A properly sized load bank should match the actual test objective, the generator’s rated power, site voltage, power factor, and the step-loading plan. In most diesel generator projects, the best load bank size is not simply “same kW as the generator” or “bigger is safer.” It should be sized to let the generator reach the required test load in a controlled way, with enough adjustment steps to verify performance at partial load and high load without making the test unnecessarily expensive or impractical.

A diesel generator load bank is typically sized based on the generator’s rated kW, not kVA. In most cases, the load bank capacity should match the generator’s full-load kW if 100% load testing is required. However, the final size also depends on site load, test objectives, and whether resistive or combined load is needed.

If you are new to load bank testing or want a broader overview, you can first review our load bank testing guide (Diesel Generator Load Bank Testing: Complete Guide) to understand how sizing fits into the overall testing process.

What Does “Load Bank Sizing” Actually Mean?

Load bank sizing means selecting a load bank capacity and configuration that can apply the required electrical load to the diesel generator under controlled conditions.

This includes more than just total kW:

• Generator rating
• Voltage and frequency
• Phase (single or three-phase)
• Load step resolution
• Power factor requirements
• Test duration
• Future expansion

In practice, load bank sizing is about building a test setup that is technically correct and operationally practical. A poorly sized load bank may still run, but it will not deliver meaningful test results.

How Do You Size a Load Bank for a Diesel Generator?

The first step is to confirm the generator’s real usable rating.

For diesel generators, load banks are sized based on kW, not kVA.

For example:

• 1000 kVA @ 0.8 PF = 800 kW
• 500 kVA @ 0.8 PF = 400 kW

This kW value is what determines your load bank capacity.

Next, define the required test level:

• 50% load (basic check)
• 75% load (performance verification)
• 100% load (commissioning / acceptance test)

If full-load testing is required, the load bank must be able to reach 100% of rated kW.

If the site already has building load, the load bank may only need to supplement it. However, in real projects, building load is often unstable, so relying on it can lead to incomplete testing.

Why Generator kVA and Load Bank kW Are Often Confused

This is one of the most common mistakes.

Generators are rated in kVA, while load banks are rated in kW.

If you match them directly, you will oversize the load bank.

Example:

• 1250 kVA generator @ 0.8 PF = 1000 kW
• Correct load bank size = 1000 kW, not 1250 kW

Many RFQs fail at this stage because they only state “generator size” without defining power factor or test requirements.

Should the Load Bank Be the Same Size as the Generator?

In many cases, yes - especially when:

• Full-load testing is required
• No stable site load is available

But not always.

Common scenarios

Full-load testing (no site load)
→ Load bank = generator kW

With building load available
→ Load bank = difference only

Routine maintenance
→ Partial load bank may be sufficient

Multiple generator projects
→ Modular load bank system is preferred

How Much Load Is Required for Diesel Generator Testing?

Diesel generators should not run at low load for long periods.

Low-load operation can lead to:

• Incomplete combustion
• Carbon buildup
• Wet stacking

If you want to understand this further, see our article:
Why Is a Load Bank Important?

In critical applications such as:

• Data centers
• Hospitals
• Telecom systems

Incorrect load bank sizing can lead to incomplete testing and hidden system risks.

Typical test levels:

• 50% load
• 75% load
• 100% load

Correct sizing ensures the generator actually reaches these levels.

How Do Load Steps Affect Sizing?

Load bank sizing is not only about total capacity.

Step resolution is critical.

Most tests require staged loading:

• 25%
• 50%
• 75%
• 100%

If step size is too large, testing becomes inaccurate.

Typical step configurations:

• Small systems: 5 kW / 10 kW
• Medium systems: 25 kW / 50 kW
• Large systems: 50 kW / 100 kW

Fine control is especially important in commissioning and troubleshooting.

Should You Add a Safety Margin?

A margin can be useful when:

• Future expansion is expected
• Site conditions are uncertain
• Multiple generators will be tested

However, oversizing often leads to:

• Higher cost
• Larger footprint
• More complex installation

In many cases, a modular load bank system is a better solution.

Resistive vs Reactive Load - Does It Affect Sizing?

Yes.

• Resistive load bank → tests real power (kW)
• Reactive load bank → simulates inductive load (kVAR)
• Combined load bank → simulates real operating conditions

For most generator testing, resistive load is enough.

For more complex systems such as UPS installations and data centers, combined load may be required.

See detailed comparison: Resistive vs Reactive Load Bank

What Information Is Required Before Sizing?

To size a load bank correctly, you need:

• Generator kVA and kW
• Voltage / frequency / phase
• Power factor
• Target load percentage
• Whether site load is used
• Load type (resistive or combined)
• Installation environment
• Ambient temperature / altitude
• Cable length / connection method
• Mobility requirement

Without this, sizing is only an estimate.

Practical Tip From Real Projects

In real projects, load bank sizing depends on multiple factors such as generator rating, site conditions, and test objectives.

Incorrect sizing is one of the most common reasons why generator testing fails to meet project requirements.

If you're not sure how to size the load bank for your generator, you can explore our load bank solutions or contact us with your generator details.

Typical Load Bank Sizing Examples

250 kVA generator
→ 200 kW load bank for full-load testing

625 kVA generator
→ 500 kW load bank, or smaller if site load is stable

1500 kVA generator
→ 1200 kW, often using a modular setup

Multiple generators
→ Modular system recommended

What Happens If the Load Bank Is Undersized?

• Cannot reach required load
• Incomplete testing
• Hidden performance issues

This is especially risky in standby systems.

What Happens If the Load Bank Is Oversized?

• Higher cost
• Larger system
• More complex installation

Usually unnecessary unless future expansion is planned.

How We Approach Load Bank Sizing in Real Projects

We do not rely on nameplate data alone.

We evaluate:

• Generator rating
• Test objective
• Site conditions
• Load type
• Installation constraints

This ensures the load bank is both technically correct and practical to use.

If you are comparing options, you can also review:
How to Choose the Right Load Bank for Your Generator

Conclusion

Load bank sizing for diesel generators should be based on real kW demand, test objectives, and site conditions.

In many cases:

Load bank size = generator rated kW for full-load testing

But real projects often require adjustments based on:

• Site load
• Test requirements
• System configuration

A properly sized load bank ensures accurate testing and reliable generator performance.

If you are working on a specific project, feel free to share your generator specifications. We can help determine the most suitable load bank configuration based on actual operating conditions.