The difference between a LaunchDaemon and a LaunchAgent is that the agent will run as any user that logs in through loginwindow, while the agent can launch even before that as the root (or another specified) user.

1. Create a plist like this
   

   <?xml version="1.0" encoding="UTF-8"?>
   <!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
   <plist version="1.0">
     <dict>
       <key>KeepAlive</key>
       <false/>
       <key>Label</key>
       <string>net.wgratz.cpunap</string>
       <key>ProgramArguments</key>
       <array>
         <string>hwprefs</string>
         <string>cpu_nap=1</string>
       </array>
       <key>RunAtLoad</key>
       <true/>
       <key>KeepAlive</key>
       <true/>
       <key>LaunchOnlyOnce</key>
       <true/>
     </dict>
   </plist>

2. Move it to /Library/LaunchDaemons
3. Load it by executing sudo launchctl load -w /Library/LaunchAgents/net.wgratz.cpunap.plist
4. Start it by executing sudo launchctl start net.wgratz.cpunap

• launchd.info
• Medium.com - Adding Startup Scripts to Launch Daemon on Mac OS X Sierra 10.12.6