[vundercind]

I'd guess it has something to do with its unusual drivetrain. It can operate: 1) fully electric, 2) fully electric but with gas used in generator-mode to supply power to the electric drive train, 3) gas engine mechanically powering the wheels (like a normal ICE car).

I'd expect it operates in mode 2 a lot when at highway speeds, but not accelerating.

[SoftTalker]

I believe the Prius is either in mode 1 or 3. Never heard that it has a generator capability, unless newer models have changed?

[numpad0]

Prius "eCVT" is a special planetary gearset that all gears are powered. ICE is connected to the planets, input and output has the alternator and traction motors. Difference in resistance between two motors is imparted to the ICE, achieving power mix and generation control.

It's a really simple and clever solution. So much so that brain hurts to think about