What Size Generator Do I Need? Practical Selection Guide

“Generator size” is one of the most common questions buyers ask. This guide provides quick-reference sizing for common applications and the methodology to calculate exact requirements.

Quick Reference: Generator Size by Application

Residential

Home Type	Generator Size	What It Powers
Small apartment	5-10kW	Lights, fridge, fans, TV, phone charging
Small house	10-17kW	Above + well pump, AC (small), microwave
Medium house	17-25kW	Above + central AC, oven, water heater
Large house	25-50kW	Whole house including pool pump, workshop

Commercial

Business Type	Generator Size	Key Loads
Small shop/office	20-50kW	Lighting, computers, small AC
Restaurant	50-100kW	Kitchen equipment, refrigeration, HVAC
Grocery store	100-250kW	Refrigeration (critical), lighting, HVAC
Office building	150-500kW	HVAC, elevators, lighting, IT
Hotel	300-1000kW	Rooms, kitchen, elevator, pool, laundry

Industrial

Facility	Generator Size	Key Loads
Small factory	200-500kW	Production machinery, lighting, HVAC
Medium factory	500-1500kW	Continuous production lines, compressors
Large factory	1500-3000kW	Heavy machinery, multiple production lines
Data center	1000-5000kW+	IT load + cooling (typically 2N redundant)

Sizing by Equipment

Equipment	Running Watts	Starting Watts
Refrigerator/freezer	600-800	2,000-2,400
Sump pump	800-1,200	2,000-3,200
Well pump (1HP)	1,000	3,000
Window AC (12,000 BTU)	1,200	3,600
Central AC (3 ton)	3,500	10,500
Electric water heater	4,500	4,500
Electric oven	3,000-5,000	3,000-5,000
Washing machine	1,000-1,500	3,000-4,500
Microwave	800-1,200	800-1,200
Computer + monitor	300-500	300-500
5HP motor (pump, compressor)	3,700	11,000+

Three-Phase Sizing

For three-phase equipment:

kW (3-phase) = Volts × Amps × √3 × Power Factor / 1000

Quick Three-Phase Sizing

Motor HP	Running kW	Starting kW
10 HP	7.5kW	22-30kW
25 HP	18.5kW	55-75kW
50 HP	37kW	110-150kW
100 HP	75kW	225-300kW
200 HP	150kW	450-600kW

Step-by-Step Sizing Method

Step 1: Essential vs. Non-Essential

First, decide which loads the generator must power.

• Essential: Must run during outage (life safety, critical equipment)
• Important: Should run (comfort, convenience)
• Non-essential: Can wait (non-critical)

Step 2: Calculate Running kW

Running kW = (Total running watts of essential loads) / 1000

Step 3: Calculate Starting kW

Starting kW = Running kW + (Largest motor starting surge)

Step 4: Add Safety Margin

Generator kW = Starting kW × 1.25 (25% safety margin)

Common Sizing Mistakes

Oversizing

“If some is good, more is better” — WRONG for generators.

Problems with oversized generators:

• Wet stacking: Unburned fuel in exhaust (light load operation)
• Poor fuel efficiency: Engine at partial load is inefficient
• Higher purchase cost: Wasted capital
• Higher maintenance costs: Engine hours accumulate but no useful work

Rule: Design for 60-80% average load for optimal operation.

Undersizing

The most embarrassing mistake — generator can’t start the loads.

Consequences: Voltage dip, frequency drop, tripped breakers, failed starts.

Special Cases

Welding Machines

Welding machines create rapidly variable loads. Size generator at 1.5-2× the welder’s rated input kVA.

UPS Systems

UPS input rectifiers draw harmonic currents. Size generator at 1.3-1.8× UPS kVA rating.

Elevators

Counter-weighted elevators can actually regenerate power when descending. Use specific generator sizing for elevator applications.

Still Unsure? Let Pauway Help

Pauway Power’s engineering team can calculate the exact generator size for your application. Contact us with your load list for a professional sizing recommendation.

Get Professional Sizing →