Ans: The area normal the direction of heat transfer is taken for the heat rate calculation formula or Fourier's law of heat conduction.

As you can see from the following figure, the heat flows from the left-right direction. The heat flows across a plane surface that is inside the screen.

The area normal the direction of heat transfer is taken for the heat rate calculation formula or Fourier's law of heat conduction. So, in this case

Area = A = H*W