Chapter 21 - AC Circuits - Learning Path Questions and Exercises - Conceptual Questions - Page 747: 3

The discussion is outlined below:

Because the voltage is multiplied by $1/2$, so is the current. Also, since the power is proportional to the square of the current, the power is multiplied by $(\frac{1}{2})^2 = \frac{1}{4}$. So the power is 25 percent of the designed power.