If you have an HDMI port marked 1.4 MHL you don't need a power cable, your TV will power it. I have two power HDMI ports on the back of my Sharp LC-60LE757 60-inch Aquos Quattron 1080p 240Hz Smart LED 3D HDTV.
"Chromecast is powered from a combination HDMI 1.4+ & MHL port, a USB port, or a power adapter, and employs an embedded version of Chrome OS. "
If you have an HDMI port marked 1.4 MHL you don't need a power cable, your TV will power it. I have two power HDMI ports on the back of my Sharp LC-60LE757 60-inch Aquos Quattron 1080p 240Hz Smart LED 3D HDTV.
"Chromecast is powered from a combination HDMI 1.4+ & MHL port, a USB port, or a power adapter, and employs an embedded version of Chrome OS. "