Thermal energy meters measure the energy content of liquid or gas flows in British thermal units (BTU), a basic measure of thermal energy. One BTU is equal to the amount of energy needed to heat one pound of water one degree Fahrenheit. Examples of energy sources include coal, electricity, natural gas, distillate fuel oils, residual fuel oils, and solar energy. Thermal energy meters are available in digital and analog designs, and differ in terms of product specifications, applications, and features.  Some products are lightweight and portable. Others are designed to work in bi-directional dual mode so that the recorded reading is displayed on a control system. When used with ovens, boilers and furnaces, thermal meters measure the thermal energy added to an area. When used with cooling systems such as air conditioners, however, they measure the thermal energy that is removed. 

Product specifications for thermal energy meters include temperature range, environment or flow temperature, temperature sensor type, differential temperature accuracy, battery life (if applicable), working current, and hibernating current. Dimensions such as length, width, and weight are also important to consider. There are several different temperature ranges for BTU meters. For liquid flows, they include liquid temperature range, optional liquid temperature range, and ambient temperature range. BTU meters for gas flows may also have multiple temperature ranges. Solid-state temperature sensors may be calibrated according to NIST-traceable temperature standards.  Electrical parameters cover input power, internal power supply, and wiring specifications.  

Thermal energy meters are used in chilled water, condenser water, and hot water systems for both commercial and industrial buildings. In office buildings, they are used to bill tenants for energy usage and to predict energy demand. In institutional settings, energy measurement devices are used for site-wide energy cost allocation. Thermal meters are also used in central plant monitoring, both on university campuses and across industrial facilities. In addition to their applications with furnaces, boilers, and heating, ventilation and air conditioning (HVAC) systems, BTU meters are used by the power generation industry. Devices such as BTU analyzers are used in the production of fungicides and herbicides to monitor the calorific value of components in the vent gas collection system.