What I don't get about Back to the Future Part II is how was Old Biff able to come back to 2015 from 1955 after he gave his 1955 self the Sports Almanac.
If you recall from Part I, the chance lightning strike over the clock tower was absolutely essential to getting Marty back to 1985 from 1955 because there are not enough megawatts in 1955 to get the flux capacitor to operate (or something like that).
So how was Old Biff able to come back? He couldn't have been able to set up that wire from the clock tower the way Doc did. There's no indication he even knew about wattage limitations in 1955.