About Generator Size Calculator
Free, accurate generator sizing tools built on industry standards from the U.S. Department of Energy, ASHRAE, IEEE, and the National Electrical Code.
Our Mission
Generator Size Calculator exists to solve one of the most common and costly mistakes in emergency preparedness: buying the wrong-sized generator. Undersized generators fail when you need them most. Oversized generators waste money and fuel.
We built this platform to make accurate generator sizing accessible to every homeowner, electrician, and preparedness planner -- not just those who can afford an electrical consultant. All tools are free, no account required.
How Our Calculator Works
Our generator sizing engine uses the same formulas electricians and engineers use in the field:
Wattage = BTU / EERFor AC-based loadsSurge = Running Watts × Motor Factor (2-4×)Per IEEE 141 motor starting standardsRecommended = (Running + Surge Headroom) / 0.8NEC 80% continuous load ruleFuel/hr = (kW Load × Load Factor) / (Energy Density × Efficiency)DOE generator efficiency curvesSolar: Panel Watts = Daily Wh Need / (Sun Hours × 0.85)NREL PVWatts methodologyAll appliance wattage data is sourced from manufacturer specifications and the DOE Appliance Standards database. Surge multipliers are validated against IEEE motor starting current standards (IEEE 141).
Editorial Standards
We hold our content to the same standards as professional electrical publications:
- All wattage data is sourced from manufacturer specifications, DOE appliance databases, and ASHRAE standards.
- Surge multipliers are validated against real-world measurements and IEEE motor starting current standards.
- Fuel consumption estimates use industry-standard generator efficiency curves calibrated to 25-50-75-100% load points.
- Solar calculations use NREL PVWatts methodology for peak sun hour estimates.
- All generator recommendations include a 20-25% safety margin per NEC and generator manufacturer guidelines.
- Content is reviewed and updated when new data becomes available or standards change.
Data Sources & Standards
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Important Disclaimer
All generator sizing calculations are estimates based on industry-standard formulas. Actual power requirements vary by specific appliance model, age, condition, ambient temperature, and usage patterns. Always consult a licensed electrician for critical installations, medical equipment backup, and whole-house standby systems. Never rely solely on an online calculator for decisions involving life-safety equipment.