Recommended Generator
4 kW
Running
1,050W
Peak
2,650W
Type
Portable
Inputs
Result
Running watts: 150 + 100 + 800 + 800 = 1,850W. Largest surge is sump pump (2,400 - 800 = 1,600W extra). Peak = 1,850 + 1,600 = 3,450W. With 25% margin: ceil(3,450 * 1.25 / 1,000) = 5 kW. Rounded to 6 kW standard size.
Inputs
Result
Running: 150+100+800+800+3,500+1,000 = 6,350W. Largest surge is AC (7,000-3,500 = 3,500W). Peak = 6,350 + 3,500 = 9,850W. With 25% margin: ceil(9,850 * 1.25 / 1,000) = 13 kW.
Most homes need a 5,000–10,000 watt generator for essential circuits (refrigerator, lights, furnace, sump pump). A whole-house standby generator typically requires 15,000–25,000 watts to run everything including central AC, water heater, and cooking appliances. Calculate your specific needs by adding running watts plus the highest surge wattage.
Running watts (rated watts) is the continuous power an appliance uses during normal operation. Starting watts (surge watts) is the extra power needed for the first few seconds when a motor-driven appliance starts up. Starting watts can be 2–3 times higher than running watts. Your generator must handle both.
| Appliance | Running Watts | Starting Watts |
|---|---|---|
| Refrigerator | 150W | 1,200W |
| Furnace Blower | 800W | 2,350W |
| Sump Pump (1/2 HP) | 800W | 2,400W |
| Central AC (3-ton) | 3,500W | 7,000W |
| Well Pump (1/2 HP) | 1,000W | 2,100W |
Portable generators (3,000–12,000W) cost $500–$2,500 and are good for essential circuits during short outages. Standby generators (10,000–25,000W) cost $3,000–$15,000 installed but start automatically, run on natural gas or propane, and can power your entire home indefinitely.
| Feature | Portable | Standby |
|---|---|---|
| Output range | 3,000–12,000W | 10,000–25,000W |
| Cost (installed) | $500–$2,500 | $5,000–$15,000 |
| Fuel type | Gasoline | Natural gas / propane |
| Auto start | No (manual) | Yes (10–30 sec) |
| Run time | 6–12 hrs per tank | Unlimited (piped gas) |
A 25% safety margin prevents overloading, which can damage the generator and connected appliances. Generators run most efficiently at 50–75% of their rated capacity. Running at full load continuously reduces lifespan and fuel efficiency. The margin also accounts for power fluctuations and appliances not listed.
A 3-ton central AC unit requires about 3,500 running watts and 7,000 starting watts. You would need at least a 10,000–12,000 watt portable generator to start the AC, and it would leave little room for other appliances. Most people use a standby generator for central AC backup.
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Last Updated: Mar 9, 2026
This calculator is provided for informational and educational purposes only. Results are estimates and should not be considered professional financial, medical, legal, or other advice. Always consult a qualified professional before making important decisions. UseCalcPro is not responsible for any actions taken based on calculator results.