Methods and systems are described for detecting a wide variety of fault conditions in a HVAC system based on analysis of current and historical energy consumption patterns, and on comparison with energy consumption patterns of other similarly situated RTUs. Energy consumption comparisons are preferably made in regard to a normalized load ratio or NLR or more preferably to a daily maximum normalized load ratio or MDNLR, which provide more robust and reliable bases for comparison of faulty and fault-free RTUs, and hence for generalized fault detection, than other previously known metrics or criteria.