Thanks for the great responses. Here's what I've noticed from driving. I'm not sure how it is measured, but seems to me like torque on the wheels that factors in. For example, at a given speed and power demand, more opposing torque is placed on the wheels, causing the threshold to drop. Conversely, going downhill puts less torque on the wheels, requiring less power to keep the car moving/speed up, so the threshold rises. Also, in watching the battery charge/discharge arrows, I've noticed when torque on the wheels/engine/whatever is below a certain point (power demand from ICE exceeds what is required to move the car at a given speed on a given incline/decline), the engine is putting charge back into the battery. I assume this threshold between discharge/charge is enough electrical power generated from the ICE to overcome the climate/accessory load in the cabin.I could be a bit off, but that's just my impressions/observations from driving it for a few weeks.