The solar roof that Toyota is offering as an option on its next-gen Prius hybrid sedean is even less efficaceous than expected, according to specialty publication EVWorld. The solar panels, reports EVWorld, will add not a nanowatt of charge to drive the Prius.

Technology Review looked at the potential impact of a solar roof on the Prius last summer when rumors of Toyota’s plans first emerged. The clear conclusion of the experts: Keep solar panels on rooftops, where they can be tilted towards the sun for maximum efficiency and multiplied to provide the kilowatts of power it takes to drive a car. 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’ 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 that. The solar roof will simply help keep the car cool when its parked. The solar power feed runs a fan to ventilate the car. For the average driver that could be somewhat useful for, say, half the year.

BTW I spoke with Otsuka at the Geneva Motor Show earlier this month, and learned that Toyota engineers are targeting 20km as 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.

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

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This post was created for the Technology Review Potential Energy blog