Introduction

A solar energy storage system should not be designed only around daily kilowatt-hour consumption.

Many system failures are caused by short-duration power demand rather than insufficient daily energy.

A house may consume only 8kWh per day but contain equipment such as:

• Refrigerator
• Freezer
• Water pump
• Air conditioner
• Air compressor
• Washing machine
• Power tool
• Irrigation pump

These appliances may require much more power during start-up than during normal operation.

To prevent inverter shutdown, the designer must prepare a surge-power budget covering the inverter, battery, BMS, cables, breakers and actual load sequence.

Begin with a Load Start-Up Survey

Create a list of every significant appliance.

Record:

• Appliance name
• Quantity
• Rated running power
• Measured running power
• Starting power
• Starting current
• Start duration
• Daily operating hours
• Whether it may start automatically
• Whether it can start simultaneously with another load
• Whether it is critical

An example survey might look like this:

Load	Running Power	Estimated or Measured Start Demand	Automatic Start?
Refrigerator	200W	900W	Yes
Water pump	1,100W	3,500W	Yes
Air conditioner	1,500W	4,000W	Yes
Freezer	300W	1,200W	Yes
Lighting	400W	400W	No
Electric kettle	2,000W	2,000W	No

The values should preferably be obtained from:

• Appliance documentation
• Manufacturer data
• Clamp-meter testing
• Power-quality analyser
• Inverter event records

Do not assume that every motor starts at exactly three or five times its rated power.

Separate Energy Sizing from Power Sizing

Battery energy is measured in kilowatt-hours.

Instantaneous system capability is measured in kilowatts and amperes.

These are separate design questions:

Energy Question

How many hours must the loads operate?

Power Question

What is the largest continuous and short-duration demand?

A large-capacity battery with a low-current BMS may provide long backup time but still fail to start a pump.

A smaller high-power battery may start the pump but provide insufficient overnight energy.

A correct system must satisfy both requirements.

Calculate the Continuous Power Requirement

Add all loads that can reasonably operate at the same time.

Do not simply add every appliance in the building if some loads are never used simultaneously.

Create realistic operating scenarios.

Evening Scenario

• Refrigerator: 200W
• Lighting: 400W
• Television and electronics: 300W
• Air conditioner: 1,500W
• Water pump: 1,100W

Total running demand:

3,500W

Daytime Scenario

• Refrigerator: 200W
• Water pump: 1,100W
• Washing machine: 800W
• Workshop tools: 2,000W

Total running demand:

4,100W

Use the highest realistic continuous scenario and include suitable design margin.

Create a Simultaneous-Start Scenario

The worst event is not always the largest individual load.

For example:

• Air conditioner is already operating.
• Refrigerator compressor starts.
• Water-pressure switch starts the pump.
• The user switches on a kettle.

The system may experience several demands at once.

Create an overlap matrix identifying which loads can start automatically.

Load Combination	Possible?	Control Required?
Refrigerator + freezer	Yes	Usually manageable
Pump + air conditioner	Yes	May require sequencing
Pump + kettle	Yes	User education or load control
Two large pumps	No by design	Electrical interlock
Air conditioner + workshop compressor	Possible	Priority controller recommended

This step often prevents more problems than simply purchasing a larger inverter.

Check the Inverter Surge Curve, Not Just the Headline Rating

An inverter datasheet may advertise a surge rating, but the useful details are:

• Maximum surge power
• Duration at surge power
• Duration at intermediate overload
• Temperature derating
• Battery-voltage derating
• Power-factor limitations
• Motor-starting guidance
• Automatic-restart behaviour

A 10kW surge rating lasting a few milliseconds is not equivalent to 8kW available for five seconds.

A pump may require enough torque for several seconds, especially when:

• The pipe is pressurised.
• The pump is starting under load.
• Supply voltage is low.
• Mechanical components are worn.
• Ambient temperature is high.
• The motor uses a basic direct-on-line start.

Select the inverter from its complete overload curve.

Convert the AC Surge into Battery Current

The battery must supply the inverter’s surge from the DC side.

Approximate current can be calculated as:

DC Current = AC Power ÷ Battery Voltage ÷ Inverter Efficiency

For a temporary 8kW load on a 51.2V system at 90% efficiency:

8,000W ÷ 51.2V ÷ 0.90 ≈ 174A

At a lower battery voltage of 46V:

8,000W ÷ 46V ÷ 0.90 ≈ 193A

This current passes through:

• Battery cells
• Internal busbars
• BMS
• Contactors or MOSFETs
• Battery terminals
• Branch cables
• Fuses or breakers
• Main busbars
• Main inverter cables

Every component must support the required duration.

Understand the Battery’s Different Current Ratings

A battery datasheet may specify:

• Recommended discharge current
• Maximum continuous discharge current
• Peak discharge current
• Short-circuit protection current

These values are not interchangeable.

Recommended Current

A preferred operating range for efficiency, temperature and service life.

Maximum Continuous Current

The highest current permitted for continuous operation under specified conditions.

Peak Current

A higher current permitted for a limited time.

Protection Threshold

The level at which the BMS begins to protect the battery. It is not a normal operating target.

Ask the battery supplier for both the peak current and permitted duration.

“200A peak” is incomplete without stating whether it is permitted for:

• 100 milliseconds
• One second
• Five seconds
• Thirty seconds

Add Battery Modules to Reduce Current per Module

Suppose a system requires 180A during motor start-up.

One Battery

• BMS continuous current: 100A
• Peak current: 150A

This battery is unsuitable for the event.

Two Identical Parallel Batteries

The current may be shared at approximately 90A per module when the wiring and batteries are well balanced.

This provides more margin, although actual current sharing will not be perfectly equal.

Three Identical Parallel Batteries

Approximate current per battery may fall to about 60A.

This can improve:

• Voltage stability
• BMS margin
• Terminal temperature
• Cycle life
• Future expansion capability

The number of batteries must remain within the manufacturer’s approved parallel limit.

Apply a Design Margin

Avoid designing the system so that every component operates exactly at its advertised limit.

Margin is needed for:

• Low SOC
• Battery ageing
• High temperature
• Low temperature
• Cable resistance
• Unequal parallel current sharing
• Inverter efficiency variation
• Future appliances
• Motor condition
• Measurement uncertainty

For critical off-grid sites, greater margin may be appropriate than for a grid-supported residential system.

Design the Cables for Surge Conditions

Cable design should check both:

• Continuous thermal current
• Short-duration voltage drop

A cable may not overheat during a two-second surge, but excessive voltage drop can still cause the inverter to shut down.

Official equipment manuals identify inadequate battery-cable cross-sectional area as a cause of excessive voltage drop, and inverter installation guidance recommends minimising battery-cable distance.

Measure or calculate resistance across:

• Positive cable
• Negative cable
• Battery branch protection
• Main protection
• Lugs
• Busbars
• Disconnects

A low-voltage 24V system is especially sensitive because it requires approximately twice the current of a 48V system for the same power.

Consider a Higher Battery-System Voltage

For higher-power applications, increasing the DC-system voltage reduces current for the same power.

For example, ignoring efficiency:

5kW at 25.6V

Approximately 195A

5kW at 51.2V

Approximately 98A

5kW at 102.4V

Approximately 49A

Higher voltage can reduce:

• Cable size
• Voltage drop
• Busbar current
• Terminal heating

However, higher-voltage systems require:

• Approved battery architecture
• Compatible inverter
• Higher-voltage protection
• Greater insulation control
• Trained installers
• Appropriate safety procedures

Do not create a high-voltage battery by externally connecting low-voltage products unless the manufacturer explicitly approves the configuration.

Use Soft Starting Where Appropriate

Motor-starting demand can often be reduced at the load rather than by increasing every battery-system component.

Possible solutions include:

• Soft starter
• Variable-frequency drive
• Inverter-type air conditioner
• Variable-speed pump
• Compressor unloading system
• Staged motor start
• Pressure-tank adjustment
• Reduced mechanical starting load

A soft starter reduces electrical and mechanical stress during acceleration.

A variable-frequency drive can provide controlled acceleration and speed, but it must be correctly selected for:

• Motor type
• Motor power
• Supply voltage
• Phase configuration
• Starting torque
• Environmental conditions

Do not add a generic soft starter to an appliance with built-in electronics without manufacturer approval.

Separate Essential and Non-Essential Loads

A well-designed backup system should not power every building circuit indiscriminately.

Create:

Essential-Load Panel

For example:

• Refrigerator
• Lighting
• Internet
• Security
• Medical equipment
• Selected outlets

Controlled Heavy-Load Panel

For example:

• Water heater
• Large pump
• Air conditioner
• Welding machine
• Electric oven
• EV charger

During grid failure, heavy loads can be:

• Disabled
• Limited
• Sequenced
• Operated only during high solar production
• Enabled only above a defined SOC
• Enabled only when the generator is available

This approach can provide better reliability than oversizing the battery for rare coincident loads.

Install Automatic Load Management

A load-management controller can prevent several heavy appliances from starting simultaneously.

Possible control signals include:

• Battery SOC
• Inverter output power
• Battery discharge current
• Grid availability
• Generator status
• Solar surplus
• Time of day
• Priority level

Example priority:

1. Medical and communication loads
2. Refrigerator and lighting
3. Water pump
4. Air conditioning
5. Water heating
6. Workshop machinery

When demand approaches the limit, lower-priority loads are temporarily disconnected.

Schedule Flexible Loads During Solar Production

In off-grid systems, heavy daytime operation can reduce battery stress because solar power supplies part of the load directly.

Suitable loads may include:

• Water pumping
• Irrigation
• Washing machine
• Water heating
• Ice production
• Workshop equipment
• EV charging

Operating these loads around peak solar hours can:

• Reduce battery discharge current
• Preserve overnight capacity
• Reduce cycle depth
• Lower generator use
• Improve total system efficiency

However, the system should still be capable of safely handling rapid changes in solar production.

Cloud cover can transfer the load back to the battery within seconds.

Coordinate Inverter and BMS Limits

In a closed-loop system, the BMS may send a dynamic discharge-current limit to the inverter.

Verify that:

• The correct battery protocol is selected.
• The inverter displays the BMS limit.
• Parallel battery quantity is detected.
• The limit increases appropriately when batteries are added.
• The inverter reduces output when the BMS limit falls.
• Communication-loss behaviour is safe.

Some inverter manuals state that abnormal battery communication or a BMS limitation can restrict charging and discharging or shut down backup supply.

Do not assume that communication automatically guarantees correct power coordination. The values should be checked during commissioning.

Avoid Excessive Low-Voltage Cut-Off Settings

Setting the inverter cut-off voltage too high can cause premature shutdown under surge.

Setting it too low can drive the battery into BMS undervoltage protection.

Configure:

• Inverter low-voltage warning
• Inverter shutdown voltage
• Restart voltage
• BMS cell-undervoltage threshold
• BMS pack-undervoltage threshold
• Minimum SOC reserve

These values should be coordinated so that the inverter normally reduces or stops the load before the battery enters hard protection.

A hard BMS shutdown should be an emergency protection event—not the normal daily control method.

Test the System at More Than One SOC

A motor may start successfully at 100% battery SOC but fail at 20%.

Commissioning should include start tests at:

• High SOC
• Medium SOC
• Minimum normal operating SOC

Monitor:

• Battery-terminal voltage
• Inverter-terminal voltage
• Minimum cell voltage
• Peak battery current
• BMS current limit
• Inverter output voltage
• Load start time
• Breaker behaviour

Do not intentionally force the battery below its approved operating limit.

Test at Realistic Temperature

Battery resistance and inverter capability can change with temperature.

Consider:

• Hot battery room
• Cold winter installation
• Direct sunlight on inverter
• Dust-blocked ventilation
• High-altitude inverter derating
• Enclosed battery cabinet

A system commissioned in mild weather may behave differently during the hottest or coldest season.

Use the manufacturer’s temperature-derating data.

Perform Individual and Combined Load Tests

Test sequence:

1. Start the refrigerator alone.
2. Start the pump alone.
3. Start the air conditioner alone.
4. Operate normal background loads.
5. Start each motor with background loads active.
6. Test approved simultaneous combinations.
7. Test load shedding.
8. Test automatic restart after overload.
9. Review the BMS and inverter event history.

Do not move directly from no load to the maximum building demand.

Record a Surge Acceptance Table

The handover report should include:

Test	Battery SOC	Peak DC Current	Lowest DC Voltage	Result
Refrigerator start	80%	Recorded value	Recorded value	Pass/Fail
Pump start	80%	Recorded value	Recorded value	Pass/Fail
Air-conditioner start	80%	Recorded value	Recorded value	Pass/Fail
Pump with background load	50%	Recorded value	Recorded value	Pass/Fail
Controlled combined load	Minimum normal SOC	Recorded value	Recorded value	Pass/Fail

This creates a baseline for future maintenance.

Explain Load Limits to the Customer

The customer should receive a simple load chart.

It should show:

• Appliances permitted during backup
• Loads that should not run together
• Minimum SOC for heavy loads
• Generator-dependent loads
• Loads controlled automatically
• Maximum continuous inverter output
• Emergency overload procedure

A technically correct system can still fail if the user does not understand its operating limits.

Common Prevention Mistakes

Buying the Battery Only by Ah or kWh

Capacity does not prove adequate starting-current performance.

Using the Inverter Headline Surge Number

Duration and operating conditions must also be checked.

Ignoring Automatic Loads

Refrigerators, pumps and compressors may start without user action.

Testing Only at Full SOC

Low-SOC behaviour may be substantially different.

Increasing the Breaker Size to Stop Trips

This may leave the cable unprotected and does not improve the battery or inverter capability.

Using Long, Undersized DC Cables

Voltage drop can cause shutdown before any component reaches its thermal limit.

Allowing Every Heavy Load on the Backup Panel

Load prioritisation is often more economical and reliable.

Assuming Parallel Batteries Share Perfectly

Branch resistance, SOC and battery condition affect current distribution.

Frequently Asked Questions

How large should the inverter surge margin be?

There is no universal percentage. Use the measured or documented starting demand and confirm that the inverter can support it for the required duration.

Can adding another battery prevent motor-start shutdown?

It may help when battery current or voltage sag is the limiting factor. It will not correct an undersized inverter or defective appliance.

Is a 100A BMS enough for a 5kW 51.2V inverter?

It may be close to or below the actual full-load requirement after accounting for low battery voltage and inverter losses. The complete operating conditions must be calculated.

Is a 24V system suitable for a large pump?

It can be, but the DC current will be higher than in a comparable 48V system. Cable and BMS requirements can become substantial.

Can solar panels directly provide the motor surge?

Solar production can reduce average battery current, but rapid load changes may still be supplied partly or completely by the battery and inverter.

Should the inverter restart automatically after overload?

Automatic restart may be useful, but repeated cycling can damage equipment or disrupt critical loads. The cause of the overload should be controlled.

Conclusion

Preventing inverter shutdown during motor start requires more than choosing a large inverter.

A reliable design should include:

• Complete appliance start-up survey
• Continuous and surge-load scenarios
• Battery-side current calculation
• BMS peak-current duration
• Suitable battery quantity
• Inverter overload-curve review
• Low-resistance DC cabling
• Correct protection
• Soft starting where appropriate
• Essential-load separation
• Automatic load management
• Testing at different SOC levels
• Clear customer operating limits

For HIZN Lithium system matching, provide:

• Inverter brand and model
• Battery-system voltage
• Largest motor or compressor
• Appliance running power
• Appliance starting current or power
• Required backup time
• Simultaneous load requirements
• Cable length
• Ambient temperature
• Grid and generator availability

This information allows the battery capacity, BMS current and inverter surge capability to be matched to the customer’s real load profile.