How is energy efficiency defined?

Energy efficiency is defined as the ratio of useful energy output to the total energy input. This measure indicates how effectively a system converts the energy it consumes into useful work or output. A high energy efficiency ratio means that a significant portion of the energy input is being effectively utilized, while minimal energy is wasted.

In practical terms, this concept is vital for assessing technologies and practices aimed at reducing energy consumption and minimizing environmental impacts. Recognizing and enhancing energy efficiency is crucial in the context of sustainable energy practices, as it can lead to reduced energy costs and lower greenhouse gas emissions.

The other options do not accurately reflect the definition of energy efficiency. For example, the total energy consumed by a system does not provide insight into how effectively that energy is being used. Similarly, the cost-effectiveness of energy sources relates to economic factors rather than the efficiency of energy conversion, while the amount of waste produced during energy conversion speaks to environmental impact rather than energy efficiency itself.