Okay... Well in that case, Id be really selective on what panel to buy...
I'd probably buy a 100W panel. And just hook it stright to the batterie on days I want it to charge... I wouldnt bother with a charge controller, one extra thing you dont need..
A charge controller disconnects the panel when the batterie is full, so the batterie doesnt boil...
Which means, without it, you'd want to disconnect the panel from the batterie if you park it in the sun for a week or longer and dont use it. But I think you'll find that you'll use the power up much faster than your panel can produce.
Thats probably bad advice for someone who's not familiar with solar already... Charge controllers are small and light weight. Ya just put it on there and play it safe... I personally wouldnt tho. Id just put a simple 12v switch. Switch it on, your charging. Switch it off, now your not charging... That way if your using the trike everyday, just leave it on all the time. If your gonna leave it parked for 3 weeks, or 6 months, turn it off so batteries dont sit there and boil.
The switch would be cheaper and lighter than the charge controller. You'd save weight, and money.
Solar was like 4-6 dollars a watt when they first came out... Now you can find them brand new on craigslist for less than a dollar a watt.
Id get on craigslist and buy a nice large 100 panel. Use some square tubing to make a shade roof over the seat, or make it into a lid for the basket.
The bottom line is, a small panel isnt going to do anything for you...
I had the 45 watt solar kit from harbor freight a few years ago... And it was enough to keep my phone charged, and charge my 18V Ryobi drill batteries one at a time.... And thats about all the power it produced. You couldnt run a skill saw. You couldnt run a vaccum cleaner. You couldnt even run a normal television and dvd player, unless it was one of those small, laptop sized, 12v dvd players. Like the kind you would put in your car for the kids to watch while you drive somewhere. And that was in Arizona by the way on a sunny day with clear blue skies.
I think 100 watt panel is really the MINIMUM to be any kind of help. 100 watt panel is usually like 4 feet by 2 feet roughly. Its big and bulky, but anything less wouldnt do you much good.