i know its summer time, but here are a couple things to look at for the fall
http://www.sportsimportsltd.com/12voltheaters.html (//%22http://www.sportsimportsltd.com/12voltheaters.html%22)
more neat stuff to look at
http://www.sportsimportsltd.com/12volap.html
This stuff looks interesting, but any item that produces heat will drain a battery real quick, unless you are running some type of engine to replace the power used. :)
Like Tracy said, 12 volts and heat sources don't mix.
150 watts equals 12.5 amps and the 500 watt mucrowave with inverter draws about 75 amps.
Plus the normal 120 volt electric heater is 1500 watts on high power so the 150 watt 12 volt versions won't put out much heat unless you sit on it.
Folks complain about the furnace's 3.4 amp draw killing a battery. Imaging how long it'll last with a 12.5 or 75 amp draw.