How to Size a Generator for Your Home or Business

1. Understanding Watts – Running vs Starting

When sizing a generator, you need to understand two types of power requirements:

• Running (Rated) Watts: The continuous power required to keep a device running.
• Starting (Surge) Watts: The extra power needed to start devices with motors (refrigerators, air conditioners, pumps, compressors). Starting watts are typically 2-3 times the running watts.

Key Rule: Your generator must be able to handle the total running watts plus the largest single starting surge.

2. How to Calculate Your Power Needs

Step 1: List All Devices You Want to Power

Write down every device you plan to run simultaneously.

Step 2: Find Running Watts

Check the nameplate on each device. Look for "Watts" or "W". If only amps (A) and volts (V) are listed, calculate: Watts = Volts × Amps.

Step 3: Find Starting Watts

For devices with motors, starting watts are typically 2-3× running watts. Check the nameplate for "Starting Watts" or "LRA" (Locked Rotor Amps).

Step 4: Add Running Watts

Sum the running watts of all devices.

Step 5: Add the Largest Starting Surge

Add the starting watts of the device with the largest surge to your total running watts.

Step 6: Add a Safety Margin

Add 20-25% to your total to avoid running the generator at 100% capacity.

3. Common Appliance Power Requirements

4. Home vs Business Generator Sizing

Home Backup

• Essential loads: 2-5kW – lights, refrigerator, small appliances
• Full home backup: 5-15kW – all essential circuits, AC, water pump
• GTFI options: SE2500CX (2kW) to S11500DS (10kW)

Business / Commercial

• Small business: 10-50kW – office equipment, lighting, small machinery
• Industrial: 50-500kW – factory equipment, production lines, data centers
• GTFI options: Industrial gensets 15-2200kW

5. Generator Sizing Examples

Example 1: Home Backup
Lights (200W) + Refrigerator (200W, 800W start) + TV (150W) + Microwave (1000W) = Running: 1,550W. Largest surge: 800W. Total: 2,350W + 25% margin = ~3kW generator.

Example 2: Small Construction Site
Concrete mixer (3kW, 9kW start) + Lighting (1kW) + Compressor (1.5kW, 4.5kW start) = Running: 5.5kW. Largest surge: 9kW. Total: 14.5kW + 25% margin = ~18kW generator.

6. GTFI Generator Range Overview

GTFI offers generators from 0.7kW to 2200kW to suit any application:

• Portable Gasoline (2-9kW): SE2500CX to SE9500CX – home backup, camping, light construction
• Portable Diesel (1.8-10kW): S2000D to S11500DS – construction, agriculture, remote sites
• Industrial Gensets (15-2200kW): Factories, mines, data centers, hospitals
• Inverter Generators (2-15kW): IGBT technology, pure sine wave, THD<3%, 40% fuel saving
• Solar + Storage: 10.6kWh to 21.2kWh LFP batteries + 550W solar panels