This utility outputs a string of system stats. The output can be piped through to xsetroot and/or used in .tmux.conf.

sudo /usr/local/bin/status | while read LINE; do xsetroot -name "$LINE"; done

#/etc/sudoers.d/local
%users ALL= NOPASSWD: /usr/local/bin/status

In order to get power (RAPL) reports you need to run the utility with elevated privileges (ie, root). The status_writer script will output reports both to xsetroot, as well as a swap file for tmux or another program to read. The script can be invoked at system boot time by issuing the command:

setsid /usr/local/bin/status_writer >/dev/null 2>&1 < /dev/null