Cyber-physical systems often include sensor devices in their structure. These devices may require firmware updates once deployed and these updates must be energy efficient for battery powered, physically inaccessible sensors. The problem of energy saving reprogramming can be split to four tasks– making old and new firmware versions more similar, generating small delta files using differencing algorithms, propagating delta files and applying updates at the end devices. This paper describes existing approaches dealing with this problem, analyzes their power consumption and introduces new optimizations for differencing algorithms. A new approach is presented that requires no external flash memory, device reboot or complex update agent at the sensor device.