A home battery stores energy as DC; what it can actually run during an outage is decided by the inverter and by which circuits are wired to its backup output. In a three-phase house running a single-phase inverter, only the loads on the backed-up phase stay live when the grid drops. Battery capacity does not change that — the phase does.
That mismatch between what a homeowner expects ("the whole house stays on") and what the wiring delivers is one of the most common sources of awkward conversations after commissioning. This guide separates the three questions that usually get mixed together, explains why phase count decides backup scope, and lists the checks that belong on the quote before anyone promises a customer anything.
Three questions that get mixed up
Installers and homeowners often use "backup" for three different things. They are decided by different parts of the system, and only one of them is about an outage.
| Question | What decides it | What it changes |
|---|---|---|
| Will the battery cut grid import? | The inverter and the metering | The electricity bill, day to day |
| Which circuits stay live in an outage? | The phase and the backup wiring | What actually runs when the grid fails |
| How long will those circuits run? | Usable kWh against the load | Time on backup, not coverage |
A battery can do the first job well and still leave most of the house dark during an outage. Quoting "a 10 kWh battery" answers none of the three on its own.
The inverter sets the phase; the battery does not
On the DC side, our storage modules sit on a low-voltage or high-voltage bus, and that bus is chosen by the inverter — the pairing logic is set out in high-voltage or low-voltage home battery. The phase question lives on the other side of the inverter, on the AC output, and it is settled there.
What we integrate on the AC side
Our hybrid inverters are single-phase 230 V units — Huawei SUN2000 and Deye SG-series, 5 kW to 10 kW, two to three MPPT trackers, CAN/RS485 battery communication, parallel operation up to six units. They are engineered for the European 230 V / 50 Hz single-phase standard. Specifications are on the hybrid inverter page.
That single-phase output is the honest starting point for every backup conversation below.
Why a single-phase system usually backs up one phase
A three-phase service delivers power across three line conductors, L1, L2 and L3, each at roughly 230 V to neutral and 400 V between phases. A household's circuits are spread across those three phases — the kitchen sockets might sit on L1, the lighting on L2, the boiler on L3.
A single-phase inverter connects to one phase. It can only supply loads that are on that phase, and in backup mode it can only energise the circuits wired to its backup output on that phase. That is a property of single-phase equipment, not a limit of the battery: the same constraint applies to any single-phase inverter in a three-phase home. An independent industry explainer sets out the same behaviour in "Do you really need a 3-phase battery to backup your 3-phase home?".
Two consequences follow, and both belong in the first conversation with the customer:
- Draw a three-phase house with a single-phase battery and only one third of the circuits can be backed up — unless you design around it.
- Capacity is not the fix. Moving from 5 kWh to 15 kWh gives longer runtime on the backed-up circuits; it does not put the other phases back on.
To cover all three phases you need either a three-phase inverter or one inverter per phase. Which configurations an inverter actually supports — and any paralleling rules — must come from that inverter's own installation documentation, not from a general assumption.
What usually stays on — and what usually does not
Backup scope is a wiring decision, so the honest answer is always "whatever you put on the backup output." In practice the split tends to look like this:
| Circuits that usually go on backup | Circuits that usually stay off it |
|---|---|
| Lighting, selected socket circuits | Heat pump and electric resistance heating |
| Fridge and freezer | EV charger |
| Router, network and home office | Oven, induction hob |
| Boiler controls and pumps | Tumble dryer |
| Phone charging, medical devices | Large power tools, workshop loads |
Two cautions sit behind that table. First, appliances with motors or compressors draw a starting current several times their running current; the inverter has to supply that surge, not only the steady wattage on the label. Second, a load you leave off the backup output is not protected, no matter how large the battery is — say so plainly rather than letting the customer assume.
The checks that belong on the quote
- Phase count at the connection point. Single-phase or three-phase service at this address is the first number to establish.
- The inverter's phase and backup output rating. Single-phase units are the norm in our range; confirm what the AC backup output can carry.
- Which circuits go on the backup output, and who re-wires them. This is the decision that defines scope, and it is an electrical design choice, not a battery choice.
- Surge loads on those circuits. Size for starting current, not only running watts.
- Usable kWh against the runtime the customer wants. See the arithmetic below.
- The network operator's technical connection rules for that address. Registration and any limits on unbalanced feed-in are local matters; for the German side, our guides to grid registration under VDE-AR-N 4105 and §14a EnWG cover the process.
Backup duration is arithmetic, not a promise
Runtime is usable energy divided by load. The example below uses a stated assumption for depth of discharge; it is illustration, not a specification.
| Step | Example |
|---|---|
| Module in the system | 5.12 kWh |
| Assumed usable share | 80% → about 4.1 kWh |
| Continuous load on backup | 300 W |
| Runtime | about 13 hours |
The calculation is honest; the load is the part people underestimate. Give the customer the arithmetic with the assumption visible, and refuse to quote a headline runtime without it.
Three ways to cover more of the house
- Essential-loads backup panel on a single-phase inverter. The most common route: pick the circuits that matter, move them to the backup output, leave the heavy loads off it. Lower cost, smaller inverter, clear expectations.
- Three-phase coverage. A three-phase inverter, or one inverter per phase, to reach circuits on L1, L2 and L3. Confirm the supported configuration in the inverter's documentation before you quote it.
- Off-grid or weak-grid systems. Where the grid is unreliable, an off-grid kit is designed around independence rather than outage backup — the off-grid solar systems range and the off-grid design trade-offs guide cover that path, and off-grid, hybrid or backup frames the choice between the three system types.
Whichever route you take, the physical format and the expansion plan usually get decided in the same visit — see wall, stacked or rack and designing for expansion.
Tell us the connection and the loads
Send us three things — the phase count at the connection point, the inverter make and model (or confirmation that it is still open), and the list of loads the customer wants to keep running — and we will confirm which configuration fits, how the storage couples to it, and which datasheet applies before you quote. If the inverter is not chosen yet, start from the site and the objective and we will work forward from there.
Start with a specification enquiry or browse the full storage range.