I have two Electric Bikes built from Phoenix Cruiser (Crystalite) systems and 48V 20AH LiFePO4 battery packs I purchased from Ping. In an effort to take these bikes off the grid I recently installed a solar charging sytem for them. My system consists of 8, 15Watt, 12V solar panels mounted on the roof and wired in parallel through a charging current regulator to a 12V 50AH SLA energy storage battery. The battery is then conneccted to a 1500 Watt inverter that produces 110V AC, and I plug my pack chargers into the inverter.
The system seems to work fine for most purposes. In normal use my wife and I draw down the bike packs about 500 WHrs on a typical ride (that's half the stored energy in the pack, enough to go about 20 miles), and while the SLA only stores 600 Watts, it is being continuously recharged by the solar panels at a rate of about 100 watts per hour in sunlight. As a result, the system can usually recharge both bikes in a single day without problems.
The entire system cost about $1000, so I didn't do it to save money on electricity, but rather to learn more about the promise and limitations of solar power for such applications. I must admit it does feel pretty good to not have to rely on the power company for good-weather transportation, though.
The inverter adds some inefficiency to the system, and I could have just wired the panels in series to charge the bike packs directly, but then I would lose the charging management provided by the LiFePO4 chargers. I can also plug in other 110V AC devices as well if I wish to.