How to Determine What Size Generator You Need
Choosing the right generator size is critical. Too small and it won't power your essentials. Too large and you waste money on fuel and equipment. This guide explains exactly how to calculate your needs.
Understanding Running Watts vs. Starting Watts
Every generator has two wattage ratings that matter:
Running Watts (Rated Watts) is the continuous power the generator produces. This is the number you'll see prominently displayed—like "5,000W generator." Your total running load must stay below this number.
Starting Watts (Surge Watts) is the extra power available for 1-3 seconds when motors start. Refrigerators, pumps, and AC units draw 2-3x their running watts during startup. Your generator's surge rating must handle your highest-surge appliance.
The Formula: Add up all running watts, add 25% headroom, and ensure surge capacity exceeds your biggest motor's starting watts. — Generator Sizing Best Practice
Common Appliance Wattage Reference
Here are typical wattages for common household appliances:
High-Priority Essentials
- Refrigerator: 100-200W running, 1,200W starting
- Chest Freezer: 50-100W running, 600W starting
- Sump Pump (1/3-1/2 HP): 800W running, 2,000-2,200W starting
- Well Pump (1/2 HP): 1,000W running, 3,000W starting
- Furnace Blower: 500W running (gas furnaces still need electricity)
Climate Control
- Window AC (5,000 BTU): 500W running, 1,000W starting
- Window AC (10,000 BTU): 1,200W running, 2,200W starting
- Central AC (2-3 ton): 3,000-3,500W running, 4,500W+ starting
- Space Heater: 1,500W (no surge—resistive load)
Kitchen
- Microwave: 1,000-1,500W
- Coffee Maker: 800-1,000W
- Toaster: 800-1,200W
- Electric Kettle: 1,500-1,800W
Important: Don't run multiple high-draw kitchen appliances simultaneously. Run one at a time to stay within your generator's capacity.
Generator Size Recommendations by Use Case
Minimal Backup (2,000-3,500W)
Powers: Refrigerator, LED lights, phone charging, WiFi router
Best for: Apartments, short outages, basic needs only
Generators: Honda EU2200i, Westinghouse iGen2500
Standard Home Backup (5,000-7,500W)
Powers: Above + sump pump OR freezer + more lights
Best for: Most homes during typical outages
Generators: Champion 5500W, Generac GP5500
Extended Home Backup (7,500-10,000W)
Powers: Above + window AC OR well pump + multiple circuits
Best for: Larger homes, rural properties with wells, summer outages
Generators: Generac GP8000E, Champion 9375W
Whole House (10,000W+)
Powers: Central AC, electric range, multiple circuits simultaneously
Best for: Extended outages where you need near-normal operation
Note: Consider a standby generator with automatic transfer switch for this level
Why the Starting Watts Matter
The most common generator sizing mistake is ignoring starting watts. Here's what happens when you undersize:
- Generator overloads when refrigerator compressor kicks on
- Lights dim or flicker
- Generator shuts down (protection circuit trips)
- Sensitive electronics may be damaged
Your generator's surge capacity doesn't need to cover all appliances starting simultaneously—motors don't all start at once. But it must handle your single highest-surge device (usually sump pump or AC unit).
Inverter vs. Conventional Generators
Inverter generators produce clean, stable power safe for electronics. They're quieter, more fuel-efficient, and typically range 2,000-4,000W. Best for: sensitive electronics, camping, moderate power needs.
Conventional generators produce more power per dollar but with less stable voltage. They're louder and less fuel-efficient. Best for: high-power needs, running motors, construction sites, whole-house backup.
For home backup, inverter generators work great up to about 4,000W. Above that, conventional generators offer better value unless you have many sensitive electronics.
Fuel Considerations
Generator size affects fuel consumption. A 5,000W generator running at 50% load uses roughly 0.5-0.7 gallons per hour. Larger generators consume more fuel even at partial load.
Use our Generator Runtime Calculator to see how long your fuel supply will last, and our Fuel Storage Calculator to plan your fuel reserves.
Next Steps
Now that you know your generator size, calculate how long it will run on your fuel and plan your fuel storage needs.
Frequently Asked Questions
For basic essentials (refrigerator, lights, phone charging), a 3,000-4,000 watt generator is sufficient. For a refrigerator plus sump pump, you need 5,000-7,500 watts. For whole-house backup including AC or heating, plan for 10,000-20,000 watts or a standby generator.
A refrigerator typically needs 100-400 running watts but can surge to 1,200-2,000 watts when the compressor starts. A 2,000-watt generator can run most refrigerators, but we recommend 3,000+ watts for headroom and to run other essentials simultaneously.
Running watts (rated watts) is the continuous power an appliance uses while operating. Starting watts (surge watts) is the extra power needed for 1-3 seconds when motors start. Your generator's surge rating must exceed your highest starting wattage appliance.
Yes. Most sump pumps need 800-1,500 running watts with 2,000-3,000 starting watts. A 3,500+ watt generator can handle a sump pump plus lights and phone charging. For sump pump plus refrigerator, get at least 5,000 watts.
House size matters less than what you want to power. For essentials only (fridge, lights, sump pump), 5,000-7,500 watts works for any size home. For central AC (which varies by tonnage, not house size), you need 15,000-20,000 watts or a standby generator.