Quick answer
Generator sizing: total your running watts, add the highest motor starting surge, then keep 25% headroom. A home with 8kW running load and a well pump needs a ~12-13kW unit. Worksheet below.
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.