the energy possibilities of devices is low light extraction efficiency. The latter is due to the strong absorption of the generated light in the chip volume and on the contacts, as well as total internal reflection on the AlGaN/sapphire and sapphire/air interfaces.