What can a plug-in battery never do? Back-up, 3-phase, power

A plug-in battery stores some solar power and gives it back at a low power on one socket. It can never take over your house in a power cut, carry three phases or start a heat pump. For that you need an installed home battery with backup, wired into the distribution board, inspected and sized for your house. Choose the plug-in only when a small saving on one circuit is all you want.

You have seen the ads: a battery you plug into a socket, no electrician, no inspection. A Belgian owner asked the obvious question on Reddit: "Plug-in battery: what's the catch?" Another one, with a 5 kWh plug-in unit, answered from experience: in the evening his battery "sometimes runs out between 3 and 6, mostly just about 1kW short". A Dutch owner looking for simple back-up wrote on a forum that he had thought his solar panels would keep the house running in a power cut. "Not so."

That is the catch, sharper than the ads say it. A plug-in battery is built to stay small and to switch off when the grid fails. In a large home with a heat pump, an induction hob, an electric car and a home office, it covers a sliver of the evening and none of the power cut. The cost of finding out later is double: the plug-in stays a gadget, and the real back-up still has to be designed, wired and inspected.

In a home in Peer, the owner (an electrician himself) chose the other route: the whole house is on no-break, on a single-phase connection, with 5 kVA and 15.4 kWh. When the public grid fails, every appliance keeps working.

What is a plug-in battery, and what is allowed in Belgium?

A plug-in battery is a mobile battery with its own inverter that you connect through a normal socket. In Flanders plug-in batteries and plug-in solar panels may be used since 17 April 2025, without an electrician and without an AREI inspection, as long as the device is on the Synergrid list.

Two details matter. If the installation must be changed to connect the plug-in battery safely, for example a new circuit for it alone, that change must be inspected. And the device must be marked on the single-line diagram of your electrical installation, according to FOD Economie.

Why does a plug-in battery switch off in a power cut?

Because its inverter must disconnect when the public grid fails. FOD Economie explains why: it prevents the device from sending power back onto the grid, which is dangerous for the people repairing it.

So in a power cut the plug-in battery stops feeding your house, and plug-in or roof solar panels on grid-tied inverters stop too. Some plug-in batteries have a separate back-up socket. A Belgian owner describes a 5.12 kWh unit with a 2.5 kW back-up outlet "that can be used in case of power failures". That socket powers what you plug into it with an extension cord: a fridge, a router, a lamp. Not the heating circuit, not the lights in the hall, not the gate.

Want to know the three levels of back-up (an emergency socket, a back-up of some circuits, an off-grid capable system) and how fast each one switches? Read our guide to a home battery with backup power.

How much power can a plug-in battery deliver?

Usually less than 800 W, and that is by design. The Flemish government advises keeping the inverter power below 800 W for the safety of users and their wiring. From 2,600 W a separate circuit is mandatory for a fixed set-up in a home.

Sibelga's technology expert put it plainly on RTBF: plug-in batteries are often limited in capacity (at most 5 kWh) and in power (often 800 W). That is not enough for an oven, a dryer or a washing machine, or to charge a car. 800 W shaves light peaks. It does not carry a house.

That explains the evening shortfall of the owner quoted above. With 800 W you cover the base load and a bit more; the moment the hob, the heat pump and the dishwasher run together, the rest still comes from the grid, at the evening price and on your capacity tariff peak.

Can a plug-in battery work on three phases or feed a heat pump?

No. A plug-in battery sits on one socket, so on one phase and one circuit. In a three-phase house the heat pump, a three-phase hob and an 11 kW car charger are out of its reach.

Many large homes in Belgium have a three-phase connection. A plug-in unit can lower what your meter counts, but it cannot deliver power on the other two phases, and it cannot supply a three-phase appliance at all. During a power cut that difference becomes absolute: an installed three-phase system keeps every phase alive, a plug-in keeps none.

In Wetteren the whole house runs through the battery on all three phases, with 9 kVA and 15.4 kWh: a "full no break". In Kortenberg the same 9 kVA is paired with 30.8 kWh, for full back-up in off-grid mode. Want the details per phase? Read do you need a three-phase home battery for full back-up.

What else can a plug-in battery never do?

It cannot grow into a system. A plug-in battery is a closed appliance; an installed home battery is the heart of an energy system that you can extend.

A long outage shows the gap best. In Maaseik the owner already had a generator; the home battery was given an off-grid mode that lets it connect. A plug-in battery has no such input: when it is empty and the grid is down, it stays empty.

Already have solar panels with an SMA or SolarEdge inverter? An installed battery can often be added to them, back-up included. See adding a battery and back-up to an existing installation.

Is a plug-in battery safe?

It is safe when you follow the rules: a Synergrid-approved device, a fixed socket, no extension cords, and a sound electrical installation. The risk lies less in the battery than in where and how it is used.

FOD Economie lists the points: never place a battery in a closed cupboard without ventilation or in a false ceiling or floor, keep it away from flammable materials, and do not put it in a bedroom. Always use a fixed socket, and be extra careful with wiring that dates from before 1981 and was never adapted. Tell anyone who works on your installation that a plug-in device is present.

Back-up mode raises its own questions. On a Belgian DIY forum, owners discussed whether a plug-in battery in back-up mode still lets the residual current device trip on a fault. One of them concluded he would go for a Victron system checked by an approved body: "heavy but safe". An installed battery with back-up is designed, earthed and inspected as part of your installation; that is the point.

When is a plug-in battery enough, and when do you need an installed home battery?

A plug-in battery is enough when you want a small saving and no back-up. An installed battery is the answer as soon as the house must keep running. Both are honest choices, as long as you know which one you are making.

How do we build an installed home battery with back-up?

We measure first, then connect your main systems and appliances to the battery, so your household does not stop when the grid does. Our team includes three certified battery experts.

Take a look at the projects: Peer, Temse, Wetteren and Kortenberg.

1. Step 1: We measure your consumption and your connection, and check which inverter you already have.
2. Step 2: Together we decide what must keep running in a power cut, and on which phase.
3. Step 3: We size the power and the storage, with room to expand and a generator input where long outages are a risk.
4. Step 4: We install and commission the system. Like every installed battery, it is inspected and reported to the grid operator. You follow everything in the free VRM app and portal.

Our home batteries come with a 10-year warranty on retained power. See what an installed home battery can do for a large home, or check first which home battery really works off-grid.