Generator Sizing Calculator: How to Size a Generator Correctly

Proper generator sizing is critical. An undersized generator won’t start your equipment. An oversized generator wastes money and can develop engine problems from light loading.

Step 1: List All Electrical Loads

Equipment	Rated Watts	Starting Watts	Type
Example: Motor pump	7,500W	22,500W	Motor
Example: Lighting	2,000W	2,000W	Resistive
Example: Computer	500W	500W	Electronic

Load Types and Their Characteristics

Resistive Loads (easiest for generators)

• Lighting (incandescent, LED), heaters, electric ovens
• Starting current = running current (no surge)
• Power factor: 1.0

Inductive Loads (require starting surge)

• Electric motors, pumps, compressors, conveyors
• Starting current: 3-7× running current
• Power factor: 0.7-0.9

Electronic Loads (sensitive)

• Computers, UPS, variable frequency drives
• Harmonic distortion can affect generator voltage
• Power factor: 0.6-0.95 (varies)

Step 2: Calculate Total Power Requirements

Running Power (kW)

Sum of all running watts:

Total Running Watts = Sum of all equipment running watts

Starting Power (kW)

The largest motor starting surge PLUS all other running loads:

Total Starting Watts = (Largest motor starting watts × 1.2) + All other running watts

Generator Size

Generator Size (kW) = Total Starting Watts / 1000
Round up to next standard generator size

Step 3: Apply Correction Factors

Altitude

Generator sets derate with altitude:

• Above 1,000m: Derate 1% per 100m
• Above 3,000m: Derate 1.5% per 100m

Temperature

High ambient temperature reduces generator output:

• Above 40°C: Derate 2% per °C
• Extreme (50°C+): May require 15-20% total derating

Future Expansion

Add 15-25% capacity for future loads:

Final Size = Calculated Size × (1 + Future Expansion %)

Common Sizing Mistakes

Mistake 1: Ignoring Motor Starting Current

The most common sizing error. A 10kW motor can draw 50-70kW during startup.

Fix: Identify the largest motor and use its starting watts, not running watts.

Mistake 2: Sizing for Total Connected Load

Not all equipment runs simultaneously.

Fix: Determine which loads run concurrently and size accordingly.

Mistake 3: Oversizing “Just to Be Safe”

Running a generator below 30% load causes:

• Wet stacking: Unburned fuel deposits in exhaust system
• Carbon buildup: On valves, piston rings, injectors
• Reduced engine life: Incomplete combustion damages engine

Fix: Size for realistic operating load, not maximum theoretical load.

Mistake 4: Confusing kW and kVA

kW = kVA × Power Factor

Most generators are rated in both kW and kVA. Use kW for resistive loads, kVA for motor loads.

Sizing Examples

Example 1: Small Workshop

Load	Running kW	Starting kW
Lighting	2kW	2kW
Welding machine	10kW	15kW
Air compressor (7.5kW motor)	7.5kW	22.5kW
Total	19.5kW	39.5kW

Required generator: 40-50kW

Example 2: Medium Office Building

Load	Running kW	Starting kW
Lighting + outlets	30kW	30kW
HVAC (20kW motor)	20kW	60kW
Elevator (15kW motor)	15kW	45kW
UPS/computers	15kW	15kW
Total	80kW	150kW

Required generator: 150-175kW

Generator Sizing Table (Quick Reference)

Facility	Running Load	Generator Size
Small home backup	5-10kW	10-20kW
Large home backup	10-20kW	20-40kW
Small office/shop	20-40kW	40-75kW
Medium office/commercial	60-100kW	100-175kW
Large commercial/factory	150-300kW	250-500kW
Data center / hospital	500-2000kW	1000-3000kW

Still Not Sure? Contact Pauway

Sizing a generator set correctly requires understanding both the electrical load characteristics and the generator’s capabilities. Pauway Power’s engineers can help you calculate the exact size needed for your application.

Get Help Sizing Your Generator →