Choosing the right generator size isn’t just about picking a big number and hoping it covers everything. Oversizing wastes capital and reduces efficiency. Undersizing leads to overloads, voltage drops and potential equipment failure.
If you’re planning backup or primary power for a commercial or industrial site, the key is calculating generator size based on actual load demand, not guesswork. Done properly, it ensures reliability, fuel efficiency and long-term performance.
At Jubilee Energy, we design bespoke power generation solutions across Australia and the Asia-Pacific, and accurate load assessment is always the starting point.
Summary: To calculate generator size correctly, you need to assess your actual load demand in kilowatts, account for starting currents, apply appropriate load factors and allow headroom for future growth. Accurate sizing improves efficiency, reduces fuel consumption and protects sensitive equipment. Professional load analysis ensures your generator matches your operational requirements precisely.
Step 1: Identify All Electrical Loads
Start by listing every piece of equipment the generator will power. This includes:
- Motors and pumps
- HVAC systems
- Lighting
- IT infrastructure and servers
- Production machinery
- Refrigeration units
- Safety and emergency systems
For each item, record:
- Running power in kilowatts (kW)
- Starting power, particularly for motors
- Whether the load is continuous or intermittent
Manufacturer nameplates and electrical schedules are useful here.
Step 2: Understand Running Load vs Starting Load
One of the most common mistakes in generator sizing is ignoring starting current.
Motors and compressors can draw 2 to 7 times their normal running current during startup. This is known as inrush current.
If your generator isn’t sized to handle these peaks:
- Voltage may dip
- Equipment may fail to start
- Sensitive electronics can be damaged
When calculating generator size, you must consider the highest combined starting load that could occur at one time.
Step 3: Calculate Total Running Load
Add together the running kW of all equipment that will operate simultaneously.
For example:
- Pump system: 40 kW
- HVAC: 60 kW
- Lighting and general power: 25 kW
- IT systems: 15 kW
Total running load = 140 kW
But this number alone isn’t enough.
Step 4: Apply Load Factor and Diversity
Not all equipment runs at full capacity at all times. This is where load factor and diversity come in.
Load factor refers to the ratio of actual load used compared to maximum possible load.
Diversity factor accounts for the fact that not all equipment will start or operate at peak simultaneously.
Accurate load profiling helps determine realistic demand patterns rather than worst-case assumptions.
Step 5: Allow for Future Growth
Commercial and industrial operations rarely stay static.
When calculating generator size, allow additional capacity for:
- Business expansion
- New machinery
- Increased production
- Regulatory upgrades
A common approach is adding 10 to 20 percent spare capacity, depending on growth projections.
Step 6: Convert to kVA and Consider Power Factor
Generators are typically rated in kVA, not kW.
To convert:
kVA = kW ÷ Power Factor
For most industrial applications, power factor ranges from 0.8 to 0.9.
Example:
If total load is 200 kW and power factor is 0.8:
200 ÷ 0.8 = 250 kVA generator required
Power factor correction may also be considered to improve efficiency.
Why Accurate Generator Sizing Matters
Correct sizing delivers:
- Stable voltage and frequency
- Reduced fuel consumption
- Lower maintenance costs
- Longer equipment lifespan
- Compliance with operational standards
Oversized generators often operate at low load, which can cause wet stacking in diesel units and reduce efficiency. Undersized units risk overload and unplanned shutdowns. Precision matters.
The Importance of Professional Load Analysis
While basic calculations are helpful, real-world generator sizing often requires:
- Detailed load studies
- Site inspections
- Harmonic analysis
- Transient response evaluation
- Synchronisation planning for multiple units
At Jubilee Energy, we design tailored power generation systems based on comprehensive load assessments, ensuring your solution is engineered for performance and reliability.
Common Generator Sizing Mistakes to Avoid
- Guessing based on building size alone
- Ignoring motor starting requirements
- Failing to account for power factor
- Not planning for future expansion
- Choosing based solely on upfront cost
Generator sizing should always be data-driven.
Get a Generator Solution Engineered for Your Exact Load
If you’re planning a new installation or reviewing your existing power setup, accurate sizing is critical. At Jubilee Energy, we deliver bespoke commercial and industrial power generation solutions across Australia and the Asia-Pacific region, engineered to match your exact operational demand.
Speak with our team today and ensure your power system is designed for performance, efficiency and long-term reliability.
Key Takeaways
- Accurate generator sizing starts with calculating actual load demand
- Running load and starting load must both be considered
- Power factor affects final kVA requirements
- Allow capacity for future growth
- Professional load analysis ensures reliability and efficiency
FAQ
What happens if a generator is too small?
An undersized generator may overload, cause voltage instability and damage connected equipment. It can also lead to frequent shutdowns.
Is it better to oversize a generator?
Not necessarily. Oversized generators operating at low load can be inefficient and experience mechanical issues such as wet stacking.
How do I know my site’s power factor?
A qualified electrical engineer or power specialist can measure your site’s power factor during a load assessment.
Do standby and prime generators require different sizing approaches?
Yes. Prime power applications often require more detailed analysis because the generator runs continuously rather than intermittently.
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