The step is worth it when your home battery keeps running into the same walls. Think of a roof with room for more panels than the connection allows. Or panels that switch off on sunny days, or a power cut that shuts everything down. A semi-off-grid system moves your panels, battery and house onto your own island. The grid becomes the back-up, like a generator you rarely need. If you mainly want to store today's sun for tonight, a home battery already does that job.
You bought a home battery to use your own solar power, and it works. But the roof has room for more panels and the connection limit says no. On a sunny afternoon the panels drop out. And in a power cut the house goes dark with the battery still full. So a bigger thought starts growing. "Do you really aim for 100% independence, or is saving part of it good enough?" one reader asked on a Dutch-language DIY forum (translated). An installer had told him that off-grid is technically possible, but financially almost never smart.
Both are right, and that is the trap. A fully off-grid home is sized on December, when 1 kWp of panels gives about a quarter of what it gives in June. A grid-tied battery stays bound by the grid rules: power limits and registration. And it must switch off when the street goes dark, unless it has back-up. Semi-off-grid sits in between, and that is where most large homes land.
A house in Heusden-Zolder hit the single-phase limit of 5 kW of solar panels, too little to cover its use through the year. Now the panels run in the home's own off-grid system, with the battery connected directly. The house only switches to the street when the battery is empty and the sun is not shining.
What changes when a home battery becomes semi-off-grid?
With a home battery, your house works together with the grid. With semi-off-grid, your house works on its own, and the grid is the reserve.
A normal home battery runs in parallel with the grid. Your solar inverter and battery inverter are production units under the Synergrid C10/11 rules. That means power limits, registration, and a protection that disconnects them when the street fails. Why a grid-tied battery stops in a power cut is explained in which home battery really works off-grid.
In a semi-off-grid home, your panels, battery and consumers form your own island grid. An automatic transfer switch connects the house either to that island or to the street, never to both. C10/11 counts off-grid units as outside its scope, as long as the switchover follows the "break-before-make" principle: first disconnect, then connect.
How a semi-off-grid home works
| Question | Home battery (grid-tied) | Semi-off-grid home |
|---|---|---|
| Who runs the house | The grid, helped by panels and battery | Your own island; the grid only as back-up |
| Panels | Within the grid limit of your connection | Panels on the island are not connected to the grid, so the grid limit does not apply to them |
| Surplus sun | Stored, then injected into the grid | Stays in your own system |
| Power cut | Off, unless the battery has a back-up function | The island keeps running |
| Switchover | Not applicable | Automatic transfer switch, very fast |
When is your home ready for semi-off-grid?
When at least two of these five signs sound familiar. One on its own can usually be solved more simply.
Your roof has room, your connection does not
In Flanders you may connect up to 5 kVA of solar inverters on single-phase. How the limits work, and all the ways around them, is explained in how many kVA of solar panels you may install.
Your panels switch off on sunny days
High voltage in the street makes the inverter trip, just when the sun is strongest. The causes and the quick fixes are in why a solar inverter trips on high grid voltage.
A power cut really hurts
Working from home, a heat pump, a sump pump or animals. Then a few back-up circuits are not enough and you want the whole house on its own power.
Your use has grown
A heat pump, an electric car, a pool or a sauna. Your battery was sized for a different house.
You want to keep the option of going fully off-grid
Our semi-off-grid installation is ready for that step. You can take it later, after a period of experience.
Tip
Only the last two signs? Then first check whether a larger or expanded home battery is enough. How that works is in expanding a home battery later.
What stays from your home battery, and what is added?
The panels and often the battery stay. What changes is mostly in the switchboard: a transfer switch, and the circuits that move behind your island.
Whether your current battery and inverter can form their own grid is the first thing we check. We connect almost all types of solar inverter to a battery, and we also revise existing batteries. Panels on a solar charger (MPPT) charge the battery directly; a grid-tied solar inverter can keep working on the island through AC coupling.
| Part | Usually | What we check |
|---|---|---|
| Existing solar panels | Stay | Whether they stay on the grid or move to the island |
| Existing home battery | Often stays, sometimes expanded | Can its inverter form its own grid? Is the storage enough? |
| Inverter-charger power | Often more | Enough power for the whole house, heat pump and car included |
| Extra panels | Added on the island | Roof, orientation, winter yield |
| Transfer switch | New | Break-before-make, automatic |
| Switchboard | Rewired | Which circuits run on the island; new AREI inspection |
Watch out
Does your older solar installation still receive green power certificates? Then leave those panels and their meter untouched on the grid, and build the island beside them. The details are in keeping green power certificates with a battery or semi-off-grid.
Want gates, lifts and alarms on the island too? The switchboard, earthing and inspection are explained in how a full no-break house is wired.
How big should a semi-off-grid home system be, and why does December decide?
Big enough to carry your house from spring to autumn, not big enough to survive December alone. That is the difference with fully off-grid, and the reason it costs less.
In Oost-Vlaanderen, 1 kWp of south-facing panels gives about 127 kWh in June and only 33 kWh in December (PVGIS, Hamme). A fully off-grid home must bridge that gap with a huge battery or a generator. A semi-off-grid home lets the grid bridge it. That is why it needs fewer panels, and in most cases less battery capacity and power, than a fully off-grid system.
A semi-off-grid home in June and in December
The size itself follows your house. What must keep running? How much do the heat pump and the car ask in winter? And how often do you accept the grid? Our semi-off-grid projects run at a much higher inverter power than a typical home battery.
Inverter power: home battery projects compared with semi-off-grid projects
Have a heat pump, two cars and a pool? The sizing is worked out in how big a home battery for a large home should be.
Which rules apply to a semi-off-grid home in Belgium?
Fewer grid rules, but not zero rules. The island is inspected like any installation, and the grid side stays registered.
- Break-before-make. When off-grid units feed an installation that can be connected to the grid, C10/11 requires break-before-make switching. The transfer switch never connects your island and the street at the same time.
- Inspection. New circuits and a transfer switch are a major change to your installation. So an approved body inspects them before use, under the federal AREI (RGIE in French).
- Flanders: grid side. A home battery connected to the grid is registered with Fluvius, with a valid AREI inspection. Panels that only feed your island are not connected to the grid: in Aalst we added 20 kW of panels without notifying Fluvius.
- Flanders: island. An installation in island operation with an inverter of 800 VA or more is notified to the Vlaamse Nutsregulator.
- Wallonia. Declaring solar panels and batteries on the grid to ORES is a legal obligation. If your panels were commissioned before 2024 (up to 10 kW), the compensation (the meter that runs backwards) continues until 31 December 2030.
That last point matters. As long as the Walloon compensation runs, the grid acts as a free reservoir for those panels. So there is less reason to take them off the grid. New panels on the island are a different story. In Brussels, ask Sibelga how your installation is registered.
When is the step to semi-off-grid not worth it?
When the walls you run into are small. Semi-off-grid is a bigger investment: more battery and inverter power, a transfer switch and rewiring.
Our verdict
Stay with your home battery if your use is modest, your roof is small, or the power rarely goes down in your street. Expand it if only the storage is too small. Take the step to semi-off-grid if you hit two or more walls at once. Think of the connection limit, dropping panels, painful power cuts or a house that now runs on electricity. Want to leave the grid completely? Then size the whole system on December, usually with a generator. That is a different project, explained in off-grid solar panels with a battery.
Only want back-up for a few circuits during a power cut? Then a home battery with backup power is the simpler answer.
Not sure whether your house is ready for semi-off-grid? Call us or send us your battery type and your use.
How did other homes make the step?
Each of these homes had its own reason, and none of them left the grid completely.
| Project | Why semi-off-grid | Storage | What the grid still does |
|---|---|---|---|
| Heusden-Zolder | Single-phase limit of 5 kW of panels, too little for the year | Not stated | Only when the battery is empty and there is no sun |
| Aalst | Wanted 20 kW of panels, only for the house | Not stated | Steps in at peak moments, when needed |
| Gingelom | The solar panels dropped out regularly | 46.2 kWh | Only back-up when energy runs short |
| Lierde | Two home owners invested together; 52 solar panels | 30.8 kWh | Back-up through one grid connection |
| Sint-Laureins | Large house expanded with a pool and a garage | 30 kWh | Partly charges the battery only when the sun is gone and the battery is empty |
Take a look at the projects: Heusden-Zolder, Aalst, Gingelom, Lierde and Sint-Laureins.
From practice
In Gingelom the solar panels kept dropping out. The semi-off-grid system of 30 kVA with 46.2 kWh of storage makes sure that can no longer happen. The street is now only used as a back-up when energy runs short.
How do we take your home from home battery to semi-off-grid?
We start from what you already have, and we measure before we add anything. Our team has three certified battery specialists, and our semi-off-grid installation is ready to go fully off-grid later.
- Step 1: We look at your current battery, inverters and panels, your connection and your use: heat pump, car, pool or sauna.
- Step 2: Together we choose what runs on the island: the whole house or selected circuits. We size the extra panels, power and storage on that choice.
- Step 3: We install the transfer switch and rewire the switchboard. The installation is inspected before use, and the grid-side parts stay registered.
- Step 4: You follow production, use and the battery live in the free VRM app and portal. The switchover between your island and the street happens automatically.
Not sure yet whether your current home battery can form its own grid? Send us its brand and type, and we will tell you honestly.
Frequently asked questions
Can I go fully off-grid later?
Yes, the semi-off-grid installation is ready for it. After a period of experience you know how often the grid still steps in. To leave it completely, you then add panels, storage and usually a generator for December.
Does my house keep power during a power cut?
Yes. When the public grid fails, your house keeps running on the island: panels and battery. How long depends on the battery level and the sun.
Do I need a heavier grid connection?
No. In a semi-off-grid system the grid works as a back-up, like a generator. Your existing connection stays as it is.
Can I keep my existing solar inverter?
Often, yes. We connect almost all types of solar inverter to a battery. Whether it can keep producing on the island is one of the first things we check.
Want to know if your home is ready for semi-off-grid?
Tell us which home battery and inverters you have, how many panels, and what runs in your house. One of our battery specialists will tell you honestly whether semi-off-grid fits, or whether your battery is already enough.
Sources
- Synergrid: technical prescription C10/11, edition 2.4
- Vlaamse Nutsregulator: direct lines and island operation
- Fluvius: registering a home battery
- FOD Economie: types of inspections of household installations
- ORES: new energy production
- ORES: compensation before 01/01/2024
- SPW Energie: the compensation mechanism
- PVGIS, European Commission JRC