What Happens to Solar Panels During a Power Cut?
Author: Steve Fairless
Originally Published: 7th August 2026 · Updated: 2nd September 2026
In a standard grid-connected solar PV system, the inverter disconnects from the public electricity network when it detects a power cut, so the solar system will not normally keep supplying your home even if the sun is shining. This is a deliberate safety function called loss-of-mains or anti-islanding protection. A correctly designed battery backup or Emergency Power Supply (EPS) system can keep part or all of a property powered, but it must isolate the backed-up installation from the grid before creating its own local supply.
At Sustainable Energy Engineering, we separate two questions that are often confused: does the property have solar? and has the property been designed to operate safely during a grid outage? Solar panels alone do not answer the second question.
This guide explains why ordinary solar shuts down, what happens to the DC power available from the panels, how a battery can form an islanded supply, how long backup may last, what loads can be supported and what we check before promising whole-home or essential-load backup.
Solar Panels in a Power Cut: The Short Answer
- Standard grid-tied solar normally stops supplying the property. The inverter detects loss of mains and disconnects from the distribution network.
- The panels may still have sunlight on them. The issue is not that sunlight disappears; it is that a normal grid-following inverter is not allowed to energise an uncontrolled island.
- A battery does not automatically mean backup. Some battery installations are configured only for bill saving and self-consumption.
- Backup needs a safe islanding arrangement. Approved switching or a gateway/changeover device separates the property from the grid before the inverter creates a local supply.
- Backup power in kW and stored energy in kWh are different. A battery can contain enough energy for hours but still be unable to start or supply a very large load.
- Solar can recharge a compatible backup system during an outage. Whether it does depends on the inverter, battery, islanding equipment, state of charge, loads and solar conditions.
Why Does Normal Solar Shut Down When the Grid Fails?
A conventional solar inverter is designed to operate in parallel with the public distribution network. It synchronises its AC output with grid voltage and frequency. If the network supply disappears, protection must detect that condition and disconnect the generation.
The safety reason is fundamental. Engineers may be working on what they expect to be a de-energised local network. If a home solar system continued feeding that network independently, it could create a dangerous unintended island. The same protection also prevents an inverter from trying to support a section of the public network that it was never designed to energise.
A Power Cut Does Not Mean the Solar Panels Themselves Become Inert
PV modules exposed to daylight can still have DC voltage present. What changes is the system's permitted operating state. The grid-connected inverter disconnects its AC output from the network, and a standard installation therefore stops supplying ordinary household circuits until the grid returns.
Why G98 Requires Normal Solar to Disconnect
Energy Networks Association Engineering Recommendation G98 covers fully type-tested micro-generators connected in parallel with public low-voltage networks up to and including 16 A per phase. Using the nominal 230 V phase-to-neutral value, that corresponds to 3.68 kW per phase — 3.68 kW on single phase or 11.04 kW aggregate across a balanced three-phase installation.
The important point during a power cut is not the size threshold itself; it is the protection philosophy. A normal grid-following inverter must detect loss of mains and stop energising the public network. That is why a sunny roof does not mean the sockets stay live when the street supply fails.
The Protection Behaviour We Design Around
G98 type-test requirements include loss-of-mains operation in less than one second in the relevant test scenario and a minimum 20-second delay before automatic reconnection after voltage and frequency return within the required limits.
For us, those timings explain the customer experience: ordinary grid-tied solar is designed to get out of the way of a network fault, not form its own household grid. Backup therefore needs a separate, controlled islanding arrangement rather than a setting that simply tells the solar inverter to “stay on”.
Technical source: Energy Networks Association, Engineering Recommendation G98 Issue 1 Amendment 7.
What Is Anti-Islanding?
Anti-islanding is the protection that stops a grid-connected generator from continuing to energise a separated part of the distribution network. An unintentional island is not the same thing as a properly designed home backup system.
A backup installation creates an intentional island on the customer's side of suitable isolation equipment. The home or selected circuits are disconnected from the public network, then the battery inverter establishes its own local voltage and frequency. Solar can operate within that controlled island only if the equipment is designed and commissioned to do so.
Normal Grid-Tied Mode
Solar inverter follows the public grid. The home uses solar first and imports or exports the balance. Loss of mains causes the inverter to disconnect.
Intentional Backup Mode
Approved switching isolates the backed-up installation from the public network. A compatible inverter/battery then creates a local supply for the selected loads.
Does Having a Solar Battery Mean My Home Will Work in a Power Cut?
No. A battery can be installed for solar self-consumption, tariff optimisation or grid services without being configured to provide outage backup. Some systems need additional switching, a backup gateway, an EPS output, a separate essential-load consumer unit or a manual changeover arrangement.
Before specifying solar PV with battery storage, we ask what the customer actually wants during an outage. Keeping broadband, lighting, refrigeration and a few sockets running is a very different design from supporting an induction hob, immersion heater, EV charger and large heat pump at the same time.
Whole-Home Backup vs Essential-Load Backup
| Approach | What is backed up | Main design issue | Typical advantage |
|---|---|---|---|
| Essential-load backup | Selected circuits such as lighting, broadband, fridge/freezer, heating controls and chosen sockets. | The backed-up board must be separated and loads selected deliberately. | Lower peak power and slower battery depletion can provide longer practical resilience. |
| Whole-home backup | Most or all circuits remain available through the backup system. | Inverter output, surge current and customer behaviour must be able to handle large simultaneous loads. | Simpler experience for the homeowner, provided the system is sized and controlled properly. |
Whole-home backup does not mean unlimited power. A backed-up house is still limited by the inverter's continuous output, surge capability, battery energy and the solar power available at that moment. Large loads may have to be managed during a prolonged outage.
kW vs kWh During an Outage
This is the most important backup distinction. kW is power: how much electrical load the system can support at one moment. kWh is energy: how long the battery can sustain that load.
A 10 kWh battery could theoretically supply a 1 kW average load for around 10 hours before allowing for reserve, conversion losses and changing load. The same battery might have a 5 kW inverter, meaning a 7 kW instantaneous household demand would still exceed the available backup power even though plenty of energy remained stored.
Illustrative Backup Runtime From 10 kWh Available Energy
Illustrative calculation only: runtime = available battery energy divided by average load. Real runtime is lower or higher depending on state of charge, conversion losses, reserve, solar input, temperature and changing loads.
Can Solar Panels Recharge the Battery During a Power Cut?
With the right architecture, yes. A backup-capable hybrid or integrated system can allow the solar array to continue operating inside the intentional island and recharge the battery while also supplying household loads. The inverter must balance generation, battery state of charge and demand because there is no public grid available to absorb unlimited surplus.
If the battery reaches its upper charge limit and household demand is low, the system may curtail or stop solar generation. When demand rises or the battery has room again, compatible systems can resume solar production.
A Current Example: How We Translate Powerwall 3 Specifications Into Backup Expectations
Tesla’s current UK information gives Powerwall 3 an energy capacity of 13.5 kWh and states on-grid and backup power of up to 11.04 kW, depending on local conditions and the installed configuration. Those two numbers answer different questions: kWh tells us how much energy is stored; kW tells us how much load the system can support at one moment.
That distinction is central when we assess a Tesla Powerwall 3 installation. We look at the Backup Gateway, permitted output, consumer-unit arrangement, solar connection, DNO position and the actual circuits the customer wants to keep running.
What 13.5 kWh Means in an Outage — Before Real-World Losses
| Steady backed-up load | Simple 13.5 kWh ÷ load calculation | How we interpret it |
|---|---|---|
| 1 kW | 13.5 hours | A useful illustration for light essential loads, but real runtime will be lower once reserve, conversion losses and changing loads are included. |
| 3 kW | 4.5 hours | Shows how quickly runtime falls when several household loads are operating together. |
| 6 kW | 2.25 hours | Demonstrates why having enough instantaneous backup power is not the same as having long-duration backup energy. |
Solar generation during the outage can extend those runtimes where the system is configured to operate and recharge while islanded. We therefore ask which loads matter and when they are likely to run rather than promising a fixed number of backup hours from the battery capacity alone.
Manufacturer source: Tesla UK Powerwall specifications and UK support guidance. Runtime figures above are simple theoretical energy calculations, not guaranteed outage durations.
What Appliances Can Run During a Power Cut?
Most normal household electronics are modest loads individually, but the combination matters. Refrigerators, broadband routers, LED lighting, TVs and laptop chargers are usually straightforward. Kettles, ovens, induction hobs, immersion heaters, electric showers, EV chargers and heat pumps can create much larger instantaneous demand.
| Load type | Backup priority | Design consideration |
|---|---|---|
| Fridge/freezer | High for many homes | Low average energy but compressor start-up creates short peaks. |
| Broadband/router | High | Small continuous load; easy to support for long periods. |
| LED lighting | High | Low load if efficient fittings are used. |
| Gas/oil boiler controls and pumps | Often high | Usually modest power but circuit arrangement must be included in the backup board. |
| Heat pump | Case-specific | Continuous and peak electrical demand must be checked against inverter output. |
| Electric oven/hob | Usually managed | Can consume several kilowatts and shorten runtime rapidly. |
| EV charger | Usually low priority during outage | Large sustained load; smart control or manual avoidance may preserve household backup. |
What Happens the Moment the Power Goes Off?
The exact sequence depends on the equipment. A standard PV inverter detects abnormal voltage/frequency or loss of mains and disconnects. A backup-capable system detects the outage, opens or confirms the grid isolation point and then establishes a local AC supply. The transfer may be very fast, but it should not be marketed as identical to a dedicated online UPS for every sensitive device unless the product specification supports that use.
Some clocks, network equipment or computers can reboot during transfer. If uninterrupted operation is critical, we assess the tolerance of the load rather than assume any whole-home battery is a universal UPS.
What Happens When Grid Power Returns?
The system does not simply reconnect the instant a voltage appears. Grid-connected generation checks that network conditions are within permitted limits and waits for the required reconnection logic. The backup system then transitions the home back to normal grid-parallel operation and resumes its configured solar, battery and tariff strategy.
This controlled transition is a safety function, not an inconvenience. It prevents generation reconnecting to an unstable supply.
Can You Add Backup to an Existing Solar System?
Often, yes, but the route depends on the existing inverter, consumer unit, meter position, phase arrangement and the battery platform being added. An AC-coupled battery can be attractive because it may allow the existing solar inverter to remain in service. A hybrid retrofit can make sense if the original inverter is due for replacement or if a new integrated architecture offers better control.
The survey must confirm whether the solar can operate while islanded, how it communicates with the backup system and what circuits can be separated safely. A battery installation designed only for self-consumption may need significant additional equipment before it can provide outage resilience.
Our Power-Cut Backup Design Checklist
What We Check Before Promising Backup
- Backup objective: essential circuits, whole home or a specific critical load.
- Continuous and surge power: the largest loads that may run at the same time.
- Battery energy: expected outage duration and minimum reserve.
- Solar recharging: whether PV remains operational in island mode and how surplus is controlled.
- Isolation method: automatic gateway, contactor arrangement or approved manual changeover as appropriate.
- Consumer-unit layout: which circuits are on the backed-up side and whether high loads need exclusion or control.
- Phase arrangement: single-phase or three-phase supply and the capabilities of the selected equipment.
- DNO/grid compliance: connection route, export limits and type-tested protection.
- Customer behaviour: a backup system lasts much longer when large discretionary loads are managed during an outage.
- Testing: backup must be commissioned and demonstrated, not simply assumed from a battery icon in an app.
Should You Buy Solar Specifically for Power-Cut Protection?
If resilience is a priority, tell the installer before the system is designed. A standard domestic solar PV system is primarily an energy-generation system. Backup adds a separate set of requirements involving stored energy, islanding hardware, load management and commissioning.
Solar plus storage can provide excellent resilience because daytime generation may extend the usable outage duration, especially in spring and summer. It should still be sized around the loads you genuinely need, not around a headline claim that the battery can "power the whole house".
Need Solar and Battery Backup Designed Properly?
Tell us which circuits matter, how long you want them supported and whether you need automatic or manual changeover. We will compare battery energy, inverter power, solar recharging and the electrical layout before recommending a system.
Request a tailored solar and backup quote and include your annual electricity usage plus any critical loads you want to keep running.
