Why Can a 5kW Inverter Fail to Start a 1.5kW Motor?
This sounds unreasonable at first.
A water pump may be rated at only:
1.5kW
while the inverter is rated at:
5kW
So why does the inverter sometimes shut down as soon as the pump starts?
Because the motor’s rated running power does not tell you its complete electrical demand.
Equipment containing motors and compressors can require much more current during startup than during steady operation.
This affects both:
- The inverter
- The LiFePO4 battery bank
A system can therefore have enough energy capacity and enough nominal inverter wattage but still fail during startup.
1. Running Power and Starting Power Are Different
Consider a motor that uses approximately:
1,500W during normal operation
During startup, the instantaneous electrical demand may be several times higher, depending on:
- Motor design
- Starting method
- Mechanical load
- Compressor pressure
- Pump characteristics
- Supply voltage
Therefore, inverter selection should not be based only on the appliance’s running wattage.
2. Which Household Loads Commonly Have Starting Surge?
Typical examples include:
- Air conditioners
- Refrigerators
- Freezers
- Water pumps
- Well pumps
- Pool pumps
- Compressors
- Washing machines
- Workshop motors
- Power tools
By contrast, many resistive loads behave more predictably.
Examples include:
- Electric heaters
- Traditional heating elements
- Some lighting loads
The exact behavior depends on the equipment, but motor-driven loads deserve special attention when selecting an inverter.
3. Do Not Use a Fixed “3 Times” Rule for Every Motor
You may see advice online such as:
“Just multiply motor power by three.”
This can be useful as a rough early estimate in some cases, but it should not replace actual equipment data.
Different motors can behave very differently.
For a serious project, check:
- Locked-rotor current
- Starting current
- Manufacturer surge data
- Soft-start capability
- Variable-frequency drive
- Compressor type
The more expensive the project, the less you should rely on a generic multiplier.
4. Check the Inverter Surge Power
An inverter datasheet may show:
Rated Power: 5,000W
but this is only part of the information.
Also find:
- Maximum surge power
- Overload power
- Surge duration
- Overload duration
For example, an inverter may be able to support:
200% load for a very short period
while another may allow only a much smaller overload.
Even if both are sold as “5kW inverters,” their ability to start motors can be very different.
5. Surge Duration Matters
A large surge rating is useful only if it lasts long enough.
Compare two theoretical inverters.
Inverter A
- Continuous: 5kW
- Very high surge
- Surge available only for milliseconds
Inverter B
- Continuous: 5kW
- Moderate overload
- Can maintain it for several seconds
For some motor applications, Inverter B may perform better despite having a lower headline surge number.
Always check both:
surge power + surge time
6. The Battery Must Support the Same Surge
Suppose the inverter can deliver the required starting surge.
The next question is:
Can the battery supply enough DC current?
Imagine a 51.2V battery system and a temporary inverter output demand of:
8kW
Ignoring transient dynamics and using a simplified 94% efficiency assumption:
8,000 ÷ 51.2 ÷ 0.94 ≈ 166A
If only one battery with a 100A BMS is connected, the BMS may detect overcurrent before the inverter completes the motor startup.
The result:
battery disconnects → inverter shuts down → motor never starts
The customer may incorrectly conclude that the inverter is defective.
7. Why the Problem Gets Worse at Low SOC
As battery voltage decreases, more current is required to provide the same amount of power.
For example, at approximately 46V:
8,000 ÷ 46 ÷ 0.94 ≈ 185A
Therefore, a system may start the pump successfully when the battery is nearly full but fail when SOC is lower.
This pattern is an important troubleshooting clue.
If a customer reports:
“The pump works in the morning after charging, but the inverter trips at night.”
check battery voltage and BMS current capability.
8. DC Voltage Drop Can Cause a Second Problem
Even if the BMS does not trip, high startup current can cause temporary voltage drop.
Voltage drop can occur across:
- Battery cables
- Breakers
- Fuses
- Busbars
- Battery terminals
- Loose connections
The inverter may then see its DC input voltage fall below the allowable threshold.
It can shut down with:
- Low battery
- DC undervoltage
- Battery disconnect
even though the battery itself still has significant SOC.
9. Example: 1.5kW Water Pump
Suppose a customer has:
- 1.5kW pump
- 5kW inverter
- 51.2V 100Ah battery
- 100A BMS
During normal pump operation:
Approximate battery current might be around:
1,500 ÷ 51.2 ÷ 0.94 ≈ 31A
This looks easy for the battery.
But suppose startup briefly requires much more power.
If instantaneous system demand approaches 6kW:
6,000 ÷ 51.2 ÷ 0.94 ≈ 125A
That already exceeds the 100A continuous BMS rating.
The actual response depends on the BMS peak-current specification and permitted duration.
10. Example: Air Conditioner
Air conditioners are a common source of confusion because not all units start the same way.
Older fixed-speed compressor systems can have significant starting current.
Modern inverter-driven air conditioners may have softer starting behavior.
Therefore, telling a customer:
“A 2HP air conditioner always needs X kW inverter”
is unreliable.
Ask for:
- AC nameplate
- Rated input power
- Maximum input current
- Compressor type
- Starting current if provided
This creates a much more accurate inverter recommendation.
11. Refrigerators Can Also Cause Unexpected Trips
A refrigerator may use only a few hundred watts during operation.
That seems negligible in a 3kW or 5kW solar system.
However, compressor startup can create a temporary surge.
This becomes particularly important when:
- Several refrigerators start at once
- A freezer is also connected
- Battery SOC is low
- The system uses only one small battery
- Other loads are already operating
The inverter should be selected for the worst realistic combination rather than one appliance in isolation.
12. Why Adding a Larger Battery Can Solve a “Small Inverter” Problem
Sometimes installers replace the inverter with a larger model when motor startup fails.
But the actual issue may be the battery.
For example:
Existing system
- 5kW inverter
- 5.12kWh battery
- 100A BMS
Problem
Pump causes battery BMS overcurrent protection.
If the inverter already has adequate surge capability, replacing it with an 8kW inverter may make the DC current problem even worse.
A more appropriate solution may be:
- More parallel batteries
- Higher-current battery
- Reduced simultaneous load
- Soft starter
- Correct cable sizing
System diagnosis should therefore identify which component is actually reaching its limit.
13. Soft Starters Can Reduce Starting Demand
For some motor loads, suitable starting equipment can reduce startup stress.
Possible solutions include:
- Soft starter
- Variable-frequency drive
- Inverter-drive motor
- Appropriate motor controller
These devices are application-specific.
They should be selected according to the motor and electrical system.
But in suitable applications, reducing startup current can be more efficient than massively oversizing the battery and inverter.
14. Consider Which Loads Can Start at the Same Time
Suppose a home has:
- 1.5kW air conditioner
- 1kW pump
- Refrigerator
- Freezer
Individually, each may be manageable.
The difficult condition occurs if several motors start simultaneously.
This can happen automatically.
For example:
- Grid outage occurs.
- Inverter transfers to battery.
- Several appliances restart.
- Compressor and pump starting demand overlap.
- Inverter or BMS protection activates.
This is why commissioning tests should include realistic load combinations.
15. Off-Grid Systems Need More Surge Margin
In a grid-connected hybrid system, the utility grid may support some demanding load conditions depending on operating mode.
In a true off-grid system, the inverter and battery bank must handle the complete electrical demand.
This makes surge performance especially important for:
- Remote homes
- Farms
- Islands
- Water pumping systems
- Rural workshops
For these systems, choosing the smallest theoretically sufficient inverter often leads to poor user experience.
16. Three-Phase Motors Need Special Planning
Commercial and agricultural systems may contain:
- Three-phase pumps
- Compressors
- Industrial motors
Here, inverter selection involves additional questions:
- Three-phase output voltage
- Phase balance
- Motor starting method
- Starting current
- VFD compatibility
- Inverter overload rating
- Battery peak power
A simple residential inverter sizing rule should not be applied to these projects.
17. Battery Capacity Still Determines Runtime
Surge current answers:
Can the battery start the equipment?
Energy capacity answers:
How long can it run?
Suppose a pump consumes:
1.5kW
and needs to run for:
4 hours
Energy requirement is already:
6kWh
before considering losses and other loads.
Therefore, even if a 5.12kWh battery can provide enough starting current, it may not provide the required operating time.
Both power and energy must be calculated.
18. Practical Motor-Load Inverter Selection Method
Before choosing an inverter, collect:
Appliance information
- Rated running power
- Starting current
- Motor/compressor type
- Quantity
- Simultaneous operation
Inverter information
- Continuous power
- Surge power
- Surge duration
- Overload curve
- Low-voltage threshold
Battery information
- Nominal voltage
- BMS continuous current
- BMS peak current
- Peak-current duration
- Battery quantity
- Minimum operating voltage
Installation information
- DC cable size
- Cable length
- Breaker/fuse
- Busbar rating
Only after these factors are checked should final inverter size be selected.
FAQ
Why does my 5kW inverter trip with a 2kW air conditioner?
Possible causes include compressor starting surge, insufficient inverter overload capability, battery BMS overcurrent protection or excessive DC voltage drop.
Does a larger inverter always solve motor-starting problems?
No. If the battery cannot provide the required current, installing a larger inverter may not solve the problem.
Can two batteries help start a water pump?
If the batteries are approved for parallel operation, additional modules may increase available battery current and reduce current per module.
Do inverter air conditioners need the same starting surge as traditional units?
Not necessarily. Starting behavior depends on the specific air conditioner design.
Is inverter surge wattage more important than continuous wattage?
Both are important. Continuous rating determines normal operating capability, while surge performance is particularly important for motor startup.
Conclusion
When selecting an inverter for air conditioners, pumps and compressors, never compare only:
appliance running watts vs inverter rated watts
Instead evaluate:
running load + starting surge + surge duration + battery peak current + DC voltage drop
This explains why a correctly designed 5kW system may start a motor reliably while a poorly matched 8kW system may repeatedly shut down.
Selecting Batteries for Motor-Load Solar Projects?
Provide HIZN Lithium with:
- Inverter model
- Motor or air-conditioner specifications
- Required running time
- Battery voltage
- Solar capacity
We can help evaluate the LiFePO4 battery bank’s required continuous and peak current before installation.