A BEMS is a Building Energy Management System and is an extension of a traditional Building Management System (BMS). A BEMS is a computer-based and controlled system that monitors and controls energy usage within a building's electrical and mechanical systems including lighting, critical power systems, critical cooling, HVAC (heating ventilation and air conditioning) fire systems, access and security. The purpose of a BEMS is to reduce operational costs and aid decision making when it comes to the optimising energy usage.