I use the Ador1 Automatic chicken door which runs off of a 6v lantern battery. I bought the higher quality lantern battery which was about $7.
It is estimated to last a year, and has a warning when the battery is low. I plan to keep an extra always on hand.
The battery is connected and stored in an enclosed area on the door.
I have been very pleased with mine.
(others use the [COLOR=660099]
Pullet-Shut Automatic Chicken Door[/COLOR] which runs off of a 12v battery, or as a plug in. Both batteries could be connected to a small solar panel).