There are 2 types of electric awnings.
One type just extends and has an automatic dumping feature that allows one end of the awning to collapse when there is a rain water build up on the awning. The problem with this type is that you never know which end of the awning is going to drop and allow the water to run off. Many of my friends, with this type, have been soaked when the awning dumps. Also you cannot adjust the height of this awning to compensate for the sun's angle, it just extends straight out.
My preference is the Dometic 9100 electric awning. There are two adjustable arms on it that permit you to slope the awning so rain water drains off the end you choose. Also these arms allow you a 24 inch adjustment on the height of the awning so when the sun is low you can lower the awning to get more shade.
The Dometic is available from Camping World for $982.