Generator Selection Motor Loads Emergency Power

Understanding Starting Currents and Correctly Sizing Generators: How to Avoid Failures with Motors, Pumps, and Compressors

SEV Generators • Status: April 2026 • Reading time: 9–11 minutes

A generator may have "enough kW" on paper – and still fail when starting a refrigerator, a submersible pump, or a compressor. The reason is almost always the same: starting current. Many consumers briefly draw several times their normal current when starting. If the generator cannot handle this, the voltage drops, the frequency fluctuates – and the consumer does not start, or the generator shuts down.

The Most Important in Brief

Not only the continuous power but especially the short-term available power during start-up is crucial.
Refrigerators, pumps, and compressors often require several times their running power as starting power.
Generators that are too narrowly sized lead to voltage drops, frequency fluctuations, and starting problems.
Often, not only "more kW" helps, but also a sensible switching sequence or reducing the starting peak.
Cables that are too long or too thin exacerbate the problem.

What is Starting Current – and Why is it so Important?
The Most Important Terms
Which Consumers Have High Starting Currents?
Rules of Thumb: How Much Reserve Do I Need?
Generator Types: Who Handles Load Peaks Better?
How to Size Correctly: Step by Step
Practical Calculation Examples
Reducing Starting Peaks: The Clever Solutions
The Most Common Sizing Errors
Frequently Asked Questions
Conclusion

1) What is Starting Current – and Why is it so Important?

Many devices require significantly more power at startup than in continuous operation. This applies especially to motors, pumps, and compressors, but partly also to electronics with power supplies or capacitors.

Motors must build up torque from a standstill.

Compressors often start against residual pressure and therefore require particularly high starting energy.

Power supplies and electronics charge capacitors abruptly, thereby generating a short inrush current.

This additional "kick" usually lasts only fractions of a second to a few seconds – but it is enough to bring an undersized generator to its knees.

Typical symptoms of excessively high starting current:
Lights flicker strongly or briefly go out, the generator "stumbles," the RPM drops, protective shutdowns trigger, or a motor just hums and does not start.

Important: If a motor does not start and only hums, switch it off immediately. This is critical and can damage both the motor and the generator.

2) The Most Important Terms (Brief and Practical)

W / kW: Active power – what is actually used.

VA / kVA: Apparent power – important for inductive loads and unfavorable power factor.

cos φ: Power factor – roughly the ratio of kW to kVA.

Rated power: Power that can be delivered continuously.

Peak power: Short-term available power – varies greatly depending on the generator.

Note: For motors and many machines, it's not just about continuous power in kW, but above all, the ability to deliver high currents cleanly for a short time.

3) Which Consumers Have High Starting Currents?

In practice, a rough classification by starting behavior helps:

Uncritical to Moderate

Incandescent and halogen lamps

Fan heaters, hotplates, and kettles as typical ohmic loads

Classic drills depending on power

Critical

Refrigerator and freezer

Workshop compressors

Garden pumps, submersible pumps, and dirty water pumps

Air conditioners

Concrete mixers, circular saws, and large power tools

Welding machines with strongly fluctuating loads

Very Critical / Special Case

Consumers with high motor starting current and simultaneously sensitive electronics

Systems with multiple motors that start simultaneously

Loads with a poor power factor and strong load jumps

4) Rules of Thumb: How Much Reserve Do I Need?

Values can vary depending on the device, age, temperature, and mechanical load. The following factors help as practical guidance:

A) Compressor and Motor Loads

Refrigerator / Freezer: approx. 3–6× the running power at start-up

Small pumps: approx. 3–5×

Workshop compressors: approx. 4–7×

Submersible pumps: often 5–8×, sometimes more

Saws and mixers: approx. 3–6×

Important Consequence: A consumer with 800 W running power can briefly demand 3 to 5 kW at startup. This is precisely why many seemingly adequately sized generators fail.

B) Electronics and Power Supplies

LED drivers, switching power supplies, and chargers usually generate shorter pulses than motors. However, in total, many devices simultaneously can still become relevant – for example, with lighting, audio, or IT equipment.

Practical Tip: For events or mixed applications, it's better to start with a stable base load and connect consumers one after another.

5) Generator Types: Who Handles Load Peaks Better?

Inverter Generators

Very clean voltage and frequency, ideal for sensitive electronics. However, the short-term available peak power is highly model-dependent.

Very good for IT, audio, lighting, and household appliances
Often quiet and efficient
Pay close attention to hard motor starts

Conventional AVR Generators

Robust and often tolerant of load changes. Depending on the design, they are very practical for motor loads.

Good for workshops and construction sites
Often strong with inductive loads
Voltage can fluctuate more strongly with load jumps

Diesel Generators

In larger classes, often very load-stable and designed for longer operating times.

Good for continuous operation and larger motors
Robust under high load
Usually heavier and less mobile

6) How to Size Correctly: Step by Step

Create a List of Consumers

Note down the running power or current draw, the type of consumer, and whether it actually needs to be operated simultaneously with other devices.

Calculate Simultaneous Continuous Load

Add up the consumers that realistically run in parallel. This is the basis for the required continuous power.

Consider the Most Critical Start-up

Identify the consumer with the highest starting requirement – often a compressor or pump – and factor in a realistic starting reserve for it.

Plan the Switching Sequence

Start the generator and let it run stably, connect uncritical base loads, start motor loads individually, and activate sensitive electronics last.

Conduct a Practical Test

Observe the actual starting behavior, check voltage and frequency if possible, and pay attention to cable cross-sections and plug connections.

Frequently underestimated: Cables that are too long or too thin amplify voltage drops and thereby exacerbate starting problems.

7) Practical Calculation Examples

Example 1: Household Emergency Power with Refrigerator

Refrigerator running power: 150 W

Start-up: approx. 5× → 750 W starting requirement

Additionally: Light + Router approx. 150 W

Recommendation: A generator with at least 1–2 kW and solid peak power is usually easily sufficient here.

Example 2: Garden Pump + Other Consumers

Pump: 900 W

Start-up: 4× → 3,600 W

Additionally: other load approx. 500 W

Recommendation: A generator of approximately 4–5 kW is sensible here – or the starting peak is reduced.

Example 3: Workshop Compressor

Compressor: 2,200 W

Start-up: 6× → up to 13,200 W possible

Recommendation: Classic "Why won't it start?" case. Either size significantly larger or technically reduce the starting peaks.

8) Reducing Starting Peaks: The Clever Solutions

Often it is more cost-effective and sensible to reduce the starting current spike rather than generally choosing a larger generator.

Start consumers sequentially: Sounds trivial, but is enormously effective.

Start compressors unpressurized: Starting against pressure massively increases the starting power requirement.

Use soft starters: Reduces starting current and extends starting time.

Use frequency converters: Especially effective for limiting starting current in pumps and motors.

Practical consideration: Anyone who regularly wants to operate a consumer with a power generator often benefits more from a clean starting solution than from generally more generator power.

9) The most common dimensioning errors

Only the continuous power is added, but the starting current is ignored.

The peak power of the generator is overestimated.

Cables that are too long or too thin exacerbate the starting problem.

Multiple loads are switched on simultaneously.

A motor hums and is not switched off immediately.

Frequently Asked Questions (FAQ)

Is it enough to simply add up all the wattages?

No. For continuous loads, yes – but for motors, pumps, and compressors, the starting current spike must also be considered.

Why doesn't a refrigerator start even though the generator should have enough power?

Because the compressor briefly requires significantly more power during start-up than during continuous operation. Exactly this spike determines success.

Are inverter generators automatically better?

Not automatically. They provide very clean voltage, but for hard motor starts, the actual peak power of the respective model is crucial.

How much reserve should be planned?

That depends on the consumer. For resistive loads, little reserve is needed, but for pumps or compressors, the starting current spike should be calculated very carefully.

Conclusion

The correct generator size depends not only on "total kW" but, above all, on starting current spikes and switching logic. Anyone who realistically assesses motor loads, plans sufficient reserve, and sensibly designs the starting sequence will get an emergency power solution that not only theoretically – but truly works at the crucial moment.

Understanding Inrush Currents and Correctly Sizing Generators: How to Avoid Failures in Motors, Pumps, and Compressors