don't care what your heater says, 1080 watts, 24 volt x 45 amps, is 3685 btu/hr. you will be very cold. for the electric heater to be 18,000 btu at 24 volt, it would have to draw 219 amps.
for your heater to be 18,000 btu it would have to be 5274 watts and that is 24 volts x 219 amps approx