Gordon Henderson wrote:

What sort of draw would an 800w microwave have on a 12v battery ?

P = I * V, or I = P / V

So 800/12 is 67. This isn't perfect for AC and inductive loads, but
but it's good enough for a rough ball-park.

Except that 800 W is the _output_ of the magnetron in the microwave oven
and you haven't allowed for efficiency of same in your calculation. The
efficiency's probably about 70% so the input power is going to be
getting on for 1.2 kW. At 100% inverter efficiency that would require
87 A from your battery (at 13.8 V, by the way, not 12 V, if it's on
charge). With a practical inverter efficiency of say 85%, your poor
battery's going have to deliver over 100 A! Forget it and use a primus
stove instead :-)

--
Andy