power refers to the volume you project. AT a minimum, be sure that your entire audience can easily hear you without straining.
-turning your voice volume up or down adds interest. use both variations when they match the emotion you want to convey. for example, speaking loud might be used to convey excitement. speaking soft might convey sadness.
-use a microphone to amplify your voice in large rooms.
-eliminate outside noises, if you can. if you can't, consider moving the audience closer yo you, or moving into the audience.