Example of usage Daemon

How can I monitor the CPU energy spent by the 123 process?

You have to configure the daemon before it launching as following (via configuration files):

powerapi.daemon.load = [
  {
    power-modules = ["procfs-cpu-simple"],
    monitors = [
      {
        all = false,
        pids = [123],
        apps = [],
        containers = [],
        frequency = 1000ms,
        agg = "SUM",
        output = "console"
      }
    ]
  }
]

After this, you can start the daemon and thus the monitoring of the 123 process, as following:

sudo ./bin/powerapid start