Generator Wattage Calculator
Plan for backup power by calculating the generator size required to support your essential loads and motor start-up surges.
Sum the running watts for appliances you need during an outage.
Use the highest running watts among devices with motors (AC, pump, compressor).
Typical motors draw 2–4× their running watts during start-up.
Most generators are rated at PF 0.8.
Adjust for fuel type (diesel ~0.07, propane ~0.3).
Load Summary
Running: 3,500 W
Starting: 3,600 W
Generator Size
Recommended: 8,520 W
Approx. 10.65 kVA @ PF 0.8
Fuel Estimate
Daily fuel: 9.2 gal
A mid-size generator will handle most essentials. Balance fuel availability with runtime needs.
How to Use This Calculator
List essential loads
Add up the running wattage for appliances you want powered during an outage—refrigeration, lighting, HVAC, pumps.
Identify the largest motor
Use the appliance with the highest starting surge (typically well pumps or AC compressors) to ensure the generator can handle startup.
Add safety margin
Increase the result by 15–25% to avoid running the generator at maximum capacity continuously.
Formula
Recommended Watts = (Running Watts + Motor Watts × Multiplier) × (1 + Margin %)
kVA = Watts ÷ (1000 × Power Factor)
Example: 3500 W running + (1200 × 3) = 7100 W. With 20% margin → 8520 W. Round up to a 9 kW generator.
About the Generator Wattage Calculator
Choosing the right generator prevents overloads and wasted fuel. This tool balances running loads, motor surges, and safety margin to provide a realistic generator size recommendation for home or jobsite use.
When to Use This Calculator
- Emergency backup planning: Prepare for storms or outages with a properly sized portable generator.
- Construction sites: Determine generator size for power tools and temporary lighting.
- RV and off-grid setups: Balance generator output with inverter and battery systems.
- Whole-home standby systems: Estimate required capacity before speaking with an installer.
Why Use Our Calculator?
- ✅ Motor surge aware: Accounts for start-up loads automatically.
- ✅ Custom margins: Add buffer for future appliances or voltage drop.
- ✅ Fuel estimation: Plan fuel storage using daily consumption estimates.
- ✅ PF adjustment: Translate watts to kVA for industrial generator specs.
Common Applications
Homeowners: Size portable generators for refrigerators, sump pumps, and essential circuits.
Contractors: Ensure jobsite generators can start air compressors or saws.
Event planners: Provide reliable power for sound systems and lighting.
Tips for Best Results
- Label critical loads and know their wattage before an outage.
- Stagger appliance startup to reduce surge requirements.
- Choose generators with automatic voltage regulation for sensitive electronics.
- Follow manufacturer maintenance schedules to ensure rated output.
Frequently Asked Questions
How do I find appliance wattage?
Check the nameplate, manual, or use a watt meter. Many appliances list running amps—multiply by voltage to convert to watts.
What if I have multiple motors?
Use the highest starting watt load and manage others sequentially. If two motors must start together, add both surges.
Should I oversize the generator?
A 20–25% margin prevents overloads. Oversizing beyond that increases cost and fuel consumption without much benefit.
Why convert to kVA?
Industrial generators and standby units are usually rated in kVA. Dividing watts by power factor helps you compare models.