When it comes to direct current (DC), the electrical power developed at a certain time by a two-terminal device is the product of the potential difference between said terminals and the current intensity that passes through the device. For this reason the power is proportional to the current and voltage. This is,
P= VI
The voltage V for us is 1.5 and the current I is 13A, then replacing,
P = 1.5*13
P = 19.5W
Therefore the power of a brand new D-Cell is 19.5W