18000 BTU is going to need a fair amount of power and you won't run that on 120V. any electric is basically 3.4 BTU per watt so 18000 is around 5000 watts which at 220V is a nominally 20A - 25A draw. You could do that with a no10 or larger extension cord off something like a dryer or welder outlet, but again, that would have to be 220V
