Hey I did a little research for you just to actually see how it all works. Just as i suspected, the prius will run off the battery/engine at their respective times. If you are doing driving that only requires the battery to be used, once thhe charge level gets below a certain point, the petrol engine will kick in and the car will run on that and recharge the battery at the same time. The prius also uses energy from braking and rolling/coasting to help recharge the battery. Hope that helps