How does the “static” 3-phase electricity meter work with solar panels?

Introduction

This article of the smart meter below is my view of the smart meter. I am not an electricity specialist, and I would appreciate hearing if there are any possible errors in my description. Below is based on the system used in the Netherlands. But electricity meters will be the same or similar in other countries. Many buildings in the Netherlands have a two phases for many decades.  But new buildings have now often 3 phases.

I have tried to obtain official responses from grid operators confirming that smart meters do indeed register the instantaneous power. Many are unable to confirm this. I also did not get any wiser through, for example, the universities of Delft and Twente. However, I am now sufficiently convinced myself to share my view.

Summary:

If solar panels are installed with a 3-phase connection, it does not matter (for the electricity bill) to which phase they are connected: the meter determines the total power of the 3 phases at any given moment. This applies now and in the future, when price differentiation is introduced between energy used and energy fed back into the grid.

Explanation

When billing for electricity, the price of electricity is determined by the amount of energy drawn from the grid and the amount generated by for example, solar panels. The energy is expressed in kilowatt-hours. 1 kilowatt-hour is 1000 x 1 Watt x 3600 seconds = 3.6 x 10^^6 Watt-seconds = 3.6 x 10^^6 Joules.

Smart electricity meters do not measure energy directly. They determine the total “instantaneous” power of the 3 phases by measuring voltages and currents. The product of current (I) and voltage (V) is the power. For the 3 phases, the power is the sum of I x V expressed in kilowatts. If solar panels supply energy, one of the terms of the sum is negative if more is produced than consumed on that phase.

The energy is now calculated by taking many measurement points of this fluctuating power on the three phases and multiplying this by the duration.

Naturally, the rule is: the more measurement points, the more accurate the meter. In fact, therefore, the integral of the power over time is calculated. It is more appealing to say that the area is calculated under the graph of the power over time. This is a familiar graph for people checking how much energy their solar panels produce. The graph for the smart meter therefore indicates the combined power of the 3 phases, and this is comparable to the graph for the solar panels. If the graph shows a negative value, it means that more energy is being produced than consumed. The meter then registers a return to the grid (and in no other situation).

If solar panels are installed, it does not matter which phase they are connected to, because the meter determines the total power on the 3 phases at any given moment.

How do the counters work with the three-phase meter?

We will now explain how the smart static meter controls the counters. We are doing this because the Dutch Consumers’ Association’s explanation is technically incorrect at the time this was written and, moreover, confusing. There are other meters on the market, but most smart 3-phase meters have 4 counters: 2 counters per tariff, 1 for supplied energy, and 1 for returned energy.

• Meter A tariff 1 = indicates the positive sum of delivered – produced energy at tariff 1
• Meter B tariff 1 = indicates the negative sum of delivered – produced energy at tariff 1 (i.e., when more is produced than consumed)
• Meter C tariff 2 = indicates the positive sum of delivered – produced energy at tariff 2
• Meter D tariff 2 = indicates the negative sum of delivered – produced energy at tariff 2 (i.e., when more is produced than consumed)

For convenience, we only use meters A and B at tariff 1 in this example.

The meter is essentially a power meter and measures the entire capacity of the network behind the meter at a given moment, including the production of the solar panels, if present. The measurements are taken, say, 60 times per second. If it is positive, this number is multiplied by the time (in this case 1/60) and added to counter A. If, at the next measurement, this sum is negative because more is produced (by solar or so) than supplied, then this number is multiplied by the time and added to counter B.

In this last case, the value is therefore NOT deducted from meter A (and thus NOT offset against previously delivered energy).

This process is repeated continuously and therefore provides, for each tariff, the delivered energy (consumed – produced energy) and the returned energy (produced energy – consumed energy).

The meter displays the values ​​in kilowatt-hours, so actually we should convert the value above into hours rather than seconds. That does not change the principle.

The calculations performed in the three-phase meter and the energy consumption of the meter itself.

A small computer (PLC) handles all calculations and communication. Naturally, the meter consumes energy, which is in the order of 10 or 20 Watts per second and is therefore not negligible.

Each volt measurement is a maximum of about 2 watts and each current measurement is a maximum of about 5 Watts (depending on the current). For each phase, there is one of each. So 3×7 plus another 10 watts for the computer. So in this case, that quickly adds up to 31 Watts. It is in the order of 1 or 2 Euros per month. Different meters use different amounts of energy. The smarter the meter, the more energy it consumes. I am just assuming for a moment that the meter registers this energy as consumption. That is not necessarily the case: it depends on whether the connection for the meter itself is located before or after the measuring equipment. Given my experience with manufacturers and network operators, it will be practically impossible to get an answer to this. But it is actually simple to measure: connect the unloaded meter to the 3 phases and see if it runs. In other words: make sure everything is turned of. the easy way to do that is to switch off all fuses and panels.

Units used

As primary sources I used:

https://www.electrical-engineering-portal.com/three-phase-power-measurement

Landis & Gyr Unofficial confirmation of 3-phase meter summary 14/2/2019

Specifications of smart meters

Unofficial responses in the Netherlands: Liander and Enenco (no official confirmation could be obtained from these sources. At Liander, they even bluntly hung up on me.)

The balance with three phases

See : https://www.hoogspanningsforum.com/viewtopic.php?t=401&start=10 ( still exists in 2026)

In the low-voltage network, 4 conductors/wires are used. The neutral is called the ‘nought’ or zero here in the Netherlands.

The transformer in a substation where medium voltage (usually 10 or 20 kV) is transformed into low voltage (400/230 V) has 3 connections on the medium-voltage side for the 3 phases. On the low-voltage side, there are 4 connections: one for the 3 phases each and one for the neutral.

The neutral not only allows for the possibility of two voltages in a network but also provides an extra conductor to compensate for differences in current if more current flows through one phase than through the other. The system then returns to equilibrium through the current via the neutral.

In short, the current in the neutral is the negative sum of the 3 phase currents. To put it simply, one could say that the user has to pay the electricity company for energy consumption if the current in the neutral wire flows in one (negative) direction, and the user pays if the current in the neutral wire flows in the other (positive) direction (energy fed back).

An example of what I consider to be incompetent advice from VoltaSolar (outdated now)

https://www.bespaarbazaar.nl/kenniscentrum/zonnepanelen/de-praktijk/zonnepanelen-aansluiten-meterkast/  the link now (2026) goes to https://www.coen-plus.nl/ It looks like they now only give product information.

They stated at the time:

If you have a 3-phase main connection, you can use a 3-phase inverter. The current from the solar panels is then nicely distributed across the 3 phases. It is not advisable to connect capacities above 5000 Wp to one of the 3 phases. You can then only supply solar power to the devices connected to the same phase and will feed more power back into the grid. This is an undesirable situation in the long term, because the compensation for feed-in may decrease over time.

Current does indeed flow back via the “solar phase”. However, this is irrelevant because the smart 3-phase meter only looks at the energy supplied to the user per unit of time (across the 3 phases combined). The meter only shows that energy is being fed back into the grid if less power is used on the 3 phases combined than the solar panels supply. The energy company is not interested in the consumption between the different phases individually. It must be noted, however, that there is protection (provided by fuses) regarding the maximum current per group. With a large number of panels, this can be a reason to distribute the energy across 3 phases. However, this costs money because the distributors consume energy, quite apart from the costs of the distributors and the installation costs. Therefore, the idea that you save money is nonsense.

See the very nice video: . https://www.youtube.com/watch?v=vh_aCAHThTQ (less than 6 min)

To put it simply, you could say that you have to pay the electricity company for energy usage at a certain moment if all your appliances together use more energy than your solar panels generate. If your solar panels generate more energy than you use on three phases, the meter registers an energy return at that moment. The meter registers either a return or usage. Therefore, the meter can register no usage and a return at the same time at a given moment.

To explain this, I will give an example using a scale:

You have a scale with two counters. Counter 1 indicates when you place something on the scale, and counter 2 indicates when you pull the scale up with a string. You create the “negative” weight using a small pulley that reverses the force: on one side of the pulley, the string goes to the scale, and on the other side hangs a container where you can place a stone, which then pulls the scale up. You place 3 stones on the scale: a 1 kg stone, a 2 kg stone, and a 3 kg stone. The meter displays 6 kg in counter1. If I attach a string to the 3 kg stone and apply a counter-“force” of 6 kg (actually 6 * 9.8 Newtons), counter1 will show zero. If I attach 3 strings to the three stones and apply a counter-“force” of 6 kg, I get the same result. The smart meter works exactly the same way, but you replace kg with power. The scale never indicates a 3 kg weight and a minus 3 kg pulling weight simultaneously. It is all abo