Generator Sizing Calculator
Calculate the right generator size for your home, job site, or RV. Enter your appliances and wattage to find the perfect generator capacity.
Formula & How It Works
Generator Size = (Highest Starting Watts or Running Watts) / Derate Factor
Generators should never run at 100% capacity continuously. Portable generators are typically derated to 80% for reliable operation, while standby units can run at 90%. Always add 20% headroom for future loads and motor surges. The highest starting watts matter most because motors like AC compressors and well pumps need 2-3x their running wattage to start.
Common Appliance Wattage Guide
| Appliance | Running Watts | Starting Watts |
|---|---|---|
| Refrigerator / Freezer | 700W | 2,200W |
| Window AC (10,000 BTU) | 1,200W | 3,600W |
| Central AC (3 ton) | 3,500W | 10,500W |
| Well Pump (1/2 HP) | 1,000W | 3,000W |
| Microwave Oven | 1,000W | 1,000W |
| Electric Water Heater | 4,000W | 4,000W |
| Furnace Fan (1/2 HP) | 800W | 2,350W |
| Lights (LED, 10 bulbs) | 100W | 100W |
| TV (LED 55") | 150W | 150W |
| Sump Pump (1/2 HP) | 1,050W | 3,150W |
Generator Sizing Tips
- Starting vs running watts: Motors need 2-3x running wattage to start - always size for the peak
- Sequential starting: Start large appliances one at a time to manage surge loads
- 20% headroom: Always add 20% to your calculated size for reliability and future expansion
- Transfer switch: A manual or automatic transfer switch is required for safe home connection
- Fuel type: Gasoline for portable, propane/natural gas for standby, diesel for heavy-duty
Frequently Asked Questions
What size generator do I need to run a house?
A typical home needs 7,500-10,000 running watts to power essentials (fridge, lights, well pump, furnace, and some outlets). For whole-house including central AC, you will need 15,000-22,000W. Calculate your total running watts and highest starting watts, then add 20% headroom.
What size generator do I need for a 3-ton AC?
A 3-ton central AC requires about 3,500 running watts but needs 10,500 starting watts. You will need at minimum a 12,000W generator to handle the surge. A 14,000-15,000W unit is recommended for reliable starting plus other loads.
Will a 7,500W generator run a house?
A 7,500W generator can run essential loads: refrigerator, lights, furnace fan, well pump, TV, and a window AC unit. It cannot run a central AC, electric water heater, and electric range simultaneously. Prioritize critical loads and use a transfer switch.
How do I calculate starting watts?
For any motor-driven appliance, multiply the running watts by 2-3x to get starting watts. For example, a 1,000W well pump needs about 3,000W to start. Add up all running watts, then identify the single largest starting surge to determine your peak demand.
Portable vs standby generator - which do I need?
Portable generators (3,000-10,000W) are affordable and flexible but require manual setup and fueling. Standby generators (10,000-25,000W) start automatically, run on natural gas or propane, and can power your entire home. Choose standby if you need seamless backup; choose portable for occasional use or job sites.