The document describes the FLUXUS BTU, a thermal energy metering system from FLEXIM that provides highly accurate measurement of flow rate, temperature, and thermal energy in applications such as district energy systems and HVAC. It uses ultrasonic clamp-on transit time flow measurement and temperature sensors with an accuracy of better than 0.1 K. The system provides measurement of thermal energy output, flow velocity, and other quantities with a repeatability of 0.15% and an accuracy of 1.2% or better.