The Prius can't be plugged in. Every bit of energy the Prius uses comes from the gas tank, just like an old fashioned car.
What the battery and electric motors do is disguise the fact that the engine is sized for steady state driving rather than being sized for acceleration. This allows the engine to mostly run in the 34% to 38% efficiency range rather than the 20% range of old fashioned cars. The motors and battery also allows the engine to be stopped when the power demand is so low that even the Prius' efficient Atkinson cycle engine would be inefficient.
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