Abstract: A system of monitoring energy usage for heating or cooling separate units in a multi-unit structure employing thermostat means to control delivery of hot or cold fluid to the unit, an elapsed time meter, a first circuit interconnecting the thermostat with the power source and meter and a second circuit in parallel with the first interconnecting the means to control delivery of the hot or cold fluid to the unit with the elapsed time meter to provide a tamper-proof, economically installable and reparable system. The system will not meter time of usage when insufficiently hot or cold fluid is delivered to the occupancy unit even when called for by its thermostat. The system also includes a method of determining energy usage in each individual unit using the elapsed time meter reading.