Real world application, not the numbers game that is used to size up these systems.
I have 6 trojan L-16 (1050ah) running through a Xantrex RS3000 (3000 constant/ 7500 surge) and a small heater will deplete these batteries in less than 4 hours @ 600watts,..less @ 1500watts.
I have the most efficient roof top A/C unit that draws 1540watts and will only run for 2 to 3 hours tops.
Now, if I use everything except a heater or A/C,microwave(for more than 10 minutes)
Fridge
tank heaters
32" lcd tv with dvd player
computer
fax
battery chargers for all the stuff (cell phones,comp)
led light strips
The batteries will last indefinitly with the power from the solar panels charging them.
I have all these batteries being charged by a 165amp alt.

I went to profesionals to size up this system, and the "numbers" said the battery bank would run a 1600watt unit for 10 to 14 hrs.
It won't and you will fry your alt. if you allow your deep cycles to drop below 11.5 volts (and this will happen quickly) by running any type of heater.
I recommend an Espar/Webasto for your heat and let your deep cycles run your electronics. Your batteries will last a long time and not be subjected to the quick discharge that high draw heaters/A/C units will put your batteries through.Hope this helps, the battery/inverter game can get expensive quick, and these are the lessons I learned from my experiment.

 

Yup, Doghouse is right. I've experienced this first hand. I have 4 yellow top Optima batteries and killed them real quick using a 1500 watt heater for only a few hours. Then, when I started my truck to recharge the batteries the alternator burned up the second time.

Your alternator is not there to recharge batteries, it's there to maintain a full charge and not to resurrect the dead. When I got my new Optima batteries there was a notice to recharge them fully before installing them to prevent damage to the trucks charging system.

There is a lot more to electricity than meets the eye like these 2 interesting facts.

An inverter is ALWAYS draining power from your batteries even though it's not being used. It takes power to create that 110v even though it's not being used.

An 800 watt microwave uses up to 1500 watts to create that 800 watt cooking power!!!

 

I'm looking at my charge controller for my solar panels right now, and the panels are bringing in 7.1 to 10.5 volts and my battery bank is at 13.4 volts when the sun is out (this will be better during the summer when the sun is higher in the sky).
My inverter is drawing 40watts, my fridge is drawing 180 watts, and my computer is drawing 20watts. All of this is more than covered by the solar panels,....but if I use the microwave (1560watts) it will bring my batteries into the 11.9 volt range and then the solar will have to pick up the charge when I'm done. If it was night time, I would have to run the truck while I ran the micro,...or the batteries would struggle to start me in the A.M., and the alt. will be hot,...way hotter than it should be (if it was under 30 degrees). Because the batteries would be down to 12.2 to 12.4 because of the constant loads I have. And this is on a 1050ah system,600lbs of deep cycle batteries!!