As I understand it, in the 2003 and earlier generation, when the draw from the battery exceeds 9 Kw (something like 12 hp) the engine fires up. In the current model it is higher, but I don't know how much higher... I think about twice the draw. Most accelerations in light traffic start the engine in our model, but I think the post-2003 version can do a lot of normal accelerations on battery.
Mike