LetUsCalc
Free Generator Sizing Tool

What Size Generator Do I Need?

Use our free generator size calculatorto estimate the exact wattage you need. Whether you're sizing a generator for a house, planning whole home generator sizing, or need backup power for your RV - select your appliances below and get an instant recommendation.

Select Your Appliances

Kitchen

150W running / 600W starting

1000W running / 1500W starting

100W running / 500W starting

1500W running / 1800W starting

1000W running / 1000W starting

900W running / 900W starting

1500W running / 1500W starting

1700W running / 1700W starting

HVAC

900W running / 2700W starting

3500W running / 7000W starting

1500W running / 1500W starting

75W running / 150W starting

100W running / 200W starting

Entertainment

100W running / 100W starting

300W running / 300W starting

Lighting

100W running / 100W starting

600W running / 600W starting

Laundry

500W running / 2250W starting

3000W running / 6000W starting

Utilities

500W running / 1500W starting

1000W running / 3000W starting

4500W running / 4500W starting

800W running / 1300W starting

550W running / 1100W starting

Electronics

60W running / 60W starting

200W running / 200W starting

10W running / 10W starting

20W running / 20W starting

50W running / 50W starting

Personal

1500W running / 1500W starting

Add Custom Appliance

Your Result

Select appliances to see your generator recommendation

What Is a Generator Size Calculator?

A generator size calculatoris an essential tool that helps you determine what size generator you need based on your appliances and devices. Whether you're determining generator size for emergency backup power, whole house generator sizing, or RV camping - knowing your exact power requirements prevents buying an undersized generator that can't handle your needs or an oversized one that wastes fuel and money.

Our free generator wattage calculator considers both running watts (continuous power) and starting watts (surge power) to accurately estimate generator size. It includes a built-in 20% safety margin, making it easy to find what size generator you need for your house, RV, or backup power setup.

How to Estimate Generator Size for Your House

Follow these steps to calculate what size generator you need for your home or determine proper whole home generator sizing:

  1. 1
    List Your Appliances

    Make a list of all the appliances and devices you want to power simultaneously. Be realistic about your essential needs during a power outage.

  2. 2
    Find Wattage Requirements

    Check each appliance's nameplate or manual for running watts and starting watts. If only amps are listed, multiply amps by voltage (usually 120V) to get watts.

  3. 3
    Calculate Total Running Watts

    Add up the running watts of all appliances you plan to use at the same time. This is your base load.

  4. 4
    Add Starting Watts

    Find the appliance with the highest starting watts and add the difference between its starting and running watts to your total.

  5. 5
    Add Safety Margin

    Add 20-25% to your total to account for power fluctuations, altitude, and future needs. This is your minimum recommended generator size.

Running Watts vs Starting Watts: Key to Generator Sizing

Understanding the difference between running watts and starting watts is crucial when determining generator size for your house or any backup power application:

Running Watts (Continuous)

Running watts, also called rated or continuous watts, represent the steady power an appliance needs to operate normally. This is the constant electrical load the appliance draws during operation. For example, a 150-watt refrigerator will continuously draw approximately 150 watts while running.

Starting Watts (Surge)

Starting watts, also called surge or peak watts, are the extra power needed to start motor-driven appliances. When a motor starts, it draws 2-3 times its running wattage for a brief moment. For example, that same refrigerator might need 600 watts to start its compressor before settling to 150 watts.

Why this matters: If your generator can't handle the starting surge, it will overload and shut down. This is why generator sizing must account for the highest starting wattage any single appliance requires, not just the sum of running watts.

Generator Sizing Examples

Small PortableBasic Emergency Kit (2,000W)

  • Refrigerator: 150W running / 600W starting
  • LED Lights (10): 100W running
  • Phone Chargers (4): 40W running
  • WiFi Router: 20W running
  • TV: 100W running

Total: 410W running + 450W surge = 860W peak 2,000W generator recommended

Medium PortableRV Camping Setup (4,000W)

  • Small Window AC: 900W running / 2,700W starting
  • Refrigerator: 150W running / 600W starting
  • Microwave: 1,000W running
  • Lights & Electronics: 200W running

Total: 2,250W running + 1,800W surge = 4,050W peak 4,000-5,000W generator recommended

Large PortableHome Essentials Backup (7,500W)

  • Window AC (10,000 BTU): 1,200W running / 3,600W starting
  • Refrigerator: 150W running / 600W starting
  • Sump Pump: 800W running / 1,300W starting
  • Washing Machine: 500W running / 2,250W starting
  • Lights, TV, Router: 300W running

Total: 2,950W running + 2,400W surge = 5,350W peak 7,500W generator recommended

Whole HouseComplete Home Backup (20,000W)

  • Central AC (3 ton): 3,500W running / 7,000W starting
  • Electric Water Heater: 4,500W running
  • Refrigerator & Freezer: 250W running / 1,100W starting
  • Well Pump: 1,000W running / 3,000W starting
  • Washing Machine: 500W running / 2,250W starting
  • Electric Dryer: 3,000W running
  • Lights & Electronics: 500W running

Total: 13,250W running + 3,500W surge = 16,750W peak 20,000W standby generator recommended

Common Household Appliance Wattage Chart

Reference this chart for typical wattage values. Always check your specific appliance's nameplate for exact specifications.

ApplianceRunning WattsStarting Watts
Refrigerator100-200W400-800W
Freezer50-150W300-600W
Window AC (Small)500-900W1,500-2,700W
Window AC (Large)1,000-1,500W3,000-4,500W
Central AC2,000-5,000W4,000-10,000W
Electric Water Heater4,000-5,500W4,000-5,500W
Washing Machine350-500W1,000-2,500W
Electric Dryer2,000-5,000W4,000-6,000W
Dishwasher1,200-1,800W1,500-2,200W
Microwave (1000W)1,000W1,500W
Electric Stove (one burner)1,200-2,500W1,200-2,500W
Coffee Maker600-1,200W600-1,200W
Toaster800-1,500W800-1,500W
Well Pump (1/2 HP)400-600W1,000-2,000W
Well Pump (1 HP)800-1,200W2,000-4,000W
Sump Pump (1/3 HP)600-900W1,000-1,500W
Space Heater1,000-1,500W1,000-1,500W
Ceiling Fan50-100W100-200W
Box Fan50-120W100-200W
LED Lights (per bulb)8-15W8-15W
Incandescent Light (60W)60W60W
Desktop Computer150-300W150-300W
Laptop30-100W30-100W
LED TV (50")80-150W80-150W
Gaming Console100-200W100-200W
WiFi Router10-25W10-25W
Phone Charger5-25W5-25W
Garage Door Opener400-600W800-1,200W
Hair Dryer1,000-1,800W1,000-1,800W
Electric Grill1,200-2,000W1,200-2,000W

Generator Buying Tips

1. Size Up, Not Down

It's better to have extra capacity than not enough. Running a generator at 50-75% capacity is ideal for efficiency and longevity.

2. Consider Inverter Generators

For sensitive electronics, inverter generators produce cleaner power. They're also quieter and more fuel-efficient, making them ideal for RVs and camping.

3. Fuel Type Matters

Gas generators are common and affordable. Dual-fuel (gas/propane) offers flexibility. Diesel is more efficient for larger units. Solar/battery options are quiet but limited in capacity.

4. Check Runtime

Look at runtime at 50% load, not just fuel tank size. A longer runtime means fewer refueling interruptions during extended outages.

5. Noise Levels

Noise is measured in decibels (dB). Most communities have noise ordinances. Inverter generators typically run at 50-60 dB, while conventional units can exceed 70 dB.

6. Outlet Configuration

Ensure the generator has enough outlets for your needs. Consider 120V standard outlets, 240V outlets for large appliances, and USB ports for convenience.

7. Transfer Switch

For home backup, a transfer switch allows safe connection to your electrical panel. Manual switches are affordable; automatic switches offer seamless switchover.

8. Warranty & Support

Look for at least a 2-3 year warranty. Consider brands with local service centers for easier maintenance and repairs.

Frequently Asked Questions