Your calculations are a bit off, because batteries are rated at the 20 hour
rate, but you are assuming about a 120 hour discharge time, so the actual
capacity will be somewhat more - around 20%.

But that won't really affect the basic calculations - all it does really is
give some safety factor.

The panel size is a bit more complicated, but not rocket science. First you
need to determine how important the load is - will it be a major event if
whatever you are running goes offline. If so, then you need to factor in a
pretty large safety factor - 35% or more - for those rare occasions when you
get much longer than usual cloudy periods. Basically, 100% uptime will cost
you a lot more than 99% uptime.

But ignore that for now - the basic calcuation is that you want your solar
panel to supply what you use each day + around 20%. That is for the WORST
case conditions - such as you might see seasonally with lots of clouds.

Assuming you look at the insolation charts, and you get worst case in
December of 4 hours full sun per day. You are drawing .5 amps over 24 hours,
or 12 amp-hours. You need to replace that 12 AH in during the 4 hours of sun
you get, so you need a panel that will supply 12/4 = 3 amps. Add 20-25% or
so safety factor, and you come up with around 3.5 amps. Most panels are
rated at 17 volts or so, so 3.5 amps x 17 volts = about a 60 watt panel.

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"Bryan Martin" wrote in message
...
The unit I will be using is 12V 0.5A

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