Formula for heat wire Kanthal or NiCr Watt consume?
Easy to work this out from first principles.

The formula for power is \$P = \frac{ V^2}{R} \$ where V is the RMS voltage.

The formula for resistance is \$ R = \frac{\rho L}{A} \$ 

So, simply find the resistivity of Kanthal A-1 at 1400'C, the diameter of AWG 18 wire (1.024mm) will give you the cross-sectional area and work out the watts, using your given RMS voltage and wire length. Pay attention to the units.