Adding to that: with careful calibration, your emonTx can probably be accurate to within 1%, but we cannot guarantee this. If that is acceptable to you and the users of the electricity, then OK. If not, then you could - provided it is acceptable to break into the circuit - install a “revenue grade” meter that has a Modbus output. Then the numbers that the Modbus provides can be sent on to your platform.
See 3 Phase Power Monitoring at an individual CNC Machine level
Long answer: Many people have tried very hard to solve this problem. No-one has yet had 100% success - or even come close. Some systems can get some of the appliances right some of the time, we have yet to hear of one that is always correct all of the time.