Yes, if the whole house must keep running in a power cut, including three-phase appliances such as a larger heat pump. A three-phase home battery backup covers all three phases; a single-phase system covers one. Only need a fridge, the router and some lights? Then a single-phase back-up on a few circuits does the job.
A large home on three phases, with a heat pump, an induction hob, an 11 kW car charger and a home office? The quotes then offer one single-phase inverter "which doesn't provide energy for the whole home", as a Belgian owner wrote on a forum.
In a home in Wetteren the whole house runs on the battery, on all three phases: 9 kVA, 15.4 kWh.
What does full back-up mean in a three-phase house?
Full back-up means every circuit, on all three phases, keeps running when the grid fails. Only a three-phase back-up does that; anything less is partial, which is fine when you choose it knowingly.
| Set-up | What runs during a power cut | Three-phase appliances? |
|---|---|---|
| Emergency socket or single-phase back-up box | One socket or a few circuits, on one phase | No |
| Single-phase inverter with the essential circuits moved to it | The essential circuits, all on one phase | No |
| Three-phase hybrid inverter with a back-up output | Whatever the brand allows in back-up | Check the datasheet |
| Three inverter-chargers set up as one three-phase system | The whole house, on all three phases | Yes, within the power per phase |
The datasheet decides: one back-up box on a three-phase grid backs up only one phase, at most 3,300 VA. Victron places at least one inverter-charger (inverter and battery charger in one) on each phase, configured as one three-phase system. Three-phase loads on its back-up output then run on the battery during a power failure.
A single-phase battery on a three-phase connection leaves two thirds of the house dark. Fixing it later often means rewiring the distribution board and a new inspection. Our guide to a home battery with backup power covers the three back-up levels: emergency socket, some circuits, off-grid capable. It also explains their switchover speed, the inspection and a night and a day on battery.
Which appliances in a large home need three phases during a power cut?
Mostly the big ones: a larger heat pump and a fast car charger. Lights, the fridge, the router and the boiler pump are single-phase.
| Appliance | Why three-phase | On the back-up? (our advice) |
|---|---|---|
| Larger heat pump | Often needs a three-phase supply because of its start-up power | Yes, if it heats the house; check its start current |
| Car charger of 11 kW | 11 kW needs three phases at 400 V and 16 A; a single phase stops at about 7.4 kW | Usually not: keep it on the grid side |
| Induction hob | A heavy load; with a heat pump and a charger it adds up fast | Yes, but plan which phase carries it |
| Lights, fridge, freezer, router, pumps | Single-phase | Yes, spread evenly over the three phases |
An 11 kW charger would empty a 15.4 kWh battery in under an hour and a half. The car can wait; the heating cannot. A Flemish owner on a community forum chose the same: the whole house on the back-up output, the charger on the grid side.
How much power per phase does a three-phase home battery backup deliver?
Each phase delivers what the inverter-charger on that phase delivers, and no more. Three Victron MultiPlus-II 48/5000 units give 4,000 W continuous per phase at 25 °C, so 12 kW for the house. That is 4 kW for the hob on phase 1, not 12.
Continuous power of a three-phase set of three MultiPlus-II units at 25 °C
Start-up peaks reach 9,000 W per 48/5000 unit; at 40 °C its continuous power drops to 3,700 W. So the battery room matters as much as the number on the quote.
The battery is shared, the power is not: in a power cut no phase can borrow from a quiet one. Some owners notice later that one phase carries far more load than the other two. A set too small per phase trips when the hob and the heat pump start on the same phase. So the distribution board spreads heat pump, hob, oven and dryer over three phases.
These five three-phase home batteries we installed each have 9 kVA. They show what stays on when the street fails: the whole house (full back-up, or "no-break") or the solar panels and battery (off-grid mode). Click a town for the details.
KortenbergFull back-up, off-grid mode
- Connection
- Three-phase
- Power
- 9 kVA
- Storage
- 30.8 kWh
Can a home battery give full back-up on a 3x230 V connection?
It is harder: much three-phase equipment is built for 3x400 V with a neutral, and 3x230 V has none. The usual three-phase back-up does not fit as it is. Three routes remain:
- A back-up on one phase, like the home battery in Beveren, connected single-phase because the street only offers 3x230 V.
- A switch to 3x400 V with your grid operator, where the street has it.
- A battery system that makes its own 3x400 V behind a weak grid. At a hunting lodge in Gedinne only 180 V was left on 3x230 V; our system brings it to 3x400 V, charges from the grid and has a generator for power cuts.
Not sure which connection you have? Our article on a 3x230 V connection that is too weak gives the grid operator's prices for a switch and how to check your voltage.
Which Belgian rules apply to a three-phase home battery with backup?
Synergrid C10/11, the rules of the Belgian grid operators, treats a grid-connected battery as a production unit and counts per phase.
- Small installation: up to 10 kVA of storage and 10 kVA of solar on three-phase (5 kVA each on single-phase) can count as small. Above that the simplified procedure does not apply.
- Imbalance: the power difference between phases may never exceed 5 kVA. A single-phase battery next to single-phase solar panels sits on the same phase.
- Multi-phase: a production installation above 5 kVA must be connected over several phases.
- Loss of one phase: a three-phase production unit must then disconnect from the grid.
Every battery is reported to the grid operator, but a typical home needs no grid study. In Flanders you report up to 25 kVA in Mijn Fluvius, with a valid AREI inspection (the Belgian electrical safety code). Above that, Fluvius runs a grid study.
Not every set-up handles losing one phase as well as losing all three, a German owner found in a forum test. Another lost a phase after a farmer mowed a meadow with loose cables; he watched his battery drain from his holiday address.
Tip
Ask how the system reacts to the loss of one phase, and test it at commissioning by switching off a single phase. The forum fix: a phase monitoring relay that disconnects the house when one phase drops or the voltage goes out of range.
How big should the battery be for a large three-phase home?
Size the power per phase on what must run at the same moment, and the storage on how long it must run without sun. A large home with a heat pump uses far more on a cold day than in spring.
One forum owner has 8 kVA and 45 kWh of batteries. With the heat pump his home "quickly" uses 40 kWh a day on cold days. If the battery runs flat at night, AC-coupled solar panels cannot restart the next morning. A long winter outage needs a plan: a reserve that is never used, or a generator.
A long winter power cut in a three-phase home with full back-up
Each step exists in our projects. In Zele the solar panels and the battery keep working when the street fails. In Maaseik the owner already had a generator; the home battery got an off-grid mode that lets it connect.
A GX controller starts a back-up generator automatically, and the battery modules are expandable. Solar panels on the back-up side follow Victron's factor 1.0 rule: solar power no higher than the inverter-charger's continuous power at 25 °C.
Three-phase or single-phase back-up: which one do you choose?
Choose three-phase when comfort must not change during a power cut, single-phase when a few essentials are enough. Single-phase in a three-phase house is fine, if chosen on purpose.
Choose a three-phase back-up when
- the heat pump must keep heating
- you work from home and cannot drop out
- power cuts are frequent in your area
- you want the whole house on the battery, not a list of circuits
A single-phase back-up is enough when
- you only want the fridge, the freezer, the router and some lights
- there are no three-phase appliances you need
- your connection is 3x230 V and the needs are small
Our verdict
For a large, all-electric home that must simply keep running, a three-phase back-up is the only real full back-up. For everything else, a well-chosen single-phase back-up costs less and does what it promises.
What should a quote for a three-phase home battery with back-up state?
A good quote answers five questions; ask them before you sign.
Power per phase
Continuous watts at 25 °C plus the peak, not only total kVA.
The back-up output
Which circuits, on which phase, and where the car charger goes.
Loss of one phase
How it is detected, and tested at commissioning.
A long outage
Solar charging, a generator input and an automatic start.
Room to grow
More modules later, and inverter power that grows with them.
Want to know what your house needs per phase? Call us or send a message.
Frequently asked questions
Three single-phase batteries or one 3-phase?
Sometimes one becomes the other: with Victron, three units of the same model run as one three-phase system, so a single-phase set-up can be extended. Model, battery and wiring decide; send us the details and we check.
Will my solar panels keep producing during a power cut with a home battery?
Only on the back-up side: the inverter-charger forms a small grid of its own, and the solar inverter accepts it and keeps working. Panels on a separate grid-tied inverter switch off with the grid, as the rules require.
Can a three-phase backup start a heat pump?
Usually yes, when the peak power per phase is higher than the start current of the heat pump. Ask the heat pump supplier for its start current and compare.
Want your whole three-phase home on back-up?
We design per phase and connect your important systems to the battery. Our three certified battery specialists connect almost any type of solar inverter to it.
- Step 1: Measure. First your consumption and your connection, 3x400 V or 3x230 V.
- Step 2: Choose. Together we decide what keeps running, on which phase.
- Step 3: Size. Power per phase, storage and a generator input where long outages are a risk.
- Step 4: Commission. The inverter takes over within 20 milliseconds, fast enough for computers. You follow it all in the free VRM app and portal.
Our home batteries carry a 10-year warranty on retained power. See our home batteries with back-up, or check first which home battery really works off-grid.
Sources
- Synergrid: technical prescription C10/11, edition 2.4 (15 October 2025)
- Victron Energy: ESS manual, multiphase regulation
- Victron Energy: MultiPlus-II datasheet
- Victron Energy: AC coupling and the factor 1.0 rule
- Huawei: Backup Box datasheet
- Eendraadschema: three-phase or single-phase connection
- PMG Charge: which connection for a charging station
- Fluvius: report a storage system