Efficient

Efficiency is a measure of how much work you get done for the amount of energy you expend. In an ideal situation every calorie of chocolate chip cookie you eat would be converted directly into the kinetic energy of your kayak moving through the water. This would be a 100% efficient system. Unfortunately, there are losses through out the conversion process. Obviously the body uses some of the cookie just to keep the heart beating, synapses firing and muscles moving. You apply your effort to the water via your paddle and there are losses in paddle efficiency, then further losses as the boat pushes its way through the water due to hydrodynamics.

Efficiency is defined as the amount of useful work or energy you get out divided by the energy you put in:

and is often expressed as a percentage. It is impossible to have an efficiency of 100% - welcome to the laws of thermodynamics.