Maplin
currently do a 60W unit for £300. Thats 5 amps at 12v
ONLY whilst the sun is shining brightly.
Provided you can run the engine for a few hours each day a second L/B would be a lot cheaper!
Its also not clear ( I'm pretty certain it doesn't have) a proper charger circuit to make the best of lower sun light levels -
this web page gives an explanation of what is needed and why, but is designed for smaller charging currents.
This site will sell you a
much better set of charger electronics or
for higher currents as well at not to bad a price, but little hard info on light levels and solar panels recommended.