A generator that's too small will overload and shut down when you need it most. One that's too large costs more to buy and run than necessary. Here's how to calculate exactly the right generator size.
Running Watts vs Starting Watts
Every generator has two ratings: running watts (continuous power) and starting watts (surge power for motor start-up). Motors — refrigerators, AC units, pumps — draw 2–3x their running wattage when starting. Your generator must handle starting watts for each motor-driven appliance.
Common Appliance Wattages
| Appliance | Running Watts | Starting Watts |
|---|---|---|
| Refrigerator | 150–400W | 800–1,200W |
| Window AC (10,000 BTU) | 1,200W | 3,600W |
| Central AC (3 ton) | 3,500W | 10,000W |
| Sump pump (1/2 HP) | 800W | 2,000W |
| Well pump (1 HP) | 1,000W | 3,000W |
| Electric stove (one burner) | 1,500W | 1,500W |
| Lights (per 100W bulb) | 100W | 100W |
| TV | 100–300W | 300W |
| Laptop/Phone chargers | 50–100W | 100W |
How to Calculate Your Generator Size
- List all appliances you need to run simultaneously during an outage
- Add up their running wattages for total running load
- Find the appliance with the highest starting wattage
- Add that starting wattage to your total running load
- Choose a generator with a starting/surge rating above that total
Pro Tip: Size your generator at 80% capacity for best fuel efficiency and longevity. If your calculated load is 5,000 watts, buy a 6,250-watt generator to run at 80% load.
Generator Size Recommendations
| Need | Recommended Size |
|---|---|
| Essential appliances only (fridge, lights, phone) | 2,000–3,500W |
| Partial home (no central AC) | 5,000–7,500W |
| Full home with central AC | 10,000–15,000W |
| Whole home standby | 20,000W+ |
Calculate the Right Generator Size
Enter your appliances and get an instant wattage recommendation.
Use Generator Calculator →Portable vs Standby Generator
- Portable generators — $500–$2,000. Manual start, must be run outdoors, store fuel. Good for occasional outages.
- Standby generators — $5,000–$15,000 installed. Automatic start, runs on natural gas or propane. Best for frequent outages or critical medical equipment.