Bryan Martin wrote:
The unit I will be using is 12V 0.5A

12V X 0.5A = 6 Watts
6W x 24hours = 144 Watt-hours per day
144W / 12V = 12 amp hours

Now given a battery with 110 amp hours
(http://www.defender.com/product.jsp?path=-1|328|51495|306219&id=51496)

Walmart has 115 ah deep cycle batteries for $55


We should only draw the battery down 50% so we will actually only get

110 / 2 = 55 amp hours

12 amp hours needed / 55 amp hours available = 4.58 days
so with this battery we have enough juice to potentially run our unit for
4.58 days w/out charging.

Now if the above is not correct then please explain. And the next question
is how would I now figure out approx how large of a panel I would need to
keep the battery charged. I am really looking for the formula and a
understanding of how you got from A to B as oppose to "this panel should
work".



Your math is correct. In NY, with about 3 full sun hours daily max, best
case scenario I'd need a 50 watt panel. 50 watts * 3 hours = 150
watt-hours daily.

--
Steve Spence
Dir., Green Trust, http://www.green-trust.org
Contributing Editor, http://www.off-grid.net
http://www.rebelwolf.com/essn.html