(ET) Solar Cell wattage needed

[Bill <wilbett earthlink net>]

    Can any of you solar cell charging guys give me a feel for how much 
solar
cell output wattage is enough to be of any use for charging or maintaining 
charge
on batteries based on your experience? Harbor Freight Tools is advertising 
a 1.5
watt 12 volt solar cell for $10. Would three of those be of any use with 
our
elec-traks? How many solar cell watts are needed to recharge in a week? 
Also my
farm tractor sits outside all winter with its 12v battery. Would 1.5 watts 
keep
the battery charged or at least safe from freezing through the winter?

Thanks for whatever info you can throw at me.
Bill