In addition to the preceding helpful comments, think of this as vectors:
If you're flying directly north, and want to fly directly east, in an aircraft which is flying at 100MPH, at the instant you begin your turn, you are flying north at 100 MPH, and east at zero MPH. By the time you have finished your turn, you'll be going 100 MPH east, and zero MPH north, so, in the radius of the turn, you have to accelerate from zero to 100 MPH. The amount of acceleration depends upon the radius (smaller equals more).
Accelerating means adding power or reducing speed (or a combination). Once this adjustment has been made, the acceleration can now be constant as long as you like, and the difference in power to fly and accelerate is now there. If you also want to climb, add more power, or slow down more. If you're happy to descend, not so much....
And no, you don't get credit for the energy you give up from flying at 100 MPH north to flying at zero north.