Kodi is actually like copying a file using samba, but in far lower speeds than your server is capable. This should have none to minimum impact to your CPU usage, thus resulting to almost no additional power usage.
Streaming with Plex is essentially the same thing.
Transcoding with Plex, however, to a device like an ipad, as per plex FAQ, requires about 2000 passmarks per 1080p stream.
Consequently, every plex transcoding session will require a portion of your CPU's horsepower, so depending on the number of transcoding jobs, your CPU usage may vary from 20-25% up to 100% usage, resulting of course to higher power consumption.
