Boltar ) gurgled happily, sounding much like they
were saying :

shrug
How often do bus engines require full power? Not very.

Agreed but when they do need it they really need it. The steep hills
around mill hill, highgate , hampstead and southgate spring to mind. A
hybrid bus with an exhausted battery going up one of those hills won't
be very popular with other motorists.

Nor would a diesel bus with an empty fuel tank...

I really don't think it's going to be an issue.

A buses diesel fuel probably lasts all day.

And...?

If these hybrid buses are anything like hybrid cars then the battery
would run down after only a few miles without topping up which means its
quite possible the bus could be relying on engine power alone on
difficult routes such as I mentioned if the engine doesn't have the
surplus to charge the battery as well as supplying the motors.

But the engine'll be keeping the batteries brimmed as much as possible, so
it'll arrive at the bottom of the hill with full charge - which will be
perfectly adequate for the ascent. Don't forget that the only real
restriction on battery capacity in hybrid cars is the physical packaging
constraints - which just don't apply to a bus.