The panel is capable of producing 180 watts of electricity, which is apparently enough to power car accessories (which weren't specified) and also charge the lithium-ion batteries. According to Toyota, this panel can add up to 3.7 miles of range a day to the car's battery while parked, and it also helps charge the battery while the car is in motion. It also fits the contours of the Prius roof and doesn't change the design.
Something that makes this rather interesting is that Panasonic has very close ties to Tesla. The companies build batteries at the Gigafactory, and they planned to work on solar cells after Tesla acquired SolarCity. That particular collaboration seemed oriented towards the home solar panel market, particularly with the presentation of Tesla's solar roof tiles. However, Elon Musk has expressed interest in automotive solar panels.
In November, Musk talked about the possibility of a solar roof option for the Model 3 on Twitter. Nothing concrete was discussed, but the possibility seems stronger now in light of the Panasonic announcement. Panasonic also said in its press release that the company is looking to expand the availability of its automotive solar technology. It would seem like the Model 3 would be the perfect opportunity for both companies. And if Tesla can engineer absurd gull-wing doors, we wouldn't be surprised if they made a pop-up solar panel a reality either.