Generator Size Calculator
Calculate what size generator you need.
Add your appliances and their wattages to see total running and startup watts required.
Generator sizing ensures your generator can power all the devices you plan to run simultaneously without overloading. Undersizing causes shutdowns and can damage equipment; oversizing means unnecessary fuel cost.
The Formula:
Required wattage = Sum of running watts + Largest single starting surge
Running watts = steady-state power draw Starting (surge) watts = 2–3× running watts for motor-driven appliances
Common Appliance Wattage Reference:
| Appliance | Running Watts | Starting Watts |
|---|---|---|
| Refrigerator | 150–400 W | 600–1,200 W |
| Window AC (10,000 BTU) | 1,200 W | 2,400 W |
| Central AC (3 ton) | 3,500 W | 7,000 W |
| Microwave | 600–1,200 W | same |
| LED lighting (10 bulbs) | 100 W | same |
| Sump pump | 800 W | 1,300 W |
| Electric stove burner | 1,200 W | same |
| Well pump (1 HP) | 750 W | 2,250 W |
Worked Example:
Emergency power needs:
- Refrigerator: 400 W running, 1,200 W start
- Window AC: 1,200 W running, 2,400 W start
- Lights: 150 W
- Phone chargers, TV: 200 W
Total running watts: 400 + 1,200 + 150 + 200 = 1,950 W Largest surge (AC): 2,400 W
Required generator: 1,950 + 2,400 = 4,350 W → choose a 5,000W generator
Generator Size Categories:
| Generator Size | Typical Use |
|---|---|
| 1,000–2,000 W | Essential devices: phone, laptop, lights |
| 3,000–5,000 W | Refrigerator + AC + lights |
| 7,000–10,000 W | Whole home, well pump, AC |
| 15,000+ W | Large home, central AC, electric stove |
Practical Tips:
- Never run a generator indoors or in a garage — carbon monoxide kills silently
- Run the generator under load — light loads can cause “wet stacking” in diesel units
- Portable inverter generators are quieter and fuel-efficient for smaller loads
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
SuperGlobalCalculator is independently built and maintained. See how we build and verify our calculators.