Your solar installation stops because its inverter needs the grid, and the rules require it to switch off. To keep producing, something on your site must form a local grid first: a battery inverter, or a generator with a battery. Our guide to the right emergency power supply for your business compares UPS, generator and battery. This article is about the solar panels you already own. The fix pays best when the same business battery system also stores solar power and shaves peaks every day.
Maybe you saw it yourself. A sunny morning, a hundred panels on the roof, and suddenly everything is dark. The inverters show an error, the cold room warms up and production stops. Somebody asks the obvious question: we make our own power, so why do we have none?
The installer answers with one word: anti-islanding. What that answer hides is the cost. The day you need your panels most is the one day they do nothing. And the quick fixes from forums, like bridging the inverter or plugging in a generator, can break it. They can also endanger the technicians on the street.
A site in Gingelom had a related problem: its solar panels regularly dropped out. Our semi off-grid system made sure that can no longer happen. The grid is now only a back-up when energy runs short. The system has 30 kVA of inverter power and 46.2 kWh of battery.
Why do solar panels stop working during a power cut?
Because a grid-tied inverter follows the grid, and the law requires it to switch off when the grid is gone. The panels themselves still make direct current in the sun. The inverter turns that into alternating current in step with the grid voltage. Without that voltage, it has nothing to follow.
On top of that comes a safety rule. Synergrid C10/11, the technical rule of the Belgian grid operators for generators on the grid, requires an automatic separation system in every inverter. It must detect an unplanned island according to the standard EN 62116 and disconnect.
The reason is simple. A technician repairing a cable must be able to trust that a dead grid stays dead. Your inverter may not keep a piece of the street under voltage. So the switch-off is not a defect: it is the inverter doing its job.
Watch out
Not every stop is a power cut. If your inverters trip on sunny days while the grid is on, the cause is often a grid voltage that is too high. Our article on why a solar inverter trips on high grid voltage explains that case.
Which solar inverters keep working in a power cut, and which do not?
Only an inverter that can form its own grid, or that works behind a device that does, keeps producing. Everything else stops, whatever the brand or the size of the roof.
| Your installation | In a power cut | Route to back-up |
|---|---|---|
| String inverter (grid-tied) | Stops | Keep it and place it behind a grid-forming battery inverter (fix 2) |
| String inverter with optimisers | Stops | Same as above; check the brand allows it |
| Micro-inverters on each panel | Stop | Behind a grid-forming battery inverter, or a separate island circuit |
| Hybrid inverter without back-up function | Stops, even with a battery | Add the back-up option and a switching device, if the model offers it |
| Hybrid inverter with back-up output | Feeds a limited back-up circuit | Size the circuit on what the inverter can deliver (fix 1) |
| Panels on a battery inverter that forms the grid | Keeps producing | This is the target design (fixes 2 to 4) |
A battery alone does not change this. Many batteries sold with solar panels stop in a power cut too, because their inverter also follows the grid. Our article on a home battery with back-up power explains the same trap for homes. For business sites, which inverter a business actually needs compares solar, hybrid and battery inverters.
Can you open the main switch and let the panels feed your site?
No. Every version of that shortcut is either useless or dangerous. Owners try it after every big outage, and forums are full of the results.
The usual ideas are these. Bridge the inverter so it "thinks" the grid is there. Feed it a fake grid from a small inverter, a car battery or a server UPS. Or plug a generator into a socket with the main switch open. The answers from electricians are always the same.
- A small inverter or UPS cannot absorb solar power. The solar inverter pushes the voltage up, the small device tries to hold it down, and one of them fails.
- A generator cannot absorb power either. Solar power that flows back into it can damage it. The solar inverters must be isolated or controlled.
- A forgotten main switch is deadly. If the grid returns, or if your site is still connected, you put voltage on a cable someone is working on.
- Your insurer and the inspection body will not accept it. An improvised connection is not an installation that passed an inspection.
Important
Never connect a generator or battery to your installation with a plug that has pins on both ends, and never bridge protections. Let an electrician install a proper changeover that separates your site from the grid first.
What are the four real fixes?
Every real fix separates your site from the grid and forms a local grid. Your solar inverters then have something to follow again. They differ in how much of the site they carry and how long.
| Fix | What keeps running | Good for | Limits |
|---|---|---|---|
| 1. Hybrid inverter with back-up output | One back-up circuit | Offices, IT, alarms, a few sockets | Power of the back-up output; often only part of the load |
| 2. Grid-forming battery inverter, panels behind it | The loads behind it, and the panels keep producing | Existing installations with string or micro-inverters | Solar power behind it may not exceed the battery inverter power |
| 3. Semi off-grid system | The site runs on its own system; the grid is a charger and back-up | SMEs and farms up to a standard 3x400 V connection | A design choice for the whole site, not an add-on |
| 4. Generator as the grid, with a battery | Everything the generator and battery can carry, for days | Long cuts, farms, sites with an existing generator | Fuel, maintenance, noise; the generator must never receive solar power |
Fix 1: a hybrid inverter with a back-up output
A hybrid inverter with a back-up function separates a back-up circuit from the grid and feeds it from the panels and the battery. Fronius, for example, offers two levels. Its basic "PV Point" feeds single-phase loads up to 3 kW, even without a battery. Its "Full Backup" needs a battery and a switching device that separates the site safely from the grid.
For a business, this is the small solution. It suits an office, a server rack or an alarm system. It will not carry a cold room, a compressor or a production line.
Fix 2: a grid-forming battery inverter with your panels behind it
Here a battery inverter, such as a Victron MultiPlus-II or Quattro, sits between the grid and your critical loads. Your existing solar inverters are connected on its output. According to Victron, the battery inverter takes over in less than 20 milliseconds when the grid fails. It then creates a local micro-grid, and the solar inverters accept it and keep producing.
When the battery is full, the battery inverter raises the frequency slightly. The solar inverters react by producing less, so the battery is not overcharged. Victron calls this frequency shifting. One rule applies: the solar power behind the battery inverter may not exceed its continuous output power (the "factor 1.0 rule").
This is the usual route for a company that already has solar panels. You keep the inverters and panels, and add the part that forms the grid. Larger sites run several units in parallel and in three phases.
Fix 3: a semi off-grid system, with the grid as back-up
In a semi off-grid system, your site normally runs on its own system: panels, battery and battery inverter. The public grid acts like a generator. It is called upon at night, with little sun or when the battery is low. An automatic transfer switch changes between the two at high speed and keeps them separated.
The consequence for a power cut is simple: your own system keeps running. That is what the site in Gingelom now does. Our semi off-grid systems go up to 75.6 kVA on a standard 3x400 V + N connection of 63 A. A farm in Tielt uses the same logic with smart control.
Fix 4: a generator as the grid, with a battery
For long cuts, a generator is the second source. A Victron Quattro has two AC inputs, for example the grid and a generator, and connects automatically to the one that is live. The battery takes over first, without a gap. The generator only starts when the battery gets low, and then it also charges the battery.
In Maasmechelen, the customer had a generator for emergencies that stood idle. Our semi off-grid system made it useful: the customer chooses the grid or the generator to charge its own system. In the woods at Gedinne, a generator is coupled to the semi off-grid system in case of a power cut. Our article on combining a generator with a battery explains how the two share the work.
Choosing between a manual and an automatic changeover for the generator itself? Read our article on a transfer switch for a business generator.
What goes on the back-up board, and how big must the battery be?
Put only what must keep running on the backed-up part of your installation. Then size the battery on those loads and the hours you want to bridge. Solar power then extends the day, but it does not replace the battery at night or under clouds.
| Load | Typical choice |
|---|---|
| Cold rooms, freezers, barn ventilation | Back-up |
| IT, network, PLCs, controls, alarms, cameras | Back-up |
| Lighting of escape routes and key areas | Back-up |
| Water pumps, milking or feeding systems | Back-up, check start current |
| Compressors, welders, heavy machines | Usually not, unless the inverter power allows it |
| Charging points, office heating, workshop sockets | Usually not |
Two numbers decide. Energy (kWh) sets how long you last. Power (kVA) sets what can start. A motor draws several times its running current for a few seconds. A small back-up output can fail on one compressor, even with a full battery.
Battery energy for 20 kW of critical loads over a 6-hour daytime cut
A 6-hour power cut on a sunny day
In a Victron system you set a minimum reserve in the energy management (ESS). The battery only goes below it when the grid fails. Above the reserve, the battery works every day for your solar self-consumption and your peak.
What happens if the battery runs empty during a long power cut?
Then solar inverters that are AC-coupled cannot start the system again, even in full sun. They need a grid to follow, and the empty battery inverter no longer forms one. This is the black-start trap.
Owners of AC-coupled systems describe it after winter storms: the battery ran empty in the night, and the next morning the panels stayed off. Victron states the fix. For off-grid and back-up systems that must survive long grid failures, it advises adding MPPT solar chargers. These connect panels directly to the battery on the DC side, so the sun can recharge an empty battery without a grid.
A back-up system that survives a cut of several days
Tip
Ask your installer three questions. What happens when the battery is empty? Which part of the solar power is DC-coupled? And which loads switch off first when the reserve is reached?
What does the grid operator require when your site runs as an island?
Your site may run as an island, if no part of the public grid is involved and the switch is safe. C10/11 sets the rules for that.
- Off-grid units that feed a part of your installation that can also be on the grid must switch "break before make". First separate, then take over.
- An emergency supply system may run in parallel with the grid only briefly: at most 5 minutes per switchover. Every parallel connection goes into a logbook.
- Every generating installation needs a lockable safety disconnect that the grid operator can always reach.
- Large industrial sites on high voltage can ask the grid operator for permission to island with critical process loads. The conditions then go into the connection agreement.
Add the inspection of the installation under the Belgian electrical code (AREI). Where the system works in parallel with the grid, it must also be notified to the grid operator. A back-up design is never a do-it-yourself job. Our article on emergency power for businesses covers the rules for back-up in general.
When is keeping your solar panels running in a power cut not worth it?
When the cuts are short and rare and nothing on your site loses money in an hour.
Our verdict
For a workshop with office work and no cooling, a real back-up costs more than the few hours it saves. Accept the stop, or keep a UPS for a clean IT shutdown. The picture changes when the same battery works every day: storing solar power, shaving your quarter-hour peak or dealing with a limit on solar injection. Then back-up comes almost free.
For cuts that you know about in advance, read our article on a planned power interruption. For the risk of a large blackout, read what a company should do about a blackout in Belgium.
Want to know which part of your solar installation can keep running in a power cut? Call us or send a message.
What happens when you ask us to keep your solar power running in a power cut?
We start from your existing installation and from what must keep running.
- Step 1: Analysis. We list your inverters, panels and critical loads, including start currents. If data is missing, we can measure your consumption first.
- Step 2: Design. We choose the fix: back-up output, grid-forming battery inverter, semi off-grid or generator, and how much solar power is DC-coupled.
- Step 3: Installation and commissioning, with the changeover and the protections the rules require.
- Step 4: Monitoring and support. You follow production, state of charge and consumption in the app and the VRM portal.
Our battery systems for businesses range from small SME installations to modular systems up to 1 MW. Back-up power and business continuity are part of their job. Our monitoring software shows what happens on your site, also during a cut.
Frequently asked questions
Do solar panels work during a power cut if we have a battery?
Only if the battery inverter can form a grid and your site is separated from the public grid first. Many batteries sold with solar panels follow the grid too and stop with it.
Do we have to replace our existing solar inverters?
Usually not. With a grid-forming battery inverter, existing string or micro-inverters can stay on its output. The solar power behind it may not exceed the battery inverter power, and the brand must allow it.
Why do the panels not simply keep feeding our own building?
Because the inverter cannot tell your building from the street. As long as your site is connected, it would feed the grid too. That is why C10/11 requires it to switch off, and why a real fix separates your site first.
Can our solar panels restart the system after a long cut?
AC-coupled panels cannot restart an empty system on their own. DC-coupled panels on MPPT solar chargers can recharge an empty battery, which is why back-up designs for long cuts include them.
Does a generator alone keep our solar panels running?
In principle the solar inverters can follow a generator. But the generator cannot absorb surplus solar power. A battery inverter between them manages the flow and keeps the generator from being pushed.
Want your solar panels to keep working when the grid fails?
Tell us what is on your roof, which inverters you have and what must keep running. One of our engineers will tell you which of the four fixes fits your site, and what the same system saves on normal days.