In this article, we suggest to optimize packet length for energy harvesting systems. The network contains three nodes A, B, and C. Node B harvests energy from electromagnetic wave transmitted by A to communicate with node C. Packet length optimization allows to increase data rates. Two adaptation protocols are studied: In the first one, the adaptation of packet and Modulation and Coding Scheme (MCS) is made with respect to Average Signal to Noise Ratio (ASNR). In the second protocol, packet length and MCS are adapted to Instantaneous SNR (ISNR). Results show that packet length optimization allow 1-2 dB gain with respected to fixed packet length.