An Alternative Method for the Assessment of the Typical Lighting Energy Numeric Indicator for Different Outdoor Illuminance Conditions

The contribution of daylight plays a key role in the evaluation of the Lighting Energy Numeric Indicator (LENT), which refers to the annual lighting energy consumption per square meter: the higher the contribution, the lower the energy consumption. Calculation of energy need for artificial lighting is defined in European Standard EN 15193-1:2017; the daylight availability is assessed by using the Daylight Factor. This parameter is easy to use and commonly applied by designers and technicians, but it has several limitations. The estimation of daylight contribution proposed in the first version of the EN 15193:2008 was discussed in literature and alternative methodologies and models have been suggested, since it was noticed that it underestimated the daylight contribution. Starting from these considerations, this study will focus on the analysis of a parameter which represents a middle way between static and climate-based variables. This new variable is the Daylight Factor Target, reference parameter of the new EN 17037. The D target will be used in the alternative methodology of LENI calculation developed in previous studies and the results will be compared with the ones obtained by applying the EN 15193-1:2017 procedure. Results showed that the Daylight Factor Target allows to assess in the first building design phase the LENI of any building which complies with the EN 17037, independently from its characteristics, just choosing the operative working hours and the lighting power installed. The LENI target resulted to be 50% lower than the values obtained using EN 15193-1:2017.