Here I need some help from someone who hopefully knows the official terms for what I’m about to describe. I will describe a scenario, and then two ways in which the total energy (bought and sold) is calculated.
Imagine a three phase system, and for the sake of simplicity I’m going to leave L3 disconnected. Now imagine I have a PV-inverter pushing 1kW into L1, and I have a 1kW load on L2 at the same time. The total power consumed is therefore zero, since the production on L1 cancels out the consumption on L2.
Accumulation method A: Take the total power over all phases (0W) and integrate over time. After one hour, I have bought 0kWh from the grid, and I have sold 0kWh to the grid.
Accumulation method B: Integrate the power of individual phases over time, and add them together to get a total. After one hour, I’ve bought 1kWh from the grid, but I have also sold 1kWh to the grid.
I want to know if there are official names for these two methods. Surely there must be…
Indeed, I noticed that some suppliers call it “total net metering”, vs “total of import/export metering”. Or something like that. That’s what I am going with for now.
Whichever way they measure it make sure you know which meter/setup the utility is using…
Once you know the power meter they are using then you can always go direct to the vendor to get the lowdown…
I’m sure you have seen this already, but these two methods describe the difference in operation of the Carlo Gavazzi EM24 and ET340 3-Phase Energy meters. See point Q9 at the following link
Any idea which method Eskom 3-Phase meters use? I have a 3-Phase connection, and an EM24, but am not sure I want to risk tripping my pre-paid meter by exporting on one phase to compensate consumption on another phase.
Edit: I have a Landis and Gyr Three-Phase Split Prepayment Meter
I have no idea what method they use unfortunately. Apparently most parts of Europe uses the net-total way, while everywhere else it is less clear.
I’ve had one case in New Zealand where a guy had a two-out-of-three situation, he had two incoming phases, but they are not split-phase like in North America, it’s just two phases out of the three that is available in his neighbourhood. In his case, he was billed per phase (not over the total).
Then of course there are several places in South Africa where three single-phase meters are installed side by side and you pay on the total. If the meters have reverse lock-up (as many do), you again cannot feed into one phase to compensate on another. Well you can, but there is no monetary advantage
From a basic principles argument, power is merely energy divided by time (a watt is literally a joule per second), so the real energy must be the actual real power you are consuming at the time, integrated over time. The real power, in a three phase system, is actually the average current multiplied by the average phase-to-phase voltage, multiplied by the power factor and the square root of 3 (Watt’s law).
When feeding power into one of the phases, the average current must be lower, and therefore your power should reduce, which must reduce the energy bill. So in my opinion the total-net method is the correct one that everyone should use…
I’m just stating the analytical solution. For the sake of argument, let’s just assume that cos(theta) is 1. My argument centers around the “average current” part of it…
Of course we don’t actually calculate it in this way in the hardware. In the hardware we use some kind of newtonian integration running at some kind of sample frequency, accumulating a total into some kind of memory location that represents the energy. In other words… we estimate it… cleverly. Especially given the non-linear nature of many of our modern loads.
