"Bryan Martin" wrote in message
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Thanks for the replies. Unless I am missing something the formula is
sound but not worded correctly?

Yep. I think you have the basic idea, but in this stuff you have to be
careful to dot your 'i's and cross your 't's. (this group is good at
pointing out mix ups like this). It may seem rather 'anal' at times, but it
*is* important to get the right results ;-)

I will try to find more info on this. The reason behind the extended
formula's is for better understanding and also the end result and second
part of this question is that I plan on hooking this up to a solar panel
if possible. I was trying to get an idea about what I would be using per
day/hour if run 24/7 and what size panel I would need for this. Then
depending on how that works out $$$ wise for the size panel I would either
stay at that or look at some kind of timer setup that would allow the unit
to run for X hours a day. I would rather have it on 24X7 but the actual
real world usage of the unit would be around 3 hours per day on a weekday
and posssibly more on the weekends.

So I am still needing some help with the panel size I will be needing to
keep this thing up and running.

Well, you know that you need about 144 watt-hours per day on average. But
how many hours a day does the sun shine in your area? That is the another
factor needed to figure out the panel sizing.

If you're drawing 144 watt-hours *out* of your battery, how many watt-hours
do you have to put in? More, unless you've found the perfect battery.
Assume 80% of the energy you put into the battery gets lost, so to get 144
watt-hours *out*, you should plan on putting 144/0.80 = 180 watt-hours *in*.

Now, perhaps the worst season would be winter, and perhaps in your area the
sun only shines, 'on average' 3 full-sun hours a day in winter time. To get
180 watt-hours of energy to put *in* your battery from a charge controller,
in just 3 hours, you would need a charge controller output of 180
watt-hours / 3 hours = 60 watts. (at 12V, that would be 5 amps).

Is this just for a year or two, or longer? The reason I ask, is that panel
output degrades over time. Some manufacturers warrant for 80% after 20
years. I suspect you aren't planning some 20-year long plan here, so lets
just *assume* panel output is 90% of rating under the conditions you'll be
operating. So to be sure of 60 watts at the charge controller output, 60
watts / 0.90 = 67 watts panel output.

Of course, some days your sunshine will be more, and that will help recharge
your battery. But if you run real close to just those 3 hours per day, then
you're going to have a hard time recharging some weeks in the winter/cloudy
season. Moral of this is, if you have another means to fully charge your
battery sometimes, you might be okay. But if the panel is the sole
charging, and no interruption of power can be tolerated, you'll need even
more panels to manage recharging on those partly-cloudy days.

Hope this helps you along.

daestrom