Answer
Island mode lets a solar and battery system disconnect from the grid during an outage and keep powering chosen loads on its own. Something in the system must create the AC voltage and frequency, such as a grid-forming battery or hybrid inverter, or a generator. A grid-tied inverter without a backup function stops when the grid fails.
A standard solar system goes dark when the grid fails, even in full sun. Island mode is the design change that keeps a chosen set of loads running instead, and it depends on one part most grid-tied systems lack: a source that sets the voltage and frequency.
This guide explains how island mode works, how grid-forming inverters differ from grid-following ones, how fast two real products switch to backup, and how to size a system with a worked example.
The product figures in this guide come from the makers’ own pages and datasheets, read on 28 October 2026. Always check the document for the exact model you plan to buy.
Last updated: November 2026
In this guide:
- What island mode is
- Why grid-tied inverters stop in an outage
- Grid-forming and grid-following inverters
- How fast the switch happens
- How to size an island system, with an example
- Two products compared
- Standards, failure modes and costs
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What Is Island Mode?
Island mode is a planned state in which a solar and battery system separates from the utility grid and keeps powering local loads. The loads sit on a small electrical “island”.
An island system needs four things:
- Isolation. A contactor or transfer switch opens and cuts the link to the grid.
- A voltage source. One device must create the AC voltage and frequency for the island. This can be a grid-forming inverter or a generator.
- Energy. Batteries, solar panels or a generator must supply the power.
- Protection and load control. The system must handle faults and start-up surges, and it must choose which loads to feed.
Our glossary page on islanding-capable inverters lists these functions in more detail. A transfer switch is one way to do the isolation step.
Why Does a Grid-Tied Inverter Stop in an Outage?
A grid-tied inverter must stop feeding power when the grid is down. This rule is called anti-islanding protection. It protects utility workers who may be repairing lines they think are dead.
The test procedure is IEC 62116. The IEC webstore describes it as a guide for testing the automatic islanding prevention measures in utility-interactive PV inverters. Read more in our anti-islanding glossary page.
A backup system is different. It makes a planned island, with a clear boundary. The grid-tie switch must open before the island starts.
What Is the Difference Between Grid-Forming and Grid-Following Inverters?
A grid-forming inverter creates its own AC voltage and frequency. A grid-following inverter reads the grid’s voltage and frequency and matches them.
| Grid-forming | Grid-following | |
|---|---|---|
| What it does | Creates the AC voltage and frequency | Matches the AC voltage and frequency it finds |
| Needs a grid or other source to run | No | Yes |
| Typical job | Backup, off-grid and microgrid systems | Rooftop and utility solar |
Most rooftop solar inverters are grid-following. In a backup system, a grid-forming battery inverter can set the voltage for the island. A grid-following solar inverter can then keep producing power on that island if the system is designed for it. This is called AC coupling.
A grid-forming label alone does not prove a product can island, switch fast or restart a dead system. Ask the maker for each function in writing. Our glossary page on grid-forming inverters and our guide to grid-forming vs grid-following inverters go deeper.
How Fast Does the Switch to Backup Happen?
It depends on the product. Here is what two makers publish.
| Product | Published switching time | Source |
|---|---|---|
| Victron MultiPlus-II | Less than 20 milliseconds | Victron product page |
| SMA Sunny Island 6.0H and 8.0H | 0 ms (high impedance) or 20 ms (low impedance), without a switch box | SMA datasheet |
Victron says this is “so fast (less than 20 milliseconds) that computers and other electronic equipment will continue to operate without disruption.” The SMA datasheet refers to an SMA technical note on switchover time for the detail.
These are the makers’ own claims. Other systems, including those with a separate transfer switch, may switch more slowly. If you have sensitive loads, ask for the switching time of the full system and test it with your real loads.
Sources: Victron MultiPlus-II page and SMA Sunny Island 4.4M / 6.0H / 8.0H datasheet.
How Do You Size an Island Mode System?
Start with the loads, not the solar array.
- List the critical loads. Write down each device’s power in kW and its daily energy in kWh.
- Add up the power. The inverter must cover the total power of everything that runs at the same time.
- Check the start-up surge. Motors and compressors draw extra current for a moment when they start. The inverter must cover the largest surge.
- Pick the backup time. Decide how many hours the battery must run the loads.
- Add allowances. Include inverter losses and the share of the battery you can actually use.
Worked example
This is a made-up home. The numbers are assumptions for the example. Use your own loads and your battery datasheet.
| Load | Power |
|---|---|
| LED lighting | 0.5 kW |
| Refrigerator | 0.8 kW |
| Internet and security | 0.2 kW |
| Well pump | 1.5 kW |
| Part of the air conditioning | 2.0 kW |
| General outlets | 3.0 kW |
| Total | 8.0 kW |
- Total power: 0.5 + 0.8 + 0.2 + 1.5 + 2.0 + 3.0 = 8.0 kW.
- Backup target: 4 hours at 8.0 kW = 32 kWh of load energy. This is a worst case. It assumes every load runs the whole time.
- Inverter losses: assume 90% efficiency. The battery must supply 32 ÷ 0.90 = 35.6 kWh.
- Usable share: assume you can use 80% of the battery. Battery size = 35.6 ÷ 0.80 = 44.4 kWh.
- Inverter: choose one with a continuous rating above 8.0 kW. Check that its surge rating covers the largest start-up surge, such as the well pump.
- Load energy for 4 hours32
- After 90% inverter efficiency35.6
- After 80% usable share44.4
Source: The made-up 8.0 kW home above, using assumed 4-hour backup, 90% efficiency and 80% usable battery share; illustrative example, not market data, checked November 2026.
Key Takeaway
In this example, 32 kWh of load energy turns into a 44.4 kWh battery once losses and the usable share are counted. Size from the loads and the datasheet, then add allowances, not the other way round.
If solar panels produce power during the outage, the battery lasts longer. But size for the worst case: a night outage with no solar.
Pro Tip
Before you finalize the backup panel, write down the start-up surge of every motor on it and ask the inverter maker for the surge rating in writing. A well pump or compressor is the usual reason a backed-up system trips.
Watch kVA as well as kW
Inverters have a kVA rating as well as a kW rating. Motors draw more kVA than their kW suggests. A 2 kW motor with a power factor of 0.7 draws 2 ÷ 0.7 = 2.86 kVA. Check both ratings on the datasheet.
AC-coupled or DC-coupled?
In an AC-coupled system, a battery inverter sits beside the existing solar inverter on the AC side. This suits a retrofit. In a DC-coupled system, the panels and the battery share one hybrid inverter. This suits a new build. Each has trade-offs in efficiency, cost and design freedom. Ask each maker which set-ups it supports.
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Which Products Can Run an Island?
The two products below are examples of systems that make an island. Do not read this as a ranking.
Victron MultiPlus-II. Victron’s page describes an inverter/charger that suits marine, vehicle and land-based off-grid systems, and ESS (energy storage) set-ups. It lists the less-than-20-millisecond UPS function above.
SMA Sunny Island 6.0H and 8.0H. SMA’s datasheet describes a battery inverter for on-grid and off-grid use. It lists up to 95.8% maximum efficiency for both models, a 10-year warranty on registration, and says SMA has installed more than 150,000 Sunny Island inverters worldwide. The datasheet lists the backup switching times shown above. It also lists a generator support feature and a parallel connection option.
Before you choose, check each product’s list of approved batteries, supported set-ups, local approvals and warranty terms. These differ by country and model.
Further Reading
For the electrical side of backup design, read about AC-coupled and DC-coupled batteries and the solar inverter sizing guide.
Which Standards Matter?
- IEC 62116 is the test procedure for anti-islanding protection, as above.
- IEC 61727. The IEC webstore says that inverters meeting IEC 62116 are considered non-islanding as defined in IEC 61727. The Sungrow datasheet in our central inverter guide lists both standards.
- Local rules. Your utility and your country set the grid-connection rules for batteries and backup. Ask for the written requirements early.
What Goes Wrong in Island Systems?
- A stuck grid-tie contactor. If it fails to open, the system cannot form a safe island. It could also feed power back to the grid. Test the contactor by simulating an outage. Follow the maker’s service manual for how often.
- Battery low-voltage cut-off. A big surge, such as a pump starting, can pull the battery voltage down. The battery management system (BMS) then opens the battery contactors and the system shuts down. The battery management system page explains the BMS. Fixes include a bigger battery, a soft starter on the motor, or removing the load from the backup circuit.
- Overload from kVA. An inverter rated by kW can trip on motor loads that draw more kVA. Check both ratings.
- Mismatched settings between several inverters. Inverters that share a load must have matching settings, or one unit takes most of the load. Follow the maker’s set-up guide.
What About Black Start and Microgrids?
Black start is the ability to restart part of a power system after a blackout, without help from the grid. The US National Laboratory of the Rockies (NLR) explains this on its black start page. NLR says inverters used for black start need to run in grid-forming mode to provide a reference AC waveform. It also says starting motor loads is hard, because they can draw large inrush currents that typical inverters may struggle to supply.
So a grid-forming label does not prove black-start ability. Ask the maker whether the product supports it, and for which loads.
What Does an Island Mode System Cost?
Prices change with the country, the brand and the date. This guide does not list a price premium, because the sources we could check do not support a single figure. Ask for quotes that price each part:
- The backup-capable inverter or hybrid inverter
- The battery
- The transfer switch or contactor
- A critical-loads panel, which separates the backed-up circuits
- Extra protection devices
- Design work and the utility application
Subsidies and tax rules differ by country and change often. Check the current rules where you work. For hybrid inverter prices in India, see our guide to the 10 kW hybrid inverter price in India.
Frequently Asked Questions
What is island mode in a solar system?
Island mode is a planned state where a solar and battery system disconnects from the grid during an outage and keeps powering chosen loads on its own. It needs a way to isolate the system from the grid, a source that creates the AC voltage and frequency, enough stored or generated energy, and protection.
How does a grid-forming inverter differ from a grid-following inverter?
A grid-forming inverter creates its own AC voltage and frequency. A grid-following inverter reads the voltage and frequency of the grid and matches them, so it needs a grid, or another source that creates one, to work.
How fast does a system switch to battery backup?
It depends on the product. Victron says the MultiPlus-II takes over in less than 20 milliseconds. The SMA Sunny Island 6.0H and 8.0H datasheet lists backup switching times of 0 ms (high impedance) and 20 ms (low impedance), without a switch box. Other systems may take longer, so ask the maker for the switching time.
Can solar panels power my house during a blackout with island mode?
Yes, if the system is designed for it. It needs a backup-capable inverter or battery system, a way to isolate the home from the grid, and enough energy. Panels alone cannot run the house, because a standard grid-tied inverter stops when the grid fails.
What is anti-islanding protection?
Anti-islanding makes a grid-tied inverter stop feeding power when the grid is down. This protects utility workers who may be repairing lines. IEC 62116 is the test procedure for this protection. A backup system makes its own planned island instead, after the grid-tie switch opens.
What is black start?
Black start is the ability to restart part of a power system after a blackout, without help from the grid. The US National Laboratory of the Rockies (NLR) says inverters used for this must run in grid-forming mode, and that starting motor loads is hard because of the large inrush current.
How do I size a battery for island mode?
List the loads you must keep on, then add up their power and their daily energy. Pick an inverter that covers the peak power and the start-up surge. Then size the battery for the hours you need, and add allowances for inverter losses and the usable share of the battery.
Next Steps
- List your critical loads. Record the power, the start-up surge and the daily energy of each one.
- Ask for written proof. Get the switching time, the island function and the approved battery list from the maker.
- Check the local rules. Confirm what your utility requires before you buy.
To see how a backup design fits into layout, yield and proposal work, book a SurgePV demo and bring one real project.
Where this fits
This article is part of SurgePV's Energy Storage hub, which works through the topic from first principles to the decisions a project team actually has to make.
