The solar roof that Toyota is offering as an option on its next-generation Prius hybrid sedan may be even less useful than expected, according to a report in the specialty publication EVWorld. The solar panels, reports EVWorld, will add not a microwatt of charge to drive the Prius.

Last summer, Technology Review looked at the potential impact of adding a solar roof to the Prius when rumors of Toyota’s plans first emerged. The clear conclusion of the experts was this: keep solar panels on rooftops, where they can be tilted toward the sun for maximum efficiency. A solar rooftop would be just a “marketing gimmick,” said Andrew Frank, a plug-in hybrid pioneer at the University of California, Davis, and chief technology officer for UC-Davis hybrid-vehicle spinoff Efficient Drivetrains.

Toyota, it turns out, won’t even bother plugging its solar rooftop panel into the 2010 Prius’s nickel-metal-hydride battery. EVWorld editor Bill Moore, citing a conversation with Akihiko Otsuka, chief engineer for the Prius redesign, writes that

Toyota tried it and apparently discovered that for not-entirely-well-understood reasons, connecting the PV panels to the battery turns them into an “antenna” of sorts, which at the very least seems to disrupt the car’s radio.

So Toyota left it at that. The solar roof will simply help keep the car cool when it’s parked by running a fan to ventilate the car. For the average driver, that could be somewhat useful for, say, half the year.

I spoke with Otsuka while reporting from the Geneva Motor Show earlier this month, and learned that Toyota engineers are targeting a range of 20 kilometers in the EV mode for the plug-in version of the Prius. The lithium-battery-equipped vehicle is to be offered to Toyota’s fleet customers by the end of this year.

That would mark a boost over the 10-to-15-kilometer range offered by the nickel-metal-hydride-powered plug-in Prius that Toyota has been testing. But it remains just a third of the 60-kilometer range that GM is promising for its Chevy Volt plug-in hybrid, which is due out next year. GM’s design has already set off a debate over the cost effectiveness and efficiency of carrying the battery capacity that such range requires.