The article deals with the design of secure communication for terminal devices in the energy sector via networks with limited resources. The key exchange and encryption method were selected in terms of security and energy consumption. Energy consumption needs to be solved for security methods to be implemented into low-power microcontrollers. An analysis of Public Key Cryptography for implementation into these microcontrollers was realized too. Finally, a library for basic and modular arithmetic operations was realized, and memory size and computational requirements were measured. The designed concept of secure communication can be used for future integration into Smart Grid networks.