LENR reactors are considered as units that operate at a temperature above ambient and need an excitation provided by a supply powered by electricity. Different modes of operation are described following the characteristics of the heat and energy balance. LENR reactors may be characterized by different parameters, Coefficient of Performance (COP) or Energy Amplification factor (A). The thermal insulation plays an important role. LENR reactors that require external heating in small units may become self-sustained for large sizes. The production of electricity involves the coupling with a thermal machine. The system is able to export power if the COP and the temperature are high enough. (C) 2016 ISCMNS. All rights reserved.