Energy measurement device

An energy measurement device (100) for a building automation system (170) is provided. The energy measurement device computes an average energy use per class for a first sequence (142) of energy measurements by averaging energy use measurements falling in the same class, and compute an average energy use per class for a second sequence (162) of energy measurements by averaging energy use measurements falling in the same class, compute a duration per class for the first sequence (144) by adding the duration of the shorter period in the first time-period assigned to the class, and computing a difference between the inner product of the average energy use per class for the first sequence and the duration per class for the first sequence and the inner product of the average energy use per class for the second sequence and the duration per class for the first sequence.