How did Biff return to 2015 from 1955 without a lightning strike?
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.
Biff didn't actually have to use the same plutonium-driven Delorean that Marty had to use in the 1st part. He used the Delorean that Doc had modified based on 2015 technology, which didn't need the high energy of plutonium to operate anymore rather than just garbage (nor did it need any roads either). We see this in the beginning of the film, where Doc just puts some garbage into the tank and is done with it.
In the same vein it's also likely that it was a lot more fuel efficient too, so Biff didn't actually have to go scanvenging dust bins either. The Delorean simply had enough fuel to go from 2015 to 1955 and back again. (Alternatively, he was aware of how the Mr. Fusion Home Energy Reactor worked from his time, too, and that you just need to put some trash into it anyway.)