1. A building management system in a building, comprising: a plurality of sensors distributed throughout the building and configured to measure one or more environmental variables, the plurality of sensors comprising a temperature sensor that measures a temperature variable and a humidity sensor that measures a humidity variable, wherein the plurality of sensors generate sensor data representing the measured environmental variables; a controller comprising a communications interface configured to receive the sensor data from the plurality of sensors and a processing circuit coupled to the communications interface; the processing circuit comprising an enthalpy module configured to calculate an enthalpy variable using the temperature variable and the humidity variable; the processing circuit further comprising a regression analysis module configured to generate an energy usage model for the building using the calculated enthalpy variable, wherein energy usage model includes one or more model coefficients based on the sensor data; the processing circuit further comprising a facility monitoring module configured to automatically identify a change in the building's energy usage model using new sensor data received from the temperature sensor and the humidity sensor, wherein the energy usage model expresses the building's energy consumption as a function of one or more predictor variables and the one or more model coefficients, and wherein the change in the building's energy usage model is a change in at least one of the model coefficients resulting from the new temperature sensor data and humidity sensor data; wherein the facility monitoring module comprises: a test statistic module configured to compare a coefficient of the energy usage model to a calculated energy model coefficient for a more recent time period and to use a difference between the model coefficients to create a test statistic, a critical value module configured to determine whether the test statistic exceeds a critical value and to determine that an identified change in the building's energy usage model has occurred in response to the test statistic exceeding the critical value, and a reporting module configured to communicate the identified change in the building's energy usage model to a module for initiating an adjustment to the energy model for the building in response to the identified change; the processing circuit further comprising a building control module that uses the adjusted energy model to generate and transmit control parameters via the communications interface; and one or more mechanical building devices that affect an environmental variable inside the building by operation of one or more electro-mechanical or hydraulic elements in response to the transmitted control parameters.