I use a small Maplin Solar panel ,, it does its job , but i cant honestly say that i can rely on it ,, the battery still needs a good charge , every week , changed Vans recently and I will get a split charger ,, but for £15 it has paid for its self.
A locksmith i know has a Panel on his roof that fits most of his Transit roof ,, he has 240 volts to use a soldering iron and his computers ..