So I've had a chromecast for a while, powered off the TV USB. Was around at my brothers place the other day and saw he had his tv setup to turn on and off with google home. Tried when I got home and a not go on turning TV on, had a bit of an Ah-ha moment when I realised the chromecast needs an always on power which it doesn't have when the TV is off.
So I've dug out a USB power adaptor and it works fine, turning TV on and off.
Next issue, turning tv off on a timer.
saying "hey google, turn living room tv off in 5 minutes" gets the acknowledgment from google saying the tv will be turned off at the time 5 minutes from now, but it never actually happens, TV stays on.
Samsung TV with HDMI-CEC turned on and chromecast plugged in to the right HDMI port.