Generator Sizing: How to Calculate the Correct KVA for Your Load

Step 1: Understand Your Load Types

Before you do any math, you must categorize the equipment you intend to power. Not all loads behave the same way when a generator starts up.

1. Resistive Loads

These are the simplest loads. The current and voltage are in phase, meaning the power factor is 1.0.

• Examples: Incandescent lighting, electric heaters, toasters, ovens.
• Impact: These require no special startup calculation; the running load is the starting load.

2. Inductive/Motor Loads

These are the most critical factor in generator sizing. Motors require a massive surge of current (inrush current) to start rotating magnetic fields.

• Examples: Air conditioners, water pumps, compressors, conveyors, elevators.
• Impact: The starting kVA (skVA) can be 3 to 6 times higher than the running kVA.

Step 2: Understand kW vs. kVA

A common mistake is confusing kW (real power) with kVA (apparent power). Generators are typically rated in kVA because the engine must supply the kW, but the alternator must supply the total kVA.

To convert between the two, you need the Power Factor (PF).

• Formula: kW = kVA × Power Factor
• Standard: Most generators are rated at a power factor of 0.8.
• Example: A generator rated at 100 kVA can typically supply 80 kW (100 kVA × 0.8 PF) of usable real power.

If you only know the kW of your load, you must account for the power factor to size the kVA properly.

Step 3: The Calculation Methodology

To calculate the required generator size, follow this three-step method: List, Convert, Apply Starting Factors.

1. Create a Load List

Compile a detailed spreadsheet of every piece of equipment you intend to run simultaneously. For each item, record:

• Running kW/kVA: The power required to keep it operating.
• Starting kVA (Locked Rotor Amps): The surge required to start the motor.
• Quantity: Number of identical units.

2. Calculate Total Running kVA

Sum up the running requirements of all loads.

• If load is listed in Amps (A) and Volts (V):
  • Single Phase: (Volts × Amps) / 1,000 = kVA
  • Three Phase: (Volts × Amps × 1.732) / 1,000 = kVA (1.732 is the square root of 3)
• If load is listed in kW: kW / Power Factor = kVA (Assume 0.8 if unknown)

3. Calculate Starting kVA (The Critical Step)

You cannot simply add the running kVA to the largest motor’s starting kVA unless you are using a complex load shedding system. The industry standard rule of thumb for generator sizing is:

Generator Size = Total Running kVA + (Largest Motor Starting kVA – Largest Motor Running kVA)

This formula ensures the generator can handle the voltage dip caused by the largest motor kicking in while the rest of the facility remains operational.

Step 4: Real-World Example

Let’s size a generator for a small workshop.

The Loads:

1. Lighting & Outlets: 10 kW resistive (PF 1.0) = 10 kVA
2. HVAC Unit: 5 kW running (PF 0.8) = 6.25 kVA running / Starting kVA = 25 kVA
3. Air Compressor: 7.5 kW running (PF 0.8) = 9.375 kVA running / Starting kVA = 37.5 kVA

Step A: Total Running kVA

• Lighting: 10 kVA
• HVAC: 6.25 kVA
• Compressor: 9.375 kVA
• Total: 25.625 kVA

Step B: Identify the Largest Starting Surge

• HVAC Starting Surge: 25 kVA
• Compressor Starting Surge: 37.5 kVA (Largest)
• Largest Motor Running kVA: 9.375 kVA

Step C: Apply the Formula

Generator Size = Total Running kVA + (Largest Starting kVA – Largest Running kVA)
Generator Size = 25.625 kVA + (37.5 kVA – 9.375 kVA)
Generator Size = 25.625 kVA + 28.125 kVA
Generator Size = 53.75 kVA

Conclusion: You require a generator rated for approximately 55–60 kVA to handle the inrush current of the compressor without causing a voltage dip that would dim lights or stall the HVAC unit.

Step 5: Important Sizing Considerations

Beyond the raw calculation, several external factors affect the final size:

1. Elevation & Derating

Generators rely on air for combustion. If your site is located at a high altitude (above 3,000 feet), the air is thinner, and the engine loses power. You must "derate" the generator.

• General rule: Derate the generator by 3% for every 1,000 ft (300m) above sea level.

2. Ambient Temperature

Extreme heat also reduces engine efficiency. If operating in environments consistently above 104°F (40°C), derating may be required. Conversely, cold-weather kits (block heaters) add to the electrical load calculation.

3. Duty Cycle (Standby vs. Prime)

Generator ratings are not universal:

• Standby (ESP): For emergency use during utility failure. Do not exceed 200 hours per year. This rating offers no overload capacity.
• Prime (PRP): For continuous power where utility is unavailable. Rated for unlimited hours with a 10% overload capability for short durations.
• Recommendation: Always size using the Standby rating if the generator is for backup. Oversize by 20-25% if using a prime-rated generator for continuous duty.

4. Future Expansion

If you anticipate adding more equipment within the next 3-5 years, factor that into your current kVA total. Buying a generator that is slightly larger now is significantly cheaper than replacing the alternator or the entire unit later.

Summary Checklist

To accurately calculate the correct kVA for your load:

• [ ] List all loads—separate resistive (heat/light) from inductive (motors).
• [ ] Calculate Total Running kVA—convert Amps to kVA or kW to kVA (PF 0.8).
• [ ] Identify Largest Motor—note its Starting (Inrush) kVA.
• [ ] Apply Sizing Formula—Running kVA + (Largest Motor Start – Largest Motor Run).
• [ ] Apply Environmental Factors—adjust for altitude and temperature.
• [ ] Select Rating—choose Standby (Backup) or Prime (Continuous) power.

By following this methodology, you ensure that your generator investment is protected, your critical systems remain operational during outages, and you avoid the costly mechanical failures associated with incorrectly sized power systems. If your load list includes variable frequency drives (VFDs), UPS systems, or medical imaging equipment, consult a professional electrical engineer, as these loads have unique harmonic and transient responses that require advanced analysis.