Generator Sizing Calculator
Determine the right generator size for your home or business based on your essential electrical loads and starting requirements.
Your Electrical Loads
Generator Sizing
Proper generator sizing requires calculating both running watts (continuous power) and starting watts (surge for motors). Undersizing risks damage; oversizing wastes fuel.
- • Starting watts: 2-3x running for motors
- • Add 20% safety margin to calculations
- • Portable: 2-12kW | Standby: 14-48kW
How to Use
- Add your electrical loads using quick-add buttons or custom entries
- Set running watts, starting watts, and quantity for each load
- Mark essential loads that must run during an outage
- Click Calculate to see recommended generator size
FAQ
What's the difference between running and starting watts?
Running watts is the continuous power needed to operate an appliance. Starting watts is the extra surge required when motors first start—typically 2-3x higher. Your generator's peak capacity must handle these surges.
Portable vs standby generator—which do I need?
Portable generators (2-12kW) are affordable and handle essential loads. Standby generators (14-48kW) are permanently installed, start automatically, and can power your whole home. Choose based on your power needs and budget.
Can I run my AC on a portable generator?
Window units (5,000-10,000 BTU) can run on 5-8kW portables. Central AC (3-5 tons) needs 7,000+ starting watts—typically requiring a large portable (10kW+) or standby generator. Always check starting watts, not just running.