The most common mistake homeowners make when buying a generator is getting the wrong size. Buy too small and it shuts down when your AC kicks on. Buy too big and you waste hundreds of dollars on a machine that burns more fuel than necessary.
Running Watts vs Starting Watts
Running watts is the continuous power a generator produces during normal operation. Starting watts is the extra burst needed to start motor-driven appliances. Electric motors in refrigerators, AC units, and well pumps require 2–3 times more power to start than to run.
A central air conditioner might need 1,500 watts to run but requires 4,500–5,000 watts to start. If your generator cannot supply that surge, it will shut down.
Common Appliance Wattage Reference
| Appliance | Running Watts | Starting Watts |
|---|---|---|
| Refrigerator | 150–400W | 800–1,200W |
| Central AC (2.5 ton) | 3,500W | 7,500–9,000W |
| Window AC (10,000 BTU) | 1,200W | 3,600W |
| Well pump (1/2 HP) | 1,000W | 2,000W |
| Sump pump (1/3 HP) | 800W | 1,300W |
| Microwave | 1,000W | 1,000W |
| LED lights (10 bulbs) | 100W | 100W |
| Television (55 inch) | 100W | 100W |
Step-by-Step Sizing Calculation
Step 1: List all appliances you want to run simultaneously and add up their running watts.
Step 2: Find the single appliance with the highest starting watt requirement.
Step 3: Use this formula: Required Generator Size = Total Running Watts + (Highest Starting Watts - That Appliance Running Watts)
Example Calculation
- Refrigerator: 300W running
- 10 LED lights: 100W running
- Window AC: 1,200W running / 3,600W starting
- Sump pump: 800W running
- Total running: 2,400W
- Required size: 2,400 + (3,600 - 1,200) = 4,800 watts
General Sizing Guide
- Small home, essential only: 3,000–4,000 watts
- Medium home with window AC: 5,000–7,500 watts
- Medium home with central AC: 8,000–12,000 watts
- Large home, whole-home backup: 14,000–20,000 watts
The 80% Rule
Never run a generator above 80% of rated capacity for sustained periods. It shortens engine life and increases fuel consumption. If your calculation shows 6,000 watts needed, buy a 7,500-watt generator to maintain safe headroom.