Can a generator recharge your home battery in a power cut?

Yes, you can charge a home battery with a generator during a long power cut. But only with a system built for it. The battery must run the house in island mode and have an AC input the generator can use. The generator then refills the battery for a few hours; the battery carries the house the rest of the time. Size the generator on the charger, not on the house.

You have a large home, a battery and solar panels, and you think you are covered. Then the power stays off for two days in winter. A Dutch owner wrote on a forum that the power had failed "6 times" in two months; he woke up in a cold house. A French owner watched his 14 kWh battery do nothing during an outage of "almost 48 hours". Others buy a generator and find their inverter will not accept it.

What no one tells you before the first long outage: a battery is sized for a night, not for three grey winter days. Solar panels give little in December. A generator you cannot connect safely stays in the garage. Adding a generator input later often means a new switchboard layout and a new inspection.

In Maaseik a generator was already there. We gave the home battery an off-grid mode that lets the owner connect the generator. The house runs on 9 kVA and 15.4 kWh of storage.

Why is a home battery alone not enough for a power cut of several days?

Because a home battery is sized for daily use, and a long outage in winter needs more energy than it holds. In summer the solar panels refill it every day; in a grey week they hardly do.

A large home with a heat pump uses far more on a cold day than a battery of 15 kWh can hold. The battery then carries the house through the night, but not through the next day and night as well. The generator is the reserve that turns one night of back-up into several days.

How much battery you need for a night and a day without the grid is in our guide to a home battery with backup power. Here we look at what happens when that is not enough.

Can every home battery be charged from a generator?

No. The battery system needs an island mode and an AC input that accepts a generator. A battery that switches off with the grid, or only has an emergency socket, cannot be refilled that way.

Victron describes both routes. The Quattro-II can connect to two independent AC sources, such as the grid and a generator, and switches to the active one by itself. Victron recommends the generator on AC input 1 and the grid on AC input 2. With a single input, a changeover switch in front of it chooses grid or generator, never both.

Is your battery not able to run the house during a power cut at all? Check first which home battery really works off-grid.

How big must the generator be to recharge the battery?

Big enough for the battery charger plus what the house uses while the generator runs. The charger power sets how fast the battery fills; a bigger generator does not charge faster.

An example. A MultiPlus-II 48/5000 charges at up to 70 A, about 3.4 kW. Putting 10 kWh back into the battery then takes about three hours. If the house uses 1 kW at the same time, the generator must deliver about 4.5 kW for those hours.

A smaller generator also works, only slower. The inverter-charger lets you set a maximum generator current; it then uses only what is left for charging, so the generator is not overloaded. With PowerAssist, the battery adds power when a heat pump or an oven starts, so the generator does not have to cover those peaks.

Why does an inverter sometimes refuse a generator?

Because the generator's voltage or frequency drifts outside what the inverter-charger accepts. It then disconnects from the generator and runs on the battery again, sometimes over and over.

The MultiPlus-II accepts 187 to 265 V at 45 to 65 Hz on its AC input. Victron calls a generator a "weak" AC source, and its manual lists settings for it. Owners on forums describe a relay that clicks in and drops out again, or a generator that trips its breaker under load. One owner of a three-phase system with a small generator found that "the full load goes directly to the generator".

Most of this is solved with the right generator and the right settings. That means a stable output, enough continuous power and an input current limit that matches the generator. A generator that does not start at all after months of standing still is a different problem. See why a back-up generator fails to start.

How do you get through a three-day power cut with a generator and a battery?

The battery carries the house; the generator runs in blocks to refill it; the solar panels help whenever there is light. The generator does not run through the night.

The advantage over a generator alone: the house never goes dark while the generator is off. And the generator runs at a good load instead of idling for small loads. How many generator hours a battery saves on sites that live off-grid is worked out in our article on an off-grid generator with a battery.

With a GX controller, the system can start a back-up generator automatically, for example when the battery drops below a set level. You follow it in the free VRM app, also from a holiday address.

What happens to the solar panels while the generator runs?

They must not push power back into the generator. In island mode, solar panels on the back-up side keep charging the battery. While the generator runs, their surplus can only go to the battery.

So the system must limit or pause the solar inverter while the generator is connected. It lets it produce again when the battery needs the energy. How this is done depends on the brands in your house. Ask your installer before you rely on it.

In a three-phase home the question is the same per phase, and a generator must fit the three-phase system. Read whether you need a three-phase home battery for full back-up before you choose one.

Where does the generator go, and which Belgian rules apply?

Outside, and connected through a changeover so that it can never run in parallel with the grid. The Belgian grid rules start from that one principle.

• Carbon monoxide: every petrol or diesel engine produces it. The Belgian Poison Centre warns that engines running in a poorly ventilated space can cause CO poisoning. Never run a generator in a garage, cellar or technical room.
• Never in parallel with the grid: the Synergrid C10/11 rules exclude units that never run in parallel with the grid. The switch between grid and island mode must work "break-before-make".
• No extension cord through a window, no cable with a plug on both ends: a fixed generator is installed by a recognised electrician. The inlet and the changeover belong on the diagrams of your installation.
• Earthing in island mode: your RCDs must keep working when only the generator or the battery feeds the house. Our guide to a home battery with backup power explains the inspection.

A generator that is only a reserve for your own house is not a grid-connected production unit. The home battery itself is, and must be reported to your grid operator as usual.

Portable generator or a fixed one with automatic start?

A portable set is a reserve you start by hand; a fixed set with automatic start makes a multi-day outage almost invisible. For a large home that must keep running, the second is the real back-up.

What should you ask before you connect a generator to your home battery?

Five questions tell you whether a generator will really keep your home running. Ask them before you buy either one.

How do we connect a generator to a home battery?

We start from what must keep running, then design the battery, the generator input and the changeover as one system. Our team has three certified battery specialists.

See the projects in Maaseik and Maasmechelen.

1. Step 1: We measure your consumption and check what must keep running for several days.
2. Step 2: We size the battery for the nights and the charger for the generator you have or want.
3. Step 3: We design the generator input, the changeover and the automatic start with the GX controller.
4. Step 4: We install, test the generator under load, and hand over. When the grid fails, the inverter takes over within 20 milliseconds, and you follow everything in the VRM app.

Our home batteries are modular and can be expanded later, with a 10-year warranty on retained power. See what our home batteries with back-up can do for a large home.