For the most part, today’s plug-in hybrid electric vehicles are a joke. I say this for a couple of reasons.points out that many PHEV owners do not plug in their PHEVs as often as they should
Studies suggest that there is a correlation between how much electric range a PHEV has and how likely the owner is to plug it in. This makes total sense. It is very intuitive. Now, on the other hand, imagine that you had a PHEV with 90 miles of electric range. You know that you can skip the gas stations for weeks or even months at a time. You’ll need gasoline only for that occasional longer trip or road trip. You’re assured that you’re going to realize substantial savings on your transportation costs by driving electric. You’ll want to plug in your vehicle as often as needed to do all your day-to-day driving.
My hypothesis is that somewhere around the 25 kWh of usable battery lies the point where the hybrid owner's likelihood to plug in falls off quite rapidly. Perhaps the true drop-off point is a little to the left or right, but I suspect that the motivation to plug in falls off quite quickly in correlation to the drop-off in the usable range.