Harvey's calculations are correct,  but with I would be slightly more
optimistic (not by much, so do not get your hopes up :-( ),  A good
deep cycle battery, (I've heard somewhere ) can store about 150AH
(ampere-hours).  Theoretically, this means that it can dish out 150
Amps continuously for one hour, or 75 amps continuously for 2 hours,
etc...
So, with Harvey's numbers, you could run such a heater for an hour
(let's say this is possible, but not probable -- 150 Amps is HIGH for
any battery not to boil it's guts out when drawn continuously).
However, 1500 watts of continuous heating is enough to bake the
insides of a VW van almost anywhere, any temp (well, maybe not where
Tobin and Christa passed through in Canada).  I would imagine a 500
watt heater would be more than enough for most reasonable climates.

Also, the 12V to 110V converter -- yes, it will lose some energy
during conversion, but as long as it's insde the cabin, that'll be
heat disspated into the cabin also....

So, one might hope to get a few of hours' heat out of a big battery
that has been fully charged.  Rember -- you'll probably have to drive
all day to get that thing charged again...

If anyone can find the AH ( ampere-hours) rating of marine batteries,
we can make the necessary calculations (this is NOT the CCA or
Cold-cranking-amps number).

Cetin

Harvey Chao writes:
 > fair guess at the steady state load would be around 150 amps at 12 volts.  
 > 
 > People, I don't think this is gonna work .  If you started with a good deep
 > discharge cycle marine battery, maybe 5 minutes before you sucked the battery
 > inside out?  
 > 
 > Harvey
 >