What is inside a Victron home battery system?

A Victron home battery is a set of parts around one inverter-charger that can form its own grid. As a 3-phase home battery it keeps the whole house running in a power cut, but only the circuits wired to its output. Which parts you need, and where each circuit goes, decides whether that works.

You live in a large house and you have a quote for a "Victron home battery set, 3-phase". The list reads like a spare-parts catalogue. A Belgian home owner shared his own list on a forum. It named three MultiPlus-II units, a Cerbo GX, two Lynx busbars, an energy meter and about 25 kWh of batteries. Then he asked the forum for the wiring diagram.

That is the real question. A parts list does not tell you what keeps running when the street goes dark. A Dutch knowledge thread on Victron off-grid systems puts it plainly. If you connect only the input, you get no off-grid at all; the house must hang on the output. That choice is made on the switchboard, long before the first outage.

In Zele we built a classic three-phase home battery with an off-grid mode. When the street loses power, both the solar panels and the battery keep working.

Which parts make up a Victron home battery system?

Victron calls a grid-connected home battery an Energy Storage System (ESS). The minimum is one inverter-charger (MultiPlus or Quattro) and one GX controller; the other parts are added as the house needs them.

Around those parts come the cables, DC fuses and busbars, and the protection on the switchboard. None of that is on the datasheet, yet it decides how safe and how strong the system is.

Why is the inverter-charger the heart of the system?

The inverter-charger works in both directions. It charges the battery from the grid or from solar power, and it discharges the battery into the house.

It also guards your connection. With PowerControl, a maximum grid current is set and the charger only uses what is left. With PowerAssist, the battery adds power for a short peak that the grid or a generator cannot deliver alone.

The transfer switch is the relay inside the unit that connects the house to the grid, or cuts it loose. Its rating limits how much current can pass from the grid to the output. In a power cut, the continuous power is what your house can draw, whatever the size of the battery.

What happens in the first 20 milliseconds of a power cut?

The inverter-charger notices the grid is gone, opens its transfer switch and forms its own grid. Victron states that this happens in less than 20 milliseconds, fast enough for computers to keep running.

Disconnecting is not optional. A home battery that runs parallel to the grid must have anti-islanding: it may never keep a dead street line live. That is why a normal solar inverter simply stops in a power cut.

Victron itself says ESS is not meant for systems without any grid connection. The same hardware can run a house fully off-grid, but with another configuration. Read which home battery really works off-grid if that is your goal.

Which circuits keep running: the output or the grid side?

Only the circuits on the output of the inverter-charger stay powered in a power cut. Everything on the grid side of the unit goes dark with the street.

Victron gives a water heater as the example for the second output. Many owners also keep the car charger on the grid side, so a car cannot empty the battery during an outage. Full back-up means the whole house board hangs on the main output. How that board is split, earthed and protected is in our guide to how a full no-break house is wired.

Why do your solar panels keep producing in a power cut?

Because the Victron unit makes a grid of its own, and solar equipment on that grid keeps working. A solar inverter on its own must stop the moment the grid fails.

Converting solar power from DC to AC and back costs energy: Victron mentions losses of up to 20 to 30%. Owners on forums also report that some brands react badly to frequency shifting. Already own an SMA or SolarEdge inverter? Read how to add a battery to an existing solar installation.

What changes in a three-phase house?

A three-phase Victron system has at least one inverter-charger on each phase, set up as one three-phase system. Three-phase loads such as a heat pump can then hang on the output and keep running in a power cut.

Each phase only gets the power of its own unit. Three MultiPlus-II 48/5000 units give 4,000 W continuous per phase, not 12,000 W on one phase. For more power, up to six units can run in parallel on each phase. How to split the loads over the phases is explained in our guide to a three-phase home battery with full back-up.

The Belgian grid rules count too. Synergrid C10/11 treats a battery in production mode as a production unit. On a three-phase connection, a small installation has at most 10 kVA of storage. Three units of 5 kVA already pass that on paper, so the design and the paperwork must take it into account. Read how to register a home battery with Fluvius.

Our three-phase home battery in Kortenberg gives full back-up in off-grid mode, with 9 kVA and 30.8 kWh. In Wetteren the entire three-phase house runs on the battery, with 9 kVA and 15.4 kWh.

What do the GX controller and the battery management system decide?

The battery management system (BMS) protects the cells; the GX controller decides how the whole system behaves. They talk to each other over a data cable.

Lithium batteries with a CAN bus connection, such as BYD or Pylontech, report their own state of charge to the GX. Victron advises against adding a second battery monitor next to them: two monitors only conflict.

On the GX you choose how much of the battery serves the house every day and how much stays ready for a power cut. Victron gives an example: 30% for self-consumption and 70% kept as back-up. You can also keep the battery full and use it only when the grid fails.

The GX also starts a back-up generator automatically and sends everything to the free VRM app and portal. For a power cut of several days, read how a generator recharges your home battery. Our energy management software runs on the same controller. Two ways to use that controller further are Dynamic ESS on quarter-hour prices and Home Assistant with a Victron battery.

What does a Victron set leave out that a working house needs?

A set gives you the parts; a working house needs a design. The questions below are not on a shop page, but they decide whether the back-up works on the day it counts.

• Which circuits go on the output? And which stay on the grid side, like the car charger.
• Is the inverter power right for the house? The battery size never raises the kW in a power cut.
• Does the solar power fit the factor 1.0 rule? Or does part of it need an MPPT charger?
• How is the earthing handled in island mode? Your residual current devices must keep protecting you.
• Are the cables and fuses sized for the current? A 48 V battery carries high currents.
• Is the inverter on the Synergrid list, and is the system reported and inspected? In Belgium, both are needed before it runs parallel to the grid.

Want to see how the pieces fit in an off-grid building rather than a house? Our overview of off-grid system components goes through them one by one.

How do we build a Victron home battery that keeps the house running?

YouPower is a certified Victron Energy partner and installer. We design the system for the house first, and then choose the parts.

1. Step 1: We measure your consumption and peaks, and list the loads that must keep running.
2. Step 2: We choose the inverter power per phase and the battery. On 3 x 400 V we install three inverters, one per phase.
3. Step 3: We split the switchboard into grid side and back-up side, and connect the main systems to the battery.
4. Step 4: We connect your solar panels, new or existing. We can connect almost all inverter types to the home battery.
5. Step 5: You follow the system in the free VRM app and portal, locally or from anywhere.

Our team has three certified battery experts, who also maintain and repair existing battery systems. Want to know what a back-up battery does first? Read about a home battery with backup power, or see our home batteries for large homes.