First, the PHEV is 100 kilograms (220 pounds) heavier than the standard Prius. The larger battery means there's no room for a spare tire. The test models can go just 13 kilometers on electric power alone, and the 1.5-liter gas engine kicks in any time the speed goes over 100 kmh (the current Prius' gas engine starts up at 68 kmh). The batteries recharge in 60-90 minutes at 200 volts or 3-4 hours at 100 volts.
If emissions are all you're concerned about, the PHEV Prius looks good. Probably. Even with the excess weight and figuring in "emissions created in the production of the electricity used to recharge the batteries" (BusinessWeek's phrase), is cleaner than current hybrids. BusinessWeek explains the differences depending on where you plug it in:
However, the level of emissions reduction varies from country to country, depending on how the electricity is produced. In France, which relies heavily on nuclear power, the projected carbon dioxide reduction could be as much 45%, Toyota estimates. But in the U.S., where most energy is created by burning fossil fuels, the benefits are far smaller, at an estimated 4%. Toyota says those benefits could be boosted by the use of biofuels, which the plug-in Prius accepts.
So, the plug-in Prius - as it's being tested right now - nets us just four percent decrease in CO2 emissions. The good news is that that number can only get greener.