Introduction

Many homeowners purchase a hybrid solar system because they expect the lights, refrigerator and internet equipment to continue operating during a utility outage.

However, a common customer complaint is:

“The battery was charged, but the entire house lost power when the grid failed.”

The customer may see:

• Battery SOC above 80%
• Solar panels producing power
• Inverter display operating normally
• No obvious battery alarm

Yet the household loads still switch off.

In many cases, the battery is not defective. The real problem is that the electrical system was not designed or configured for backup operation.

A grid-connected solar system, a battery energy storage system and a true backup system are not automatically the same thing.

To provide power during an outage, the system must include the correct:

• Hybrid or battery inverter
• Backup or EPS output
• Critical-load distribution panel
• Grid-isolation function
• Battery reserve
• Inverter operating mode
• Neutral and grounding arrangement
• Load capacity
• Transfer settings
• Commissioning test

A Battery Does Not Automatically Make Every Solar System a Backup System

A standard grid-tied inverter is designed to operate while the utility grid is available.

During a grid outage, it normally disconnects from the grid for safety. This prevents the solar system from energizing utility lines that technicians may be repairing.

This protective behaviour is commonly known as anti-islanding.

Therefore, adding a battery near an existing grid-tied inverter does not automatically allow the home to operate during an outage.

The system requires equipment capable of creating its own stable AC voltage and frequency when the utility grid is absent.

This operating condition may be described as:

• Grid-forming operation
• Backup mode
• EPS mode
• Off-grid mode
• Island mode

The exact terminology varies by inverter manufacturer.

Grid Output and Backup Output Are Different

Many hybrid inverters contain separate AC connections.

Grid Connection

The grid terminal connects the inverter to the utility supply and main electrical system.

Its functions may include:

• Exporting solar power
• Importing grid power
• Charging the battery
• Synchronizing with utility voltage and frequency
• Measuring household consumption

Backup or EPS Connection

The backup terminal supplies selected loads when the grid is unavailable.

Its functions may include:

• Creating local AC voltage
• Supplying critical loads
• Isolating the backup circuit from the grid
• Using battery and available solar power
• Restarting loads after an outage

The backup output must not be incorrectly connected to the utility grid. GoodWe installation documentation, for example, warns that the backup side must not be connected to the grid.

If all household circuits are connected only to the normal grid side, they may lose power even though the inverter and battery support backup operation.

What Is a Critical-Load Panel?

A critical-load panel is a separate distribution board containing the circuits that should remain powered during an outage.

Typical critical loads include:

• Refrigerator
• Freezer
• Internet router
• Basic lighting
• Security equipment
• Medical equipment
• Selected wall sockets
• Small water pump
• Communication equipment

Non-critical loads may include:

• Electric water heater
• Large air conditioner
• Electric oven
• EV charger
• Pool heater
• Workshop machinery
• Large irrigation pump
• Sauna
• Electric boiler

The critical-load panel connects to the inverter’s approved backup or EPS output.

Solis documentation similarly describes connecting critical loads to the backup port while managing non-critical or sheddable loads separately.

Why Whole-House Backup Is More Difficult

Some systems support whole-house backup, but the inverter and battery must be designed for the complete property load.

Whole-house backup may require support for:

• Large continuous power
• Several motor-starting loads
• High short-duration surge power
• Three-phase loads
• Unbalanced phase loads
• Electric heating
• Air-conditioning systems
• Water pumps
• Large neutral current
• Automatic transfer equipment

A house may normally consume only 1kW or 2kW, but several appliances can start simultaneously after a grid outage.

For example:

• Refrigerator restarts.
• Freezer restarts.
• Water pump starts.
• Air conditioner restarts.
• Electric kettle remains switched on.
• Water heater reconnects.

The combined demand may exceed:

• Backup-port output rating
• Inverter surge capacity
• Battery BMS discharge limit
• Main battery cable capacity
• Backup breaker rating

A properly selected critical-load panel reduces this risk.

Reason 1: The Loads Are Connected to the Wrong Inverter Output

This is one of the most common installation causes.

The battery may power the inverter, but the household loads remain connected to the grid-only distribution side.

During normal grid operation, everything appears correct.

During a power failure:

• Grid output is isolated.
• Backup output becomes active.
• No loads are connected to the backup output.
• The customer experiences a complete blackout.

How to Prevent It

Before installation, prepare a single-line diagram showing:

• Utility grid
• Main distribution board
• Hybrid inverter grid terminal
• Hybrid inverter backup terminal
• Critical-load panel
• Battery bank
• PV input
• Generator connection, if applicable
• Meter and current transformers
• Grounding arrangement

Every backup circuit should be identified clearly.

Reason 2: Backup Mode Is Disabled

Some hybrid inverters require backup functions to be enabled during commissioning.

Possible settings include:

• Backup enable
• EPS enable
• Off-grid mode
• Backup reserve
• Battery discharge permission
• Black-start enable
• Grid-forming enable
• Backup-port voltage
• Backup-port frequency

The inverter may be physically wired correctly but still not energize the backup output because the required mode is disabled.

Prevention Checklist

Confirm:

• Correct country or grid code
• Backup output enabled
• Battery discharge enabled
• Correct AC output voltage
• Correct frequency
• Correct battery protocol
• Minimum backup SOC
• Black-start function
• Automatic restart setting

Record the final configuration in the commissioning report.

Reason 3: Battery SOC Is Below the Backup Reserve

Many hybrid systems reserve part of the battery for emergencies.

Example:

• Battery SOC: 35%
• Configured backup reserve: 40%

The inverter may refuse to discharge the battery because the available SOC is below the protected reserve.

The customer sees energy in the battery but cannot use it during the outage.

How to Prevent It

Define separate operating levels for:

• Daily self-consumption
• Time-of-use discharge
• Backup reserve
• Generator start
• Controlled shutdown
• BMS protection

The reserve should reflect:

• Grid reliability
• Required backup duration
• Critical-load consumption
• Weather conditions
• Generator availability
• Expected outage length

Do not set the reserve so high that the customer rarely uses the battery, or so low that no useful emergency energy remains.

Reason 4: Battery Discharge Is Disabled

Battery discharge may be disabled by:

• Inverter schedule
• BMS alarm
• Low temperature
• High temperature
• Low SOC
• Cell undervoltage
• Communication failure
• Time-of-use setting
• Manual battery switch
• Remote shutdown input
• Emergency stop

Check both the inverter and battery displays.

A system may show the battery online while the BMS reports:

• Discharge current limit: 0A
• Discharge MOS off
• Contactor open
• Discharge forbidden
• Battery protection active

The battery must provide a positive discharge permission and sufficient current limit for backup operation.

Reason 5: The Backup Load Exceeds the EPS Rating

The backup output may have a different rating from the inverter’s normal grid-connected output.

For example, an inverter may provide:

• 10kW grid-connected output
• Lower continuous backup output
• A specific overload duration
• A limited motor-starting capability

Check:

• Continuous EPS power
• Peak EPS power
• Overload duration
• Maximum current per phase
• Three-phase imbalance allowance
• Neutral current
• Half-wave load support
• Motor-load limitations

Do not assume that the inverter’s model name represents its full backup capability.

Reason 6: The Battery Bank Cannot Supply the Backup Current

Even when the inverter supports the load, the battery must provide sufficient DC current.

Approximate battery current can be estimated using:

Battery Current = AC Load Power ÷ Battery Voltage ÷ Inverter Efficiency

For an 8kW backup load using a 51.2V battery and assuming 92% efficiency:

8,000W ÷ 51.2V ÷ 0.92 ≈ 170A

A single battery with a 100A BMS cannot normally support this continuous demand.

The battery bank must be sized according to:

• Backup energy
• Continuous power
• Surge power
• BMS current
• Minimum battery voltage
• Temperature
• Parallel current sharing
• Future load growth

Reason 7: Inverter Transfer Time Is Too Long for Sensitive Equipment

When the grid fails, the inverter needs time to:

1. Detect the outage.
2. Disconnect from the grid.
3. Establish local AC voltage.
4. Transfer the backup loads.

This transfer may be fast, but it is not always completely uninterrupted.

Some equipment may restart even after a very short interruption:

• Desktop computers
• Servers
• Network switches
• Medical equipment
• Sensitive control systems
• Industrial PLCs
• Digital clocks

A product described as having backup capability is not automatically equivalent to an online UPS.

How to Prevent It

Before purchasing the inverter, check:

• Specified transfer time
• UPS mode availability
• EPS mode transfer time
• Supported load types
• Whether the critical equipment has its own UPS

For highly sensitive loads, install a dedicated UPS between the backup panel and the equipment where appropriate.

Reason 8: The Inverter Does Not Support Battery-Only Black Start

Black start means starting the inverter without an available utility grid.

Some systems can start using:

• Battery only
• Battery plus PV
• Grid only
• Generator only

Other systems require a specific sequence or minimum battery voltage.

Before installation, test:

• Grid disconnected
• PV disconnected
• Inverter fully powered down
• DC capacitors discharged
• Battery at medium SOC
• Battery-only start

A system that works after a brief grid interruption may still fail after a complete overnight shutdown.

Reason 9: The Battery Is in Sleep Mode

After prolonged low SOC or shutdown, the battery may enter sleep mode.

When the grid fails, the inverter may not detect an active battery.

Recovery may require:

• Battery power-button operation
• BMS reset
• Grid charging
• Generator charging
• Approved external charger
• Remote wake-up signal

The correct recovery method should be included in the customer manual.

For remote sites, avoid a design that requires manual battery activation unless local access is always available.

Reason 10: Incorrect Neutral and Grounding Arrangement

Backup operation changes the AC source.

When the grid is present, the utility normally provides the system reference.

During island operation, the inverter may need to provide or control the neutral-earth relationship.

Depending on the inverter and local electrical rules, the system may use:

• Internal neutral switching
• External transfer switch
• Neutral-earth bonding relay
• Permanently connected neutral
• Switched neutral
• External isolation transformer

Incorrect neutral or grounding arrangements may cause:

• RCD or GFCI tripping
• Backup output failure
• Voltage on exposed metal
• Unstable appliance operation
• Generator conflicts
• Multiple neutral-earth bonds

This work should be designed and tested by qualified electrical personnel.

Reason 11: Incorrect Grid-Isolation or Transfer Equipment

The backup system must prevent power from feeding into the failed grid.

Isolation may be provided by:

• Internal inverter relay
• External automatic transfer switch
• Backup interface
• Contactor system
• Gateway or system controller

If the required transfer equipment is missing or incorrectly wired, the inverter may block backup operation for safety.

The system diagram must match the inverter manufacturer’s approved architecture.

Reason 12: The Generator Is Connected to the Wrong Port

Some customers add a generator to extend battery backup.

The generator cannot automatically connect to any AC terminal.

Depending on the inverter, it may connect to:

• Grid port
• Dedicated generator port
• Smart port
• External transfer switch
• Separate AC charger

Solis specifically warns that generators should not be connected to the backup port; supported connections use the appropriate grid or smart port architecture.

Incorrect generator connection can prevent backup operation or create an unsafe AC source conflict.

How to Design the Critical-Load Panel

Step 1: List Every Essential Appliance

Record:

• Running power
• Starting power
• Daily energy
• Operating time during outage
• Phase
• Whether it starts automatically

Step 2: Calculate Maximum Simultaneous Power

Include realistic overlapping loads.

Step 3: Calculate Required Backup Energy

Use:

Required Energy = Load Power × Backup Time

Then include:

• Inverter losses
• Battery reserve
• Maximum depth of discharge
• Battery ageing allowance
• Temperature allowance

Step 4: Separate High-Power Loads

Consider excluding or controlling:

• Electric water heating
• EV charging
• Large air conditioning
• Electric oven
• Large pumps
• Workshop machinery

Step 5: Label Every Circuit

Use labels such as:

• BACKUP SUPPLY
• GRID ONLY
• CRITICAL LOAD
• NON-CRITICAL LOAD
• DO NOT CONNECT TO GRID OUTPUT

Example Backup Design

Customer requirements:

• Refrigerator: 200W
• Freezer: 250W
• Lights: 300W
• Router and CCTV: 100W
• Small pump: 800W
• Miscellaneous sockets: 500W
• Required backup time: 10 hours

Estimated normal running load:

2.15kW maximum when all listed loads operate

The system must also consider:

• Pump starting surge
• Refrigerator compressor surge
• Inverter standby consumption
• Nighttime energy
• Battery reserve

A suitable design might use:

• Dedicated critical-load panel
• Hybrid inverter with adequate EPS output
• Multiple parallel battery modules
• Pump start control
• Minimum backup reserve
• Separate non-essential-load circuits

The final equipment size must be calculated from actual product specifications.

Commissioning Tests Before Handover

Test 1: Simulated Grid Failure

Open the approved grid isolator and confirm:

• Backup output remains active.
• Grid output is isolated.
• No reverse power reaches the grid side.
• Critical loads continue operating.

Test 2: Battery-Only Black Start

Turn off grid and PV, shut down the inverter fully, and restart from the battery using the approved procedure.

Test 3: Low-SOC Backup

Test at the minimum normal backup SOC.

Test 4: Maximum Critical Load

Operate the approved critical loads together.

Test 5: Motor Start

Start the largest pump, refrigerator or compressor.

Test 6: Automatic Grid Return

Restore grid power and confirm smooth reconnection.

Test 7: Repeated Outage

Simulate a second outage after the system has returned to normal.

Test 8: Communication Failure

Where approved, confirm the safe response if battery communication is lost.

Customer Handover Information

The customer should receive:

• Critical-load circuit list
• Maximum backup power
• Expected backup duration
• Backup reserve SOC
• Loads prohibited during outages
• Grid-failure operating procedure
• Battery wake-up procedure
• Generator procedure
• Emergency shutdown procedure
• Inverter and BMS alarm guide
• Technical-support contact information

Common Prevention Mistakes

Assuming Every Solar Circuit Has Backup

Only circuits connected to the approved backup architecture will remain powered.

Connecting the Backup Output to the Main Grid Bus

This can create a dangerous backfeed path.

Placing Every Household Load on the Backup Panel

The inverter or battery may overload immediately.

Ignoring Motor Starting Current

A small pump can require substantial short-duration power.

Setting Backup Reserve to Zero

No emergency energy remains when the grid fails.

Testing Only While the Grid Is Available

A true outage test is required.

Ignoring Transfer Time

Sensitive equipment may still restart.

Failing to Test Black Start

The system may depend on grid power for initialization.

Using the Wrong Generator Port

Generator and backup outputs serve different electrical functions.

Frequently Asked Questions

Why do the solar panels stop working when the grid fails?

A standard grid-tied inverter shuts down for anti-islanding safety. A compatible hybrid or grid-forming system is required for solar production during an outage.

Can the whole house use battery backup?

Possibly, but the inverter, battery and transfer system must support the complete load and surge demand.

Why does only one part of the house have power?

Those circuits are probably connected to the critical-load or backup panel.

Can I connect the backup output to the main distribution board?

Only through an approved whole-house backup or transfer architecture. Direct connection can create dangerous grid backfeed.

Why does the inverter show battery power but the sockets are off?

The inverter may be operating while the loads are connected to the grid-only output, or the backup output may be disabled.

Will solar panels charge the battery during an outage?

They can in a properly designed hybrid or off-grid system, subject to inverter mode, battery condition and available solar power.

Conclusion

A LiFePO4 battery does not automatically guarantee power during a grid outage.

Reliable backup requires:

• A grid-forming hybrid or battery inverter
• Correct use of the backup or EPS output
• A properly designed critical-load panel
• Adequate battery current and energy
• Correct reserve settings
• Approved grid-isolation equipment
• Correct neutral and grounding design
• Black-start capability
• Real outage commissioning tests
• Clear customer operating instructions

For HIZN Lithium system matching, provide:

• Inverter brand and model
• Battery voltage
• Battery quantity
• Critical-load list
• Maximum load power
• Largest motor load
• Required backup time
• Single-phase or three-phase system
• Grid and generator arrangement
• Country of installation

This information allows the battery bank and backup architecture to be evaluated before installation.