When I first read your comment, I thought that there was no way that solar panels could be heavy enough to affect your mileage. So I did some quick calculations.

Estimates of solar panel weight I found say that solar panels tend to weigh 2-4 lb per sqft.[1] Note that estimate is for roof-mounted solar panels, but if anything, car-based solar panels should be lighter.

A Prius has dimensions of 180"x69".[2] The sqft of the solar panels will be less than that, since the panels don't cover the windshield, and they don't go all the way to the edge of the hood, etc. That's 86.25 sqft, so that puts an upper bound on the weight of the solar panels at 345 lb. If we assume the lighter end of the scale for solar panel weight, that becomes 172.5 lb. That's 6-11% of the curb weight of a Prius. That's pretty substantial! For reference, a 50 lb spare tire kit can reduce fuel economy (and presumably mileage, for EVs) by 1%.[3]

Although that does imply that you're breaking even if the solar panels can increase your range by ~5%, not accounting for the weight. If you live somewhere like Arizona, that might work out. If you live in Seattle, maybe not.

[1] https://sunmetrix.com/is-my-roof-suitable-for-solar-panels-a....

[2] https://www.toyota.com/prius/features/dimensions/

[3] https://www.tirerack.com/tires/tiretech/techpage.jsp?techid=...