Public key cryptography is widely considered as the best building block for key exchange; different public key algorithms are standardized and used in many applications. Among them, ECC (Elliptic Curve Cryptography) is considered the best solution in terms of security, computational requirements and storage need for secret and public keys. Energy consumption is among the main constraints to be considered in wireless sensor networks. In the case of sensor networks, the typical approaches of minimizing latency via a complete hardware coprocessor or reducing area overhead via an efficient implementation of finite field operations might not provide the best solution. In this paper a coprocessor for minimizing both additional resources and power consumption is presented for elliptic curve over binary extension fields, The costs and performances of such new coprocessors are compared with known results, showing that space exists for the reduction of energy consumption without degrading the other performance figures.