I once used an 85W portable solar panel to charge my battery while driving after the alternator failed. It was surprisingly effective at keeping the car operab

I once used an 85W portable solar panel to charge my battery while driving after the alternator failed. It was surprisingly effective at keeping the car operable, at least until the sun went down. I didn't have any accessories running, so it was just the ECU/fuel pump/injectors/ignition it had to keep up with. The experience left me wondering what MPG improvement it would be to embed solar cells in the trunk lid and hood for charging the battery in the daytime, offloading the alternator.