Don't meter the house. Meter the circuits that cost you money.

The bill arrives and the number is bigger than last month. You look at the total and learn nothing.

A single meter at the main DB answers one question: how much did the whole house use? It cannot tell you whether the geyser is running at 3am, whether the pool pump is doing 14 hours a day, or whether the guest wing air conditioner has been left on since December. The total hides the culprit.

That is the case for submetering. You measure the circuits that matter, not the house.

Why KNX is a good place to do it

KNX already runs through the DB. The actuators are there, the bus is there, and the logic is there. A meter that talks KNX puts its readings on the same bus as everything else, so a number can trigger something without a second system in between.

Think of it as hiring an intern to read meters versus having a manager who reads them and acts. The intern writes the number on a clipboard. The manager sees the geyser pulled energy outside its schedule and sends an alert.

Three kinds of meter, three kinds of job

I read through the KNX Association’s energy management series, the Ekinex and Hager product documentation, and a KNX metering overview document. The products fall into three groups.

Direct-connect meters. The Hager TE360 is a three-phase kWh meter with a KNX output, rated for 100 A direct connection, seven DIN modules wide. The Hager datasheet lists 230/400 V AC, a bus current of 8 mA, and PTB certification. The metering overview document I used describes the TE360 as Class B, with 1% accuracy for direct measurement up to 100 A. It can send total, partial and current consumption to the bus periodically. This is the one you use at the incomer or on a big dedicated feed.

Current transformer (CT) meters. The Ekinex EK-MS1-TP-TA is a sub-meter that measures through external CTs, and it does not connect directly to the load. It is a four-module DIN device that measures active, reactive and apparent power, power factor, current, voltage, and imported and exported energy. It can run as three independent single-phase channels or one three-phase line. The measuring range is 0.2 A to 120 A, depending on which CT you order (5 A, 20 A, 60 A or 120 A versions are available, ordered separately). The Ekinex documentation puts the CTs under 2% for active energy at a power factor between 0.75 and 1. The meter classifies as PMD-II under IEC 61557-12.

Pulse and bus interfaces. If a meter already exists, a pulse counter or an interface can bring its readings onto KNX. Hager lists the TXF121 as a KNX interface for electrical energy meters. The metering overview document also describes an S0 pulse counter that handles up to two meters, and an M-Bus gateway for heat, water and gas meters. A pulse counter arrives unprogrammed, so you configure it in ETS. Each M-Bus value needs its data format set correctly, otherwise it can be sent in the wrong form.

What I would actually meter in a Johannesburg home

Not everything. A meter on every circuit is expensive and nobody reads the output. Start with the loads that are big, scheduled or easily forgotten:

1. The geyser (or heat pump) circuit.
2. The pool pump and any water features.
3. The air conditioning or the HVAC feed.
4. The solar or inverter incomer, if there is one, so you can see import and export.
5. Any detached unit: garden cottage, guest wing, workshop.

A CT meter with three channels covers three of these from one DIN device. That is a sensible budget answer for most houses.

Measure, then let KNX act on it

This is where a KNX meter earns more than a standalone display. The Ekinex meter has up to 8 comparators that trigger when any measured value crosses a threshold, and 8 logic channels that combine them. So you can build a rule such as: if the pool pump circuit draws above its normal current, send a fault flag. If the geyser draws power outside its heating window, flag it. The same threshold can drive a visualisation alert or a scene.

Cumulative energy can also be sent at the end of each day, week, month or year, which gives you a clean log without flooding the bus. Which brings me to the part people forget.

Bus load and sending rates

Meters are chatty if you let them be. Every power reading sent every few seconds is a telegram the line has to carry. Set sensible cyclic intervals and use change thresholds, rather than sending everything constantly. I covered how many telegrams a line can carry in an earlier post on bus load, and the same rule applies here: decide what really needs to be live. Instantaneous power for an alert might. A monthly kWh total does not.

Things to settle before the DB is built

CT meters need space for the CTs around the conductors, and the CT range has to match the circuit. A 5 A CT on a 40 A circuit will not work. The Ekinex meter also takes no measurement for the first 10 seconds after power-on, so do not be alarmed by a blank reading after an outage.

Decide the following with the electrician before the board is wired:

• Which circuits get a CT, and which CT size each one needs.
• Where the meter lives and how its mains supply is protected. The Ekinex device has a 230 V logic supply as well as the bus connection.
• What happens with the data: a visualisation, an alert, or both.
• Who owns the thresholds, so the rules reflect how the home is really used.

What I have not verified

I did not find datapoint types listed in the Ekinex or Hager product pages I read, so check the ETS product database before you promise a specific integration. I have also not tested these meters against Eskom or City Power billing, and a KNX sub-meter is not a replacement for the utility’s own meter. It is a way to understand where your usage goes.

The conversation to have early

If you are building or renovating, ask the question while the DB layout is still on paper: which circuits do you want to understand? Adding a CT around a cable at design stage costs very little. Retrofitting it into a finished board costs a lot more.

Just a conversation about what the project needs. wayne@knxlogic.co.za | 082 564 3982 | www.knxlogic.co.za

Sources: KNX Association, “Energy Management with KNX: Part 1” and “Why you should add a smart meter to your smart home” (knx.org); Ekinex EK-MS1-TP-TA datasheet (ekinex.com); Hager TE360 and TXF121 product pages (hager.com); “Smart Metering with KNX” overview (KNX/Chipkin PDF).