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But what if Toyota routed energy from the solar panels directly into the batteries, instead of just pushing the power to a small fan? That has genuine potential. asked Daniel Sherwood, a solar energy engineer and the founder of 3Prong Power, a Berkeley-based plug-in hybrid conversion start-up, to do some “back of napkin” calculations. Let’s assume that you could cover a full third—very optimistic—of the Prius’s 7.6-meter top surface with the Kyocera panels that Toyota is planning to use. According to Sherwood, an “average solar day” in California produces 10 kilowatt-hours of energy per square meter and the Kyocera panels are 16 percent efficient. That means a car parked all day in the driveway could produce 1.6 kilowatt hours of energy—more than enough to top up the Prius’s 1.3 kilowatt-hour battery pack. Because the Prius can go about three or four miles on a kilowatt-hour, you could conceivable enjoy as much as five miles of all-electric driving on the full battery packs—a fuel-efficiency boost of 10 percent of more.

But hold on. Unlike a plug-in hybrid or a conventional hybrid with additional battery capacity, today’s Prius is not set up to take advantage of this opportunity. Even with an EV button—like the one available on the Toyota Highlander Hybrid—the Prius’s computer control system will baby the battery packs and only give you a mile or so of gas-free driving. And that’s with a light touch on the accelerator. The solar panels could push the upper limit of that capacity, but again, that’s only after a full day of charging and very careful driving mostly in the city.

Using the power from the rooftop solar panels to run the air-conditioning might hold more promise. Most hybrid drivers know that blasting AC kills mileage. But given the considerable power required by AC systems, and the need to generate power while the car is driving, the best you could hope for is a small “offset” of the AC’s power requirements.

Where does this leave us? Not very far from where we are today. For Toyota, bringing the solar panel option to the market adds a little more green sheen to the Prius halo, but is unlikely to significantly improve the vehicle’s actual environmental and energy performance.