Think of the power supplies as battery chargers. The 13" MBA has a 54Wh battery, and Apple claims it get 12 hours of "wireless web" use. The 13" MBP has a 72Wh

Think of the power supplies as battery chargers. The 13" MBA has a 54Wh battery, and Apple claims it get 12 hours of "wireless web" use. The 13" MBP has a 72Wh battery and claims 9 hours of "wireless web" or "iTunes movie playback". In practice, people find the Apple claims fairly reasonable. While that does not prove how precise or accurate their projections are, I don't think they are absurd. Software-based monitors of power consumption on MacBooks show variation between low single digits and low teens under light use, and shoot up dramatically if you are doing something intensive.