The 180-watt panel will now charge both the 12-volt battery and the primary drive battery.

Panasonic has detailed its new contoured solar panel that was previously announced as an option for the Prius Prime plug-in hybrid.

Covering nearly the entire roof, the photovoltaic cells provide up to 180 watts of charging power to both the 12V and drive batteries -- more than three times the output of the solar panel available on the previous-generation Prius PHEV. The previous 50-watt setup was only used to charge the 12V battery.

"Panasonic will make efforts to expand the use of the 'HIT Photovoltaic Module for Automobile' and contribute to the achievement of an environmentally friendly society in the automotive as well as the housing and industrial fields," the company said in a statement.

To be clear, 180 watts is still a small fraction of the power provided by a basic charging system connected to the grid. For comparison, the Prime takes 2.5 hours to fill its 8.8-kWh pack when connected to a 3,300-watt charger. A completely depleted battery would likely take more than a week to charge via the solar panel alone.

The announcement suggests Panasonic may be the supplier, or manufacturing partner, for the Tesla Model 3's optional solar roof. Without a breakthrough in panel efficiency, however, the relatively small panel would likely require several months to fill a 60-kWh battery used in a long-range EV.