BAS (Building Automation System) and BMS (Building Management System) are terms for the same thing. A computer based centralized system that monitors and controls a buildings heating, ventilation, and air conditioning systems (HVAC). Energy Management systems (EMS) are typically a system of computer-based tools used by operators of an electric utility grid. However, years ago the term started being used in building control when a system monitored a buildings electrical demand and performed energy reduction functions.
A BAS/BMS usually consists of a few to many distributed computer-based controllers/modules that have “Input Points” to monitor temperatures, pressures, and the status of open or closed contacts. They also have “Output Points” to either vary (modulate) a signal to devices such as valves, variable speed drives, and dampers or make/ break binary contacts to turn something on or off. They have “Variables” such as setpoints, alarm points, and timers that programs use to control the HVAC equipment.
Today almost every BAS/BMS can also function as a building’s EMS by just adding a few more points (Inputs/Outputs). Input points are required to monitor the building utility usage (gas and electric) and the BAS/BMS requires the output points to reduce its use by turning off noncritical equipment or changing setpoints to reduce the load of operating equipment.