Locate the required wattage of the electronic device normally located at the bottom or back.
Calculating the correct gauge
Result is the Gauge.
Total wattage = 1200W / 110 = 10.9
Round down = 10 Gauge.
Most devices will do fine with 12G(gauge) extension cords. Power hungry devices such as lawnmowers, heavy machines, require a lower gauge such as 10G. Keep in mind, the lower the gauge means the thicker the cord.