On occasion I use the ChargePoint station at my work which reports the energy usage history. According to ChargePoint, my plug-in Prius has a maximum capacity of 2.9 kWh when charging from "empty". I thought that the PIP battery capacity was closer to 4.4 kWh?
it's programmed to not empty all the way and not charge fully to protect the battery. most people have been reporting 3.1-3.3 kwh for a full charge due to charging losses. the car has about 3 kwh available for use.
Don't forget that PHV is still a killer hybrid even when depleted. Some of that remaining electricity is used to deliver the 50 MPG afterward. The rest is the buffer, a portion left always held onto for battery longevity (to avoid deep discharging).
