Generator Size Calculator
Calculate the recommended generator wattage capacity (running and starting surge watts) needed to power your home during an emergency blackout.
Select Essential Home Loads
Running / Surge WattsGenerator Wattage Sizing Formula
Peak Starting Watts = Total Running Watts + Largest Single Motor Surge Delta | Recommended kW = Total Watts × (1 + Safety %)- Running (Continuous) Watts: Constant power consumed while the appliance is operating.
- Starting (Surge) Watts: Instantaneous inductive motor surge (2 to 3× running watts) required for 2-3 seconds to start compressors and pumps.
- Safety Margin: 20% to 25% capacity reserve to protect generator alternator and engine from overheating.
- Informational Aid: This tool is an estimation guide only and does not constitute certified electrical engineering design.
Step-by-Step Worked Example
Calculation Steps:
How to Use This Calculator
Assumptions & Limitations
- Only one major electric motor is assumed to cycle on and start at any single split-second.
- Sizing estimates provide general consumer planning guidance.
Frequently Asked Questions
What is the difference between Running Watts and Starting Watts?
Running watts (continuous watts) is the steady electrical power needed to keep an appliance operating. Starting watts (surge watts) is the high burst of extra power needed for a few seconds to start electric motors in refrigerators, air conditioners, and sump pumps.
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