We assume the following:
Using 12V system.
PC9+plus operation hours: 7 hours
Inverter efficiency: 85% (Good quality modified sine wave inverter)
Depth of discharge: 95% (deep-cycle battery)
Constant power demand = 300W.
With a 12V battery, the current will be 300 / 12V = 25 Amp (Make sure the wire you use will can carry 25A constant current).
Peak power demand is about 1800W for 5-15 milliseconds.
1800 / 12V = 150 Amp ( Make sure your battery has a CCA rating equal or higher than 150 Amp)
The battery size can be roughly estimated with the following equation:
(duration x steady battery current) / (depth of discharge * inverter efficiency)
Based on the above figures, the equation gives:
(7 hours x 25 Amp ) / ( 0.95 depth discharge x 0.85 inverter efficiency ) = 216.7 Amp-hours
Round it up to 220Ah. This is the smallest battery size it requires to run the AC for 7 hours. Please note that using Close Comfort with the Igloo tent can extend the operation time by about 30 to 40 percent.
Please also allow extra capacity for other appliances that could possibly consuming power at the same time.