Lighting
Modern LED lighting typically uses relatively little power and can be extremely useful during an outage.
Solar backup and power cuts explained
What happens to solar panels during a power cut?
It is a perfectly reasonable question: if the sun is shining and your roof is generating electricity, why should your house go dark when the grid fails?
The answer is safety. An ordinary grid-connected solar inverter deliberately stops supplying the grid during a power cut. Keeping your home powered requires a different, carefully designed backup mode.
The short answer
Ordinary solar shuts down. Properly designed backup solar can keep selected parts of the house alive.
During a normal power cut the electricity network disappears from the inverter's point of view.
The inverter detects that loss of mains and stops feeding its normal grid-connected AC output.
The panels on the roof may still be exposed to daylight and may therefore still have DC voltage present.
The panels have not necessarily stopped being photovoltaic.
It is the safe connection between the solar system and the normal household/grid AC system that has deliberately been interrupted.
Tom Solar visual guide
Normal solar versus solar with properly designed backup.
Why shut the inverter down?
Because somebody may be working on the supposedly dead electricity network.
Imagine a storm damages an electricity cable.
The network operator isolates the affected network so engineers can repair it.
If thousands of rooftop solar systems simply continued feeding power into those wires, the supposedly isolated network could remain dangerously energised.
Network off must mean customer generation stops feeding the network too.
That is why grid-connected generation incorporates loss-of-mains protection.
Anti-islanding
The inverter constantly watches the electricity network.
Under normal operation the inverter works in parallel with the grid.
It monitors electrical conditions such as voltage and frequency.
If the network disappears or moves outside the permitted operating conditions, the system's interface protection disconnects generation from the distribution network.
This automatic behaviour is commonly called anti-islanding.
It prevents an ordinary grid-connected inverter from accidentally creating its own uncontrolled electrical island on the public network.
Important safety point
“The solar has shut down” does not mean the rooftop DC side is safe to touch.
Photovoltaic cells produce electrical voltage when exposed to sufficient light.
Switching off the grid or isolating the inverter's AC connection does not magically stop sunlight reaching the modules.
Solar DC conductors must be treated as potentially live in daylight.
Electrical work on a PV installation should be carried out using appropriate isolation procedures by people competent to work on solar DC systems.
So how can backup work?
First disconnect from the public network. Then create a private local supply.
A deliberately designed backup system behaves differently from ordinary grid-connected solar.
When the public network fails, suitable switching isolates the backed-up electrical installation from the grid.
Compatible power-conversion equipment can then establish the voltage and frequency required to supply selected household circuits.
The home becomes a controlled electrical island — hence the term “island mode”.
Normal mode versus island mode
The electrical architecture changes when the grid disappears.
| Normal grid operation | Island / backup operation |
|---|---|
| House is electrically connected to the public network. | Backed-up circuits are electrically separated from the public network. |
| Grid establishes normal system voltage and frequency. | Suitable backup power-conversion equipment establishes the local supply. |
| Grid can supply loads beyond the inverter's own output. | Loads are limited by backup inverter power and available stored/generated energy. |
| Surplus solar can normally export. | There must be no export into the failed public network. |
A battery is not enough
“I have 10kWh in the garage” does not prove the lights will stay on.
The battery is simply the store of energy.
Backup operation also depends on:
a compatible inverter or power-conversion system;
safe electrical isolation from the public network;
correct switching arrangements;
suitable earthing and protection during island mode;
properly selected backed-up circuits;
and equipment configured and commissioned for backup operation.
Battery storage and blackout protection are separate features.
Never assume one automatically gives you the other.
EPS
What does the “EPS” connection on an inverter mean?
Manufacturers frequently use the term EPS — Emergency Power Supply.
The exact meaning varies between products.
It might describe:
one dedicated backup socket;
a separate output supplying selected essential circuits;
or part of a larger automatic whole-house backup arrangement.
Do not compare systems using the word “EPS” alone.
Ask exactly what is powered, at what maximum output, for how long, and what happens automatically when the grid fails.
Essential loads
Sometimes the clever solution is not to back up everything.
Fridge & freezer
Keeping food refrigeration alive is often far more valuable than trying to run every appliance in the property.
Internet & communications
Router, phone charging and selected sockets can keep the household connected.
Heating controls
Depending on the heating system, keeping controls or circulation equipment alive may be important.
Whole-house backup
“Everything stays on” sounds wonderful — until somebody turns on several big loads at once.
When connected to the grid, the house can draw power from the grid whenever household demand exceeds solar and battery inverter output.
During island mode that safety net has disappeared.
The backup inverter has a finite output.
Electric shower: potentially a very large load.
Electric hob or oven: several kilowatts.
EV charger: commonly around 7kW single phase.
Heat pump: variable demand plus starting considerations.
Kettle: often around 3kW.
Battery capacity does not tell you how much power you can use at once.
That is determined by the inverter's backup output, battery discharge capability and system design.
kW versus kWh — again
One tells you how hard the system can work. The other tells you how long.
kW is power.
It limits how much electrical load the backup inverter can support at a moment.
kWh is energy.
It determines how much stored energy is available before the battery reaches its permitted discharge limit.
A huge battery behind a small inverter still has a small instantaneous power limit.
Simple illustration
Imagine a 5kW backup inverter and 10kWh of usable stored energy.
5kW
Illustrative maximum backup power available at a moment.
10kWh
Illustrative stored electrical energy available.
2kW
Illustrative constant household backup load.
Ignoring conversion losses and reserve limits for simplicity:
10kWh ÷ 2kW ≈ 5 hours.
But if the household suddenly attempts to draw 8kW, a 5kW island-mode inverter cannot simply supply it merely because plenty of energy remains in the battery.
Can solar keep generating?
In a good backup design, sometimes yes — but this is not automatic.
Once the property has been safely isolated from the public network, suitable equipment can create a local AC electrical system.
Depending on the architecture, compatible solar generation may then continue operating inside that local island.
Solar could:
power household loads directly;
reduce the amount being drawn from the battery;
or recharge the battery when generation exceeds demand.
But something must control surplus generation.
If the battery is full and the house is consuming little power, the system cannot export excess electricity into a dead public network. Compatible controls must reduce or stop generation as required.
Black start
What happens if the battery reaches empty before daylight returns?
This is where system capability becomes particularly important.
Some equipment supports what is commonly called black start.
That means the system can establish or re-establish its local electrical supply without first relying on the public grid.
Other systems may require sufficient battery state of charge, a live grid reference or a particular startup sequence.
Solar panels on the roof do not guarantee that a completely flat backup system can wake itself up at sunrise.
This is an equipment-specific feature and should be checked before purchase.
Automatic changeover
Will I even notice the grid has failed?
Possibly.
Some backup systems change operating mode very quickly.
Others have a noticeable interruption while switching.
The exact transfer time depends on the equipment and system architecture.
“Backup” does not automatically mean “uninterruptible”.
If equipment genuinely cannot tolerate even a brief interruption, check whether the proposed system meets the required continuity standard rather than assuming any battery backup behaves like a UPS.
EV chargers
Your car can empty a domestic backup battery remarkably quickly.
A normal single-phase EV charger may draw around 7kW.
That is larger than the total island-mode output of many domestic battery inverters.
Even where the inverter could support it, using precious backup energy to charge a vehicle may make little practical sense during an outage.
EV charging is commonly a load you deliberately shed during backup operation.
Keep the lights, fridge, communications and genuinely important household services running instead.
Heat pumps
Backup heating needs more thought than simply reading the battery label.
A heat pump can be a significant electrical load.
Its backup suitability depends on:
normal running power;
compressor starting behaviour;
auxiliary or backup heaters;
circulation pumps and controls;
battery inverter output;
and how long the outage may last.
Resistive backup heating can be particularly demanding.
A system that comfortably runs the heat-pump compressor may not necessarily support several kilowatts of auxiliary electric heating as well.
Earthing and protection
Disconnecting from the grid changes more than where the electricity comes from.
The normal electricity network forms part of the property's electrical protection and earthing environment.
Once an installation deliberately operates as an island, the designer must ensure that protective measures still operate correctly.
Current MCS battery-storage requirements include specific provisions for:
isolation of the islanded live conductors from the grid;
neutral arrangements;
earthing;
protection against electric shock;
fault protection;
and correct protective-device operation.
Backup is an electrical-system design problem — not an optional software tick box.
It needs to be designed and installed as such.
When the grid comes back
The inverter does not simply reconnect blindly.
Grid-connected equipment monitors the restored electricity network.
Before normal parallel operation resumes, the system must see acceptable network conditions and satisfy its protection and reconnection requirements.
A backup-capable installation then leaves island mode and returns to its normal grid-connected operating state.
In a properly designed automatic system, most of this happens without the homeowner doing anything.
Exact behaviour varies between manufacturers and system architectures.
Questions worth asking before buying
“Does it have backup?” is only the first question.
What actually stays powered?
One socket, selected circuits or the majority of the house?
What is the backup power rating?
The normal grid-connected inverter rating and island-mode rating are not always identical.
Can solar operate while islanded?
Ask whether PV continues supporting loads and battery charging during an outage.
Can it black start?
Ask what happens if the battery reaches its minimum state of charge overnight.
How fast is changeover?
Determine whether sensitive equipment will experience an interruption.
Which loads are excluded?
EV chargers, showers, ovens, immersion heaters and heat pumps may need special treatment.
Frequently asked questions
Solar, batteries and blackouts answered.
Do solar panels work during a power cut?
The panels can still develop DC voltage in daylight, but an ordinary grid-connected inverter stops supplying its normal AC output when the public network fails.
Why does solar shut down?
Loss-of-mains protection prevents local generation from continuing to energise a public network that should be isolated.
What is anti-islanding?
It is protection that disconnects local generation from the public network when the network supply disappears.
Will having a battery keep my house powered?
Not automatically. Backup requires compatible inverter equipment, switching, isolation, earthing, protection and a system designed for island operation.
What does EPS mean?
It commonly means Emergency Power Supply, but exactly what the EPS output can supply varies significantly between manufacturers.
Can I back up the whole house?
Potentially, but the design must stay within backup inverter power, battery discharge capability and stored-energy limits. Large loads may need to be excluded.
Can solar recharge the battery during an outage?
Some systems can. It depends on the inverter architecture, system compatibility and the controls available in island mode.
What is black start?
It is the ability of compatible equipment to establish or re-establish a local islanded electrical supply without first relying on the public grid.
Can I charge my EV?
Only if the backup installation was designed for such a large load. EV charging is often deliberately disabled during a domestic outage.
Can a heat pump continue running?
It may be possible where the inverter and battery can support its running and starting requirements, but it should be deliberately assessed rather than assumed.
Is battery backup the same as a UPS?
Not necessarily. Some home battery systems have a transfer delay. Equipment requiring truly uninterrupted power should have its continuity requirements checked separately.
What happens when grid power returns?
The inverter monitors the restored network and reconnects only after acceptable conditions and its required reconnection process have been satisfied.
Technical basis checked
Backup power is useful precisely because the safety rules are strict.
This homeowner guide was reviewed against current material including:
- Energy Networks Association — Engineering Recommendation G98 and loss-of-mains protection requirements.
- MCS — MIS 3012 Battery Storage Systems and island-mode requirements.
- IET — 2026 guidance on island-mode prosumer electrical installations.
- IET — Code of Practice for Electrical Energy Storage Systems.
- Ofgem — consumer guidance for planning for power cuts.
Backup capability varies substantially between manufacturers and installation architectures. Product documentation and the design of the specific electrical installation take precedence over the simplified homeowner examples used here.
Related Tom Solar guides
Follow the electricity through the system.
Tom Solar view
If backup matters to you, specify it before the system is designed.
This is one of those subjects where a customer's perfectly reasonable assumption can be completely different from what the equipment actually does.
“Solar plus battery” sounds like a house that keeps running through a blackout.
Sometimes it does.
Sometimes it powers only one emergency socket.
Sometimes it has no blackout functionality at all.
Decide what you want to survive a power cut before choosing the inverter and battery.
Then the switching, backup power, battery capacity and essential circuits can be designed around that requirement instead of being discovered after installation.