Which measurement indicates the energy efficiency of a building element?

The U value is the correct measurement that indicates the energy efficiency of a building element. It represents the rate of heat transfer through a building component, such as walls, roofs, or windows, and is expressed in terms of thermal resistance. The lower the U value, the better the insulation performance of the element, as it signifies reduced heat loss in winter and heat gain in summer. This makes it a critical factor for assessing the energy efficiency of a building, as elements with low U values contribute to lower energy consumption for heating and cooling.

Each of the other measurements has different applications that do not directly pertain to the energy efficiency of specific building elements. Peak load refers to the maximum demand for energy during a specific time period, while demand reflects the actual energy use over time but does not provide information on the efficiency of individual components. The efficiency ratio, on the other hand, generally compares inputs and outputs of energy systems but is not a specific measure for evaluating the energy efficiency of building elements such as insulation or window performance.