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In my home I have an 3-phase electronic energy meter. I have both solar panels and consumers in the house.

I am wondering what is registered when for example I have this situation:
On L1-L2 I use 2 kW
On L1-L3 I feed back 1 kW

After one hour, has the meter registered:
Consumption from the grid 2 kWh, feed back into the grid 1 kWh, or
Consumption from the grid 1 kWh.

Both consumption and feeding back occur at the same instant. I know that summation over a certain period of time does not work with electronic meters. If I consume 2 kWh during the night and feed back 1 kWh during the day, it is clear that the consumption is 2 kWh.

Technically, an electronic energy meter is able to measure and register both options.

My home installation is 127V/220V, 3 phase. The solar inverter is connected to 220V, so between L1-L2. Almost all consumers are connected to 220V as well, so L1-L2, L2-L3 or L1-L3. This is not a situation like in Europe where you only have 1 phase 220V on which everything is connected. Or the USA where 120-0-120 split phase.

I cannot ask my power company. They won't disclose this information as they discourage the use of solar panels. (Yeah, right!)

FWIW, the energy meter is a Landis+Gyr E230. In their brochure L+G does not provide an answer to this question. So I assume it is a common method these meters use. But which?

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Modern energy meters take advantage of Blondel's theorem which allows them to measure energy with one less current measurement than the number of wires in the system.

So in your three phase, four wire system, only three current transformers (CTs) are used. This means that the meter directly measures and records the total energy in the system, whether incoming or outgoing. Most modern meters have two registers, one which records incoming energy, and the other records outgoing.

Technically, an electronic energy meter is able to measure and register both options.

Not quite. Because there's one less CT than the number of wires, the meter can only calculate and infer the per-phase energy usage, not measure it directly. As a calculation after-the-fact it does not have the required accuracy to serve billing purposes.

In your example, if one phase is using a constant 2 kW and another phase is generating a constant 1 kW, after an hour the meter will register 1 kWh of energy usage by your home.

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  • \$\begingroup\$ So what you are saying is that because the meter uses N-1 current measurement, at any instant it can only calculate the total power. It cannot calculate the per-phase power. If the total power is negative for a period of time it is added to the outgoing register. If it is positive, it is added to the incoming register. But always the total over all 3 phases. Not per phase. \$\endgroup\$ Commented Jan 29 at 18:53
  • \$\begingroup\$ @JohannesLinkels that's basically correct, except that the meter is directly measuring energy in fixed time intervals. If power is recorded or reported, it is calculated by integrating over the sample time. \$\endgroup\$ Commented Jan 29 at 20:16
  • \$\begingroup\$ OK, it is the per-phase vs totals-over-all-phases that is important. Thanks. \$\endgroup\$ Commented Jan 29 at 23:19

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