Battery current draw

I think the proper measurement instead of watts or amps is phlogiston.  It's much easier to quantify.  

Originally posted by offgrid Ok GlueGuy, we will disagree and move on. But ha ha, my 38 years beats your 10  If you spent your decade designing grid tie PV systems than watts are the way to go. But if you are designing battery based systems, amp hours works better and is much simpler. The nominal voltage is known and is established by the battery, so there is no reason for confusion on that. If you do use watts and are doing it correctly,  and I certainly agree that can be done, then you must use derates for battery round trip efficiency and PV module temperature. I expect that if you take a look at those derates you will find that they match up very closely with the amp hour approach. If you are not derating when working in watts and watt hours then you will be overestimating system performance.

Well. I did learn electrical engineering in 1969. So that is 49 years. Touche! I just had a brief interlude in the computer business.

Ya got me beat in years since graduation but I won’t yield on solar experience...

Like I said, you can use watts and watt hours to do solar sizing, as long as you use the appropriate derate factors for battery and PV module performance. It’s just much easier using amp hours.

BTW, I've interviewed numerous EE grads who couldn't explain ohm's law to me. They could write C++ code in their sleep though, but what that has to do with electrical engineering is beyond me. Do they even train anyone in electrical power systems engineering anymore? 

Originally posted by Colorado r-pod guy This is all very educational, but, I'm still not sure how much power I need.  Based on my desire to run the set the thermostat to 50 degrees on 20-30 degree nights it sounds like a 100 watt panel is not going to be enough.

Correct. If you are going to be out for more then 2 nights, you need more storage and more charge capacity.

The most storage you can put on the tongue, without modifications, is a pair of GC2 6v batteries wired in series. Very popular mod. For recharging, a 120watt or larger system, preferably portable so you can do as much as you can to maximize sun angle.

What I would recommend, is to do the GC2's first, $225 or so for the whole shebang, and go out and see how well just that mod will do for the way you intend to camp. Remember during the testing and learning phase, a set of jumper cables and your truck will recharge you to 80-85% pretty darn fast if needed. You may find that for the number of days you want to be out and the conditions, just the battery upgrade will be enough. Going to dual GC's from the standard group 24 12v battery will net you nearly 4x the usable amps, or watts, or blower fan revolutions, or whatever method of measuring your needs you choose to use.

Always remember, there is always more then one way to skin a cat, and while there may be a "perfect and correct way", as long as YOU are happy with how the cat got skinned, it's all good. It's camping, not a Mars mission. Good enough, is in fact, good enough.