Whole House Generator Sizing Guide: How To Choose The Right Size For Your Home

Choosing the right standby generator for your home isn't as simple as matching wattage numbers. Whole-house generator sizing determines how much power your system needs to run essential appliances or your entire home during an outage. Selecting the wrong size can lead to system failure, equipment damage, or wasted money.

This guide from Southern HVAC walks you through real-world sizing, accurate calculations, and expert recommendations, helping homeowners in Hammond, LA, and across Southeast Louisiana confidently choose the correct generator size for their home.

What Are The Power Requirements Of A Whole House Generator?

The total power demand of a home depends on which appliances and systems you need to run during an outage. The largest load in most homes is the HVAC system, which also creates the highest startup surge.

A typical central AC unit requires:

• 3,500 – 5,000 running watts
• 6,000 – 10,000 starting watts
• 
  

A compressor creates a surge load through inrush current. This surge lasts 1–3 seconds during startup, but your generator must handle peak demand without a voltage drop.

Most central AC units draw 60–100 Locked Rotor Amps (LRA), making HVAC the most critical factor when sizing a whole-house generator.

Why Are Starting Watts Critical For Generator Sizing?

Starting watts determine generator capacity because compressors require a high inrush current. If the generator lacks surge capacity:

• AC fails to start
• Generator overloads or shuts down
• Electrical components degrade faster
• 
  

Starting watts enable operation, while running watts sustain continuous performance

Step-By-Step: How To Calculate Generator Size

Generator sizing requires adding the running load and the largest surge load, then applying a safety margin

Step 1: List Your Essential Appliances

Include only essential loads to reduce generator size:

• HVAC system consumes the highest power
• A refrigerator preserves food safety
• Lighting circuits provide visibility
• A well pump supplies water
• Medical devices ensure continuity
• 
  

Reducing non-essential loads lowers the required kW capacity.

Step 2: Calculate Total Running Watts

Add up the continuous (running) wattage of all selected appliances.

Step 3: Add The Largest Starting Surge

Include only the largest single surge load (typically AC or well pump).

Step 4: Apply A Safety Margin

Add 20–25% extra capacity to prevent overload and allow future expansion.

• Prevents continuous 100% load operation
• Supports additional appliances
• Improves generator lifespan
• 
  

Example Calculation

A typical 2,000 sq ft home in Southeast Louisiana with central AC, refrigerator, lighting, and a well pump:

Recommended generator size: 20 kW standby generator for whole-home backup

What Size Generator Do You Need For Your Home?

If you need a quick estimate, square footage can provide a starting point:

• Under 1,500 sq ft: 10 – 15 kW
• 1,500 – 3,000 sq ft: 15 – 20 kW
• Over 3,000 sq ft: 25 – 48+ kW
• 
  

Keep in mind: Homes in Southeast Louisiana with electric heating, EV chargers, or multiple AC units will require more capacity.

For homeowners in Hammond, LA, common generator sizes include:

• 13kW – Essential circuits for smaller homes
• 18kW – 22kW – Partial to full coverage
• 26kW – Whole-home coverage including HVAC

What Happens If Your Generator Is Too Small Or Too Large?

Choosing the wrong generator size can lead to performance issues, higher operating costs, and long-term reliability problems. Both undersized and oversized generators come with distinct disadvantages.

Undersized Generator

A generator that is too small cannot handle the required load, especially during startup surges. This can result in:

• Failure to start high-demand systems like HVAC
• Frequent overloads and unexpected shutdowns
• Increased wear and potential damage to electrical components

Oversized Generator

A generator that is too large may seem like a safe choice, but it can lead to inefficiencies such as:

• Higher upfront purchase and installation costs
• Reduced efficiency when operating at partial load
• Increased fuel consumption over time

Why Proper Sizing Matters

Proper generator sizing ensures the system operates efficiently, handles peak demand reliably, and maintains safe operating conditions for all connected equipment.

What Are Alternative Ways To Estimate Generator Size?

Using Electricity Bills

Peak usage determines generator size.

• Identify the highest monthly kWh usage
• Convert to kW demand
• Add 20–25% margin

Using Load Management Systems

• The system automatically disconnects non-essential loads
• Smaller generator handles critical loads

Using Transfer Switch Prioritization

Transfer switches separate essential and non-essential circuits.

• Automatic Transfer Switch (ATS) improves reliability
• Manual switch reduces cost
  

What Are Best Practices For Generator Sizing?

• Operate the generator at ≤80% capacity
• Add 20–25% safety margin
• Plan for future loads (EV, HVAC upgrades)
• Consider Southeast Louisiana’s high cooling demand
• Soft starters can reduce AC startup surge by 30–70%, allowing smaller generator sizing

What Installation And Code Requirements Apply?

Standby generator systems require:

• Automatic Transfer Switch (ATS)
• NEC compliance
• Local permits
• 
  

Licensed professionals ensure safe, code-compliant installation and long-term reliability.

Maintenance Tips For Long-Term Performance

Routine maintenance is essential to ensure your generator works when you need it most.

A commonly recommended guideline is the 20/20/20 rule:

• Run the generator for 20 minutes
• At a minimum of 20% load
• Every 20 days
• 
  

Additional best practices:

• Check oil and coolant levels regularly
• Test the battery monthly
• Schedule professional servicing at least once per year
• 
  

How To Choose The Right Generator For Your Home

When comparing generator options, consider:

• Total capacity (kW rating)
• Fuel type (natural gas, propane, or diesel)
• Transfer switch compatibility
• Warranty coverage
• Availability of local service and support
• 
  

Before making a final decision, it’s always best to request a professional load calculation for maximum accuracy.

Get Professional Help With Generator Sizing

Choosing the right generator doesn’t have to be complicated. A professional assessment ensures accurate sizing, safe installation, and long-term performance.

Southern HVAC’s Home Standby Generators Service in Hammond, LA, provides detailed load calculations and tailored recommendations based on your home’s actual power needs.

Schedule a professional generator sizing assessment in Hammond, LA, and get a system specifically designed to match your home’s power requirements and outage needs.

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