How to meter thermal energy on a vrf fan coil

We developed a tenant sub-metering thermostat which is used in apartment buildings to meter potable water, electricity use as well as heating and cooling using 2 or 4 pipe hydronic terminal devicesor radiant floors. We've been approached by a major VRF manufacturer's distributor to adapt our product to do the thermal energy metering for every DX head in large apartment building installations. So I need to find an economical BTU meter for DX terminal devices - any ideas on different ways this might be accomplished?