What is the 'right' way for a 3-Phase Watt Meter to determine import-export Watt-hours?

Question

What is the correct way for a 3 Phase Watt-Hour meter to measure total import/export watt-hours?

I ask because I have seen this done in 2 different ways both in 'approved' meters:

Assume that:
Red Phase - importing 1kW
White Phase - importing 2kW
Blue Phase - exporting 1.5kW
Assume this is constant for 1 hour.

What should the Watt-Hour register record?
Option A - Add up phases before adding to totaliser:
1 + 2 - 1.5 = 1.5 kWh Imported

Option B - Add import to import, export to export:
1 + 2 = 3 kWh Imported AND 1.5kW Exported

I have seen meters and solar-loggers use option A and more recently meters doing option B.

What is 'right'? I can't seem to find where this calculation is specified in IEC 62053 - but I don't have $1000 to spend getting each part of it, which may or may not have the calculation specified.

Answer 1

You are correct, their are two ways 3-Phase Meters can determine Import/Export Wh totals. I am a Electrical Engineer and have programmed EDMI smart power meters for customers in NZ who provide reconciled Import/Export kWh data to retailers for billing purposes.

Option A is sometimes referred to as Net Metering and Option B, Gross Metering.

Net metering is only really useful if you have a deal with your Power Company where you can buy energy off them for the same rate that they will buy it off you. This doesn't happen very often, as in practice it costs them more to generate and distribute the energy than it will cost you. It is also illegal in some countries to take the net totals in the meter. In my opinion you are better keeping the gross totals in the meter and then performing the net calculations downstream in the software reading the meters i.e. Option B.