The 2015 Toyota Prius Plug-In Hybrid has the lowest electric range of any plug-in vehicle sold in the United States: just 11 miles.
It's also among the top-selling cars that plug in, with more than 40,500 delivered since it went on sale in February 2012.
Now out of production, the plug-in Prius will be replaced by a new model that will go on sale sometime after the conventional 2016 Toyota Prius hybrid is unveiled this fall.
But the new-generation Prius Plug-In (which might conceivably be called Prius Prime) will be even more important than its predecessor, which lagged the 2010 model it was based on by two years.
Developed in parallel with the new 2016 Prius hybrid, the new plug-in will likely have a longer electric range.
In August 2013, Satoshi Ogiso, Toyota's chief engineer and the man in charge of developing the next Prius range, acknowledged that Prius Plug-In drivers wanted greater electric range.
Fourth-generation Toyota Prius hybrid prototype testing in Thailand [from Headlight Magazine forum]
"We have been listening very carefully to Prius [plug-in] owners over the past two years," Ogiso said, "and [we] are considering their request for additional all-electric range."
But the plug-in Prius will play a much greater role both for Toyota and for two smaller Japanese makers in the years to come.
Of more than 750,000 Priuses of all models sold in the U.S. since January 2012, the plug-in model accounted for 5.4 percent.
That number may have to rise to meet stricter zero-emission vehicle sales requirements in California and a dozen other states, starting in 2018.
While the math is complex, Toyota must sell not only fully zero-emission vehicles but also partially zero-emission vehicles, i.e. plug-in hybrids.
While the full ZEVs will be the Toyota Mirai hydrogen fuel-cell vehicle, the company must still sell some number of Prius Plug-In Hybrids to meet its overall goals.
2016 Toyota Prius camouflaged test car, Southern California, Jun 2015 [photo by Cars Direct]
And starting in 2018, the percentage of total sales represented by ZEVs and plug-in hybrids together rises annually.
The plug-in version of the Prius is also likely to donate its powertrain to new plug-in hybrids from both Mazda and Subaru, which fall under the requirements for the first time starting in 2018.
While small numbers of the Subaru XV Crosstrek Hybrid are sold in the States, its mild-hybrid system isn't likely to be converted into a plug-in hybrid system with enough power to carry the car 10 miles or more solely on electricity.
Mazda briefly sold a Tribute Hybrid in the U.S. only that was a rebadged version of the Ford Escape Hybrid.
It also now sells a Mazda 3 hybrid in Japan only that uses Toyota Hybrid Synergy Drive system paired to Mazda's highly-efficient SkyActiv gasoline engine.
Both companies have signed technology-sharing agreements with Toyota that are widely interpreted to mean licensing of hybrid and plug-in hybrid technology.
In other words, the next Prius plug-in is key not only for Toyota's substantial California sales, but also to keeping Mazda and Subaru to stay legal in California starting in 2018.