Generator Sizing Guide

Why generator sizing matters

Choosing the correct generator size helps ensure your home remains safe, comfortable, and functional during an outage. A properly sized generator helps prevent overloads, avoids unnecessary fuel cost, and supports the appliances you depend on.

For residential use, generator sizing generally falls under optional standby systems described in NEC Article 702—systems intended to supply selected loads, not entire premises by default. For more on scope and limitations, see our calculator disclaimer.

Key factors that influence generator size

1. Running watts vs. starting watts

Many appliances draw significantly more power when starting than when running. Motors—found in refrigerators, AC compressors, well pumps, and blower motors—often require 2–3× their running wattage for a few seconds during startup.

• The total running watts of everything operating
• The highest starting watts of the largest motor load

A generator that cannot support an appliance’s inrush current may allow it to run but fail to start it reliably.

2. Essential vs. optional loads

Most homeowners focus on a core set of essential circuits:

• Heating or cooling equipment
• Refrigerator and freezer
• Well pump or sump pump
• Basic lighting and outlets
• Internet/Wi-Fi
• Select kitchen appliances

For deeper guidance on one of the most common high-surge loads, see well pump starting watts.

3. Fuel type and power output

• Natural gas (NG): usually 8–15% lower output
• Propane (LP): full rated output, long storage life
• Gasoline: portable units; limited storage life

Always check whether the kW rating is listed for NG, LP, or both—this affects sizing.

4. Climate and startup loads

Cold weather increases demand on furnace blowers and well pumps; hot weather increases AC compressor load. Allowing a modest buffer helps account for these seasonal conditions.

5. Panel and installation considerations

Your generator must match both your load and your electrical equipment. NEC 702 requires a listed transfer switch or interlock device to safely isolate generator power.

For a comparison of safe switching options, see transfer switch vs. interlock.

Typical wattage ranges for common home appliances

These are typical running (R) and starting (S) wattage ranges:

• Refrigerator: approximately 150 W R / S varies; check nameplate or measured startup
• Chest freezer: approximately 120 W R / S varies; check nameplate or measured startup
• Gas furnace blower: 400–900 R / 1,000–1,600 S
• Well pump (½ HP): 800–1,000 R / 2,000–3,000 S
• Well pump (1 HP): 1,500–2,000 R / 4,000–6,000 S
• Sump pump (⅓ HP): 600–900 R / ~1,300 S
• Window AC (10k BTU): 900–1,200 R / 2,000–3,600 S
• Central AC (3-ton): ~3,500 R / 10,000–15,000 S
• Electric range: approximately 8,000 W as a planning value; actual nameplate ratings and simultaneous element use vary
• Electric dryer: approximately 5,500 W as a planning value; use the appliance nameplate

The refrigerator and chest-freezer figures are modern-appliance planning defaults. Compressor startup, defrost heaters, ice makers, anti-sweat heaters, fans, and other operating modes can increase demand. Actual nameplate or measured values take precedence.

How to estimate generator size

1. List the loads you want to operate

Start with essential loads only. Many homeowners skip high-draw loads like electric dryers and ovens.

2. Identify running and starting watts

If an appliance lists amps only:

• 120 V → watts = 120 × amps
• 240 V → watts = 240 × amps

3. Add total running watts

Example essentials:

• Refrigerator: 150 W
• Chest freezer: 120 W
• Furnace blower: 700 W
• Lights + electronics: 200 W

Total running watts = 1,170 W

4. Add general lighting and small appliance allowances

• Lighting: ~3 W per sq ft
• Two small appliance circuits: 1,500 W each
• Laundry circuit: 1,500 W

For a 2,000 sq ft home, this adds: 6,000 + 4,500 = 10,500 W to your plan.

5. Determine peak starting watts

Refrigerator startup varies by equipment and operating mode. For this example, assume a measured starting demand of 2,100 W for the refrigerator. Peak:

• 11,670 W total running load
• Subtract refrigerator running: 11,520 W
• Add the assumed measured refrigerator start: 13,620 W

6. Add a safety buffer

A 20–25% buffer allows for seasonal variation and small future loads. 13,620 ÷ 0.8 ≈ 17.0 kW, so most homeowners would choose an 18 kW standby generator.

Recommended generator sizes by home type

• 1,000–2,000 sq ft: 7–12 kW
• 2,000–3,000 sq ft: 12–18 kW
• 3,000–4,500 sq ft: 18–22 kW
• 4,500+ sq ft: 22–26 kW

For service-specific planning, see our guides to generator sizing for 100-amp service, generator sizing for 150-amp service and generator sizing for 200-amp service.

Generator types to consider

Portable generators (≈2–12 kW)

Basic backup, manual fueling, outdoor-only operation.

Inverter generators (≈1–7 kW)

Clean, stable power; good for electronics; quieter; typically smaller capacity.

Standby generators (≈7–26 kW)

Automatic NG or LP systems powering selected or whole-home loads.

Solar + battery systems

Silent and fuel-free but limited by battery capacity. Often best for partial-home backup.

Common sizing mistakes to avoid

• Sizing only by square footage instead of wattage
• Ignoring starting watts for motors
• Assuming everything will run at once
• Not adjusting for NG derate
• Undersized transfer switch selection
• Oversizing leading to low-load inefficiency
• Not planning for future additions (EV charger, hot tub, etc.)

When to consult a professional

A licensed electrician should evaluate panel capacity, transfer equipment, grounding, clearances, and local codes. Every home’s motor loads behave differently.

For broader generator safety guidance, see the U.S. Consumer Product Safety Commission. Installation requirements may also be influenced by standards from the National Fire Protection Association (NFPA).

Conclusion

Generator sizing starts with understanding your wattage needs, accounting for starting loads, and selecting a system that supports your home safely with a reasonable buffer.

For an instant personalized estimate, use our free generator sizing calculator.

Still have questions? Visit our Generator FAQs.

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