Power naps are useful for those who don't get enough sleep at night. I find a 20 minute meditation also has the effect of refreshing the mind and boosting energy during the day. The Transcendental Meditation technique has been found to produce a state of rest that is even deeper than sleep , and allows the body to dissolve deep rooted stress that sleep alone can't remove.
Although meditation is not a replacement for sleep at night, it could be an alternative to napping. Neuroscientist have found that during meditation when a person transcends thought and experiences inner silent wakefulness, brain wave patterns show synchronous alpha throughout the entire brain. This particular meditative state is associated with coherent, orderly brain function, which is necessary for peak mental performance. Sleep on the other hand, produces mostly theta and delta waves, which scientists associate with lack of alertness and dullness. Naps are great for catching up on sleep, and meditation is great for developing higher states of consciousness. Reference: Levels of Rest during Transcendental Meditation: American Psychologist 42: 879–881, 1987.