Choosing the wrong generator size is the most common mistake shoppers make. Too small and it won't handle the load. Your breakers trip, your fridge shuts off, and you're left in the dark anyway. Too large and you've spent $500 to $2,000 more than you needed to. This guide walks you through exactly how to find your number in three steps.
Free printable generator sizing chart (PDF)
The three-step method, the use-case and appliance wattage charts, and a fill-in worksheet on two printable pages.
Download the sizing chart PDFHow to Size a Generator in Three Steps
List What You Need to Power
Write down every appliance you plan to run at the same time during an outage. Refer to the appliance table below for wattage values.
Add Up the Running Watts
Add the running watt value of each appliance on your list. This is your minimum wattage requirement.
Add 25% for Startup Surges
Motors (fridges, AC units, pumps, power tools) draw 2 to 3x their running watts on startup. Multiply your total by 1.25 to get your safe generator size.
Generator Size by Use Case
Use this as a starting point. Your actual needs depend on which appliances you run simultaneously.
| Use Case | Generator Size | What It Powers | Example Products |
|---|---|---|---|
| Camping and Tailgating | 500W to 2,000W | Phone, laptop, small appliances, camp lights | EcoFlow RIVER 3 Max, Anker SOLIX C1000 Gen 2 |
| Apartment Essentials | 2,000W to 3,500W | Fridge, lights, TV, phone charging, fans | Cummins Onan P2500i, Pulsar PG2300iSCO |
| Home Essentials (no AC) | 3,500W to 5,000W | Fridge, lights, microwave, sump pump | Cummins Onan P4500i, Pulsar PGD40iSCO |
| Home with Window AC | 5,000W to 7,500W | Everything above plus one window AC unit | Cummins Onan P5000iDF, Pulsar PG7750BCO |
| Whole Home (Central AC) | 10,000W to 15,000W | Central AC, well pump, full kitchen, HVAC | Cummins RS13A 13kW, Briggs & Stratton 13kW PowerProtect |
| Whole Home plus EV | 15,000W to 20,000W | Everything above plus EV Level 1 charging | Cummins RS17A, Cummins RS20AC, Briggs & Stratton PowerProtect+ 18kW |
| Commercial and Industrial | 20,000W and up | Full facility, large HVAC, machinery | Cummins Quiet Connect RS25 to RS150, Briggs & Stratton Fortress 30kW to 60kW (quoted individually) |
Common Appliance Wattage Reference
Running watts = what the appliance uses while operating. Starting watts = the surge when it first turns on. Size your generator to handle starting watts.
| Appliance | Running Watts | Starting Watts | Notes |
|---|---|---|---|
| Refrigerator | 200W | 1,200W | Most common outage priority. Older or side-by-side models can draw more |
| Chest Freezer | 100W | 500W | Critical for food preservation |
| Window AC (10,000 BTU) | 1,200W | 3,600W | Size up if running AC |
| Central AC (3 ton) | 3,500W | 9,000W | Plan on a 10,000W+ generator. A soft-start kit can lower starting watts to about 5,000 to 6,000W |
| Electric Furnace | 5,000W | 7,500W | Consider propane alternative |
| Sump Pump (1/2 HP) | 800W | 2,150W | Essential in flood-prone areas |
| Well Pump (1 HP) | 750W | 2,100W | Required for rural water supply |
| Electric Water Heater | 4,500W | 4,500W | High draw, consider gas alternative |
| Microwave | 1,000W | 1,000W | Short-duration use |
| Space Heater | 1,500W | 1,500W | High draw, use sparingly |
| CPAP Machine (with heated humidifier) | 100W | 100W | Medical priority. About 30W or less with the humidifier off |
| LED Lights (10 bulbs) | 100W | 100W | Negligible load |
| TV (55 inch) | 100W | 100W | Low draw |
| Laptop and Phone Chargers | 100W | 100W | Low draw |
| Power Drill | 600W | 1,800W | High starting surge |
| Circular Saw | 1,400W | 4,200W | Very high surge, job site use |
These numbers are typical estimates, not exact values. Real running and starting watts vary by make, model, age, and installation. Check the nameplate on each appliance, and have a licensed electrician confirm your running and starting loads before you buy or install a generator.
Which Fuel Type is Right for You?
Gasoline
Best for: Occasional outages, portability
Pros: Widely available, lower upfront cost
Cons: Fuel goes stale in 30 days, harder to store long-term
Shop Gasoline GeneratorsDual Fuel (Gas + Propane)
Best for: Emergency preparedness, extended outages
Pros: Flexibility, propane stores indefinitely, often quieter on propane
Cons: Slightly higher cost
Shop Dual Fuel GeneratorsPropane and Natural Gas Standby
Best for: Whole home backup, automatic operation
Pros: Starts automatically, runs on home gas line, no manual refueling
Cons: Higher installation cost, requires licensed installer
Shop Standby GeneratorsReady to Find Your Generator?
Browse by size, use case, or brand. Free shipping on orders over $250, plus 30-day returns with no restocking fees. Large or heavy generators ship by freight for a flat delivery charge shown on the product page, with liftgate service included. Call us at (888) 775-9048 if you want a recommendation.
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