The main advantage of a hybrid system is that the engine can run at a more efficient speed, which is independent of the power being delivered to the wheels. Say

The main advantage of a hybrid system is that the engine can run at a more efficient speed, which is independent of the power being delivered to the wheels. Say you are driving through a town, accelerating from stopped to 30mph then cruising in traffic, the engine will start shortly after you start to move, run at 3000rpm the whole time (or whatever is the most efficient speed), then cut off when you begin the cruise. This is why automatic transmissions now often have 7 or more gears, as it allows the engine to run at a more efficient speed. A hybrid system in comparison is mechanically a lot simpler though: In a Prius the clutches, transmission and alternator have all been replaced by a simple mechanical device with fixed gears and two electric motors. As for the battery the Prius has a 1.3kWh battery, but due to battery longevity (the system tries to keep it charged around 60%) only around 400Wh is actually available for use.