Toyota is testing the plug-in version of the Prius in fleets, in advance of sales to the public in 2012.

Early in the Toyota Prius' history, U.S. owners noticed it lacked the EV mode switch included in the Japanese market car. Thus was born the first Prius hack, restoring that button's functionality in the U.S. cars. Then, in 2004, CalCars took Prius hacking even further, adding the capability of charging up the battery pack from the grid. CalCars boasts that this hacked Prius gets 100 mpg.

That Toyota is now promising the public a plug-in Prius as a 2012 model should only beg the question of what took so long. As with most car companies, Toyota moves at a conservative pace, not wanting to risk a manufacturing line on a product that could turn out to be a dud.

The launch of the official Toyota Prius Plug-in Hybrid Electric Vehicle (PHEV) began with concept vehicles shown at car shows, and now test vehicles being added to fleets. This launch program culminates in August 2012, when Toyota will make the Prius Plug-in available to the public.

The Prius PHEV plugs in to a 110- or 220-volt wall outlet.

As part of its roll-out, Toyota let CNET drive one of these Prius PHEV's for a week to assess the technology. Although this car offered essentially the same specifications as the model Toyota will release to the public, there will probably be a certain amount of refinement based on data from the fleet testing. Consider the current Prius PHEV a beta.

Besides the stickers along the sides proclaiming this Prius' plug-in status, the exterior sports a charge port on the left front fender. Covered by a hatch similar to the fuel filler hatch toward the rear, the charge port uses the JA1772 standard plug for electric cars. Nobody should have a problem distinguishing the fuel and charge ports.

Inside the Prius PHEV, the cargo area suffers a tiny bit from the conversion. The cargo floor is raised by about half an inch to make room for a stack of lithium ion batteries underneath. This battery pack replaces the nickel metal hydride pack used in the standard Prius. And although lithium ion has greater energy density than nickel metal hydride, the battery pack still takes up more space, as it has to store enough electricity to drive the car in EV mode.

The cargo floor is up about half an inch to make room for the battery pack.

This bigger battery pack not only knocks out the spare tire, but it feels like it makes the car heavier, as well. Toyota makes up for the spare with a patch kit.

Where the Nissan Leaf has a row of colored lights that let you know its charge state when plugged in, the Prius PHEV has a single amber light set in the dashboard that turns on when it is charging. This light isn't designed into the general instrumentation of the car, and looks more like something an engineer knocked into place with a Dremel tool.

Short range It does not take long to charge up the Prius PHEV, maybe a couple of hours. But instead of any great charging efficiency on the part of Toyota, that charging time has more to do with the fact that the battery pack is not particularly robust, only capable of driving the car for 13 miles.

That's right, only 13 miles. Once that less than copious amount of electric range gets depleted, the car switches to hybrid mode, where it operates very similar to a standard Prius.

But while the car is in electric mode, indicated by a green EV icon in the eyebrow display, it feels much like a Nissan Leaf or Chevy Volt. Push the accelerator and it jolts forward, making a turbinelike whirring sound. Acceleration continues with that linearity peculiar to electric cars. At least up to 62 mph, at which speed it automatically switches over to standard hybrid mode.

This display shows what percentage of time the car has been driven in EV mode.

Along with the green EV icon in the instrument cluster, the Prius PHEV also sports an extra graph among its various power use animations. This simple bar graph shows how much time the Prius PHEV has been driven in EV mode and how much in hybrid mode.

When the Prius PHEV arrived in the CNET garage, it showed only 2 percent EV driving, and fuel economy down at 43 mpg. Over our testing period, EV time was boosted up to 15 percent, with final mileage about 58 mpg.

Commuting within the confines of San Francisco, a city measuring only 7 by 7 miles, the car was able to make round trips under electric power the entire distance. For a commute as short as this, a driver might not use a drop of gas all week.

When the battery became depleted, the Prius PHEV almost unnoticeably switched over to its hybrid mode. Toyota has always done a good job of making the engine kick in very smoothly in its hybrids. Where in a normal Prius, you might see the battery meter rise to full after a few downhill runs, the Prius PHEV takes much longer to fill, as the battery pack is bigger. And even when the battery meter showed a quarter full after some careful driving, it would not switch back into EV mode, still operating as a normal hybrid.

There is little change to the handling feel with the Prius PHEV--the car still feels wobbly, as if it would much prefer to go in straight lines rather than bother turning. The suspension, though not rough, has more of an economy than luxury car feel. The steering feel is on the numb side, the electric power-steering unit giving off its characteristic whir when the wheel is turned.

Toyota has not released full specifications on the Prius PHEV yet, but it feels heavier than a standard Prius. The addition of the bigger battery pack would account for the mass increase, and leads us to suspect that the Prius PHEV would get worse gas mileage than the standard Prius when driving in hybrid mode because of the extra weight. That loss of efficiency can be mitigated merely by charging it up and driving it in electric mode.

Given the nature of its plug-in system, the phrase "your mileage may vary" applies to the Prius PHEV in a big way. Whatever numbers the EPA comes out with for the car's mileage will apply to few owners. Someone driving the car in EV mode 50 percent of the time will get much different mileage than another person only taking advantage of EV mode 20 percent of the time.

As the electric range is not that great, few owners would likely go to the extra expense of installing a dedicated charging station at home. But the car recharges reasonably fast from a 110-volt outlet. One thing not offered in the Prius PHEV is the ability to schedule charging times, as in the Nissan Leaf and Chevy Volt.

Toyota has not yet released pricing for the Prius PHEV.