Well, you
can run a space heater off of auxiliary batteries. All you need is, as Bruno said, a few deep cell marine batteries. Except, marine batteries won't really work, since they aren't designed for high amp draws, and "a few" becomes very subjective. But as long as you have a battery bank with enough amp hours in them, you can run a 1500 Watt space heater with no problem. How many batteries might that be, you ask? Let's do the math, shall we?
1500 Watt heater draws 12.5 amps at 120 volts, which inverts to 125 amps on a 12-volt battery system. Add in another 10% for the loss in the conversion from 120 to 12 volts, and it will draw 137.5 amps from the batteries.
If you want to run the heater for 8 hours, that's 8 x 137.5 = 1100 amp hours. Because you don't want to discharge the batteries below 50%, you need to double that to 2200 amp hours for the capacity of the battery bank. Because of the Peukert Effect, a 137.5 amp draw will actually require a battery bank of 2300 am hours, which will give you 2215.29 amp hours available.
L16 size batteries have about 410 amp hour each. They are 6 volts, so you need two for each 12-volt cell. 2300 amp hours divided by 410 amp hours is 5.60, rounded up to 6 batteries, so you'll need twelve of the 6-volt L16s in your battery bank.
Each of the L16s weighs about 125 pounds, so for twelve of them you're looking at 1500 pounds of batteries, plus the weight of AWG 2 cables and lugs and fuses, which will add about 100 pounds.
If your alternator puts about 30 amps back into the battery bank, after you've discharged 1100 amps out of them it will take approximately 47 hours to fully recharge the batteries, assuming you have no other amp draws off the batteries while they are being recharged. If you have cables large enough to handle the current, and a charging source capable of outputting a constant 150 amps into the batteries, then it should take only about 12 hours to fully recharge the batteries at 1100 amp hours.
Each of the twelve L16s will cost about $300 if you use Trojan wet cells, or about $500 if you use AGM batteries. So, $3600 for Trojans, $6000 for AGM. Add another roughly $280 for cable lugs, and $200 for AWG 2 Flex Cable, plus $300 or so for Class-T fuses and fuse blocks, heavy duty busbars and a master switch, and you're looking at either $4380 or $6780 depending on the type of battery bank you use. To run a space heater. The TV will hardly be noticed by the battery bank.
Seems like an awful lot of money to run a small space heater, not to mention the weight involved. But, to each his own.
Orrrrr, you would spend $300 on a couple of decent deep cycle batteries, hook an inverter to it, install an Espar heater, and run the TV and all sorts of other things for days without going near the 50% discharge depth.
