What happens to your smart home in a power cut?

A Loxone or KNX home keeps working in a power cut only on a battery with back-up. Its controls, its network and the loads you need must all run on that back-up output. A smart home battery then does two jobs. It keeps the house powered, and it tells the smart home that the grid is gone, so the automation can switch off heavy loads.

You have a large house with Loxone or KNX. Lights, blinds, heating, the gate and the alarm all run through one system. Then the power fails for an hour. The switches on the wall do nothing, the blinds stay where they are, and the app on your phone shows "offline". On Reddit, a Belgian owner runs "everything that can be automated" on KNX. The next reader asked the real question: which battery inverter can you actually control?

What most installers do not tell you is that a smart home is more fragile in a power cut than a classic house. In a classic house, a light switch works as soon as the circuit has power. In a smart home, that switch is a bus device: without the bus supply, the light stays dark even when the circuit has power. After the power returns, KNX actuators may need seconds to recover, and scenes restart in a state nobody chose.

In Kapellen the owner works in the sector and wanted the latest technology. Think of phone alerts when the battery is full, charging from the grid and full, clear data. The 5 kVA system has a 15.4 kWh battery.

What happens to a Loxone or KNX smart home in a power cut?

Without back-up, the whole smart home stops at once: controller, bus, network and app. With back-up that only covers the 230 V loads, the power is there but nothing can switch it.

The behaviour of KNX devices on a bus failure is set by the integrator. In one ABB switch actuator manual, each output can be set to a fixed position when the bus fails. After the bus returns, those actuators need about 10 to 30 seconds before they can switch all contacts again.

Which parts of your smart home must be on the back-up board?

Start with the parts that let you control the house, then add the loads you want to keep. The controls draw little power but decide whether anything else can be switched.

• The bus power supply of KNX, or the 24 V supply of Loxone with the Miniserver and extensions.
• The network core: router, switches and Wi-Fi access points that the app and the battery link use.
• The battery controller itself, so the smart home keeps reading the battery (on Victron: the GX device).
• Alarm, cameras and access control, so they do not depend on their small internal batteries.
• The loads you really need: lighting circuits, gate, blinds, pumps, heating controls and, if power allows, the heat pump.

In a full no-break house, the whole distribution board sits behind the inverter, so this question solves itself. Read how a full no-break house is wired. It covers the back-up board, the changeover and the earthing in island mode (running on its own, without the grid). Want the heat pump on the battery too? First check how a heat pump runs on a home battery in a power cut.

Is a small UPS or the Loxone back-up supply enough?

For short cuts and for the controls only, yes. For a house that must keep working, no. A UPS or a 24 V back-up supply keeps the brain alive, but your lights, gate and heating run on 230 V.

Loxone describes its Power Supply & Backup as a DIN-rail supply of up to 40 A with seven 24 V outputs. It takes a 36 V lead-acid battery for back-up and switches off below 31 V to protect it. It keeps the automation alive, not your 230 V loads. The two can be combined: the 24 V back-up protects the controls, and the home battery carries the house.

How does a home battery talk to Loxone or KNX?

Over the local network, with an open protocol such as Modbus TCP or MQTT. On a Victron system, the GX device (the controller of the battery system) offers both.

Both services are switched off on the GX device until someone enables them. For Modbus TCP, you choose read-only access or write access. Give the smart home read-only access unless it really must write settings. The battery keeps its own protection and charging logic; the smart home reads the data and switches loads. For the same reason, keep the battery link on your own network and do not open it to the internet. Our article on inverter and battery cyber security explains why, for companies and for homes.

KNX is an open standard with more than 500 manufacturers. That is its strength, and also why every link is designed per project. The Reddit reader quoted above put it plainly: can you only read the battery, or also really control how much it charges and discharges?

How does the smart home know the grid is gone?

The battery system tells it. Victron has a grid failure alarm that goes off when the system has not been connected to the grid for more than 5 seconds.

The alarm shows as a notification on the system and on the VRM portal, and it is available over Modbus TCP and MQTT. Victron recommends it for back-up systems. That signal is the trigger your smart home needs. Without it, the automation does not know it now runs on a limited battery.

Prefer a hard-wired signal as well? An electrician can add a simple relay on the grid side that drops out when the grid is gone. It feeds a digital input of the Miniserver or a KNX binary input. Some Victron systems are set to ignore the grid on purpose. There, the alarm reports a grid failure while the grid is there, so check this with your installer.

Which loads should the smart home switch off when the house runs on the battery?

Everything that empties the battery without being needed in a power cut. Think of the sauna, the pool heating, the EV charger and an electric water heater.

The Victron MultiPlus-II has a second output that is only live when the grid is there. It suits loads such as a water heater that should not discharge the battery. The smart home adds the finer choices on top. Read whether your home battery should charge your electric car before you decide on the charger. For outages of several days, a generator can recharge the home battery.

Who programs what: integrator, electrician or battery installer?

Three trades touch the same board, so agree on one diagram and one test before commissioning. Most problems come from each party assuming the other one handles the power cut.

How do we connect a home battery to a smart home?

We start from your loads and your automation, not from the battery. Then the back-up board, the battery link and the power-cut scene follow.

In Hasselt we connected the battery to existing solar panels and built its control panel into the distribution board. In Wetteren the whole house runs on a three-phase full no-break system of 9 kVA and 15.4 kWh. Everything keeps working when the grid fails. In Kapellen, the owner follows the system with phone alerts and clear data.

1. Step 1: We measure your consumption and list what must run in a power cut, including the controls of the smart home.
2. Step 2: We decide with you and your electrician which circuits and bus supplies go on the back-up board.
3. Step 3: We size the inverter and the battery, and set up the GX device with the grid failure alarm.
4. Step 4: We give your integrator the data link, read-only unless more is agreed, and the list of values to use.
5. Step 5: We test a real power cut together. You then follow the system in the free VRM app and portal.

Our team has three certified battery specialists. Want the battery to react to prices as well? See what our energy management software does, or see how our home batteries keep a large home running.