About This Site
About Generator Size Calculator
Free, accurate generator sizing tools for homeowners, electricians, and emergency preparedness planners — built on formulas from the U.S. Department of Energy, ASHRAE, IEEE, and the National Electrical Code.
Who Writes This
Daniel Hart
Power Systems Engineer & Generator Sizing Specialist
15+ years in electrical and aviation power systems. Certified in NEC compliance and generator system design. Has sized backup power for everything from single-family homes to off-grid cabins and commercial facilities.
Areas of Expertise
Our Mission
Generator Size Calculator exists to solve one of the most common and costly mistakes in emergency preparedness: buying the wrong-sized generator. An undersized generator trips under load when you need it most. An oversized generator wastes hundreds of dollars in unnecessary purchase cost and burns more fuel than necessary.
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, and the methodology is fully transparent.
Generator sizing is not guesswork. It follows deterministic formulas derived from published electrical engineering standards. Our calculators implement those formulas exactly as a licensed electrician or engineer would apply them in the field.
Calculation Methodology
Our generator sizing engine uses the same formulas professional electricians and engineers apply in the field. Every formula is sourced from a published standard:
Wattage = BTU ÷ EERFor AC-based loads (ASHRAE)Surge = Running Watts × Motor Factor (2–4×)IEEE 141 motor starting currentsRecommended = (Running + Surge Headroom) ÷ 0.80NEC Article 220 — 80% continuous load ruleFuel/hr = (kW Load × Load Factor) ÷ (Density × η)DOE generator efficiency curvesSolar Watts = Daily Wh ÷ (Sun Hours × 0.85)NREL PVWatts methodologyEditorial Standards
We hold our content to the same standards as professional electrical publications. These standards apply to every calculator, guide, and product recommendation on this site:
- All wattage data sourced from manufacturer specifications and the DOE Appliance Standards database — never estimated.
- Surge multipliers validated against real-world measurements and IEEE 141 motor starting current standards.
- Fuel consumption modelled using industry-standard generator efficiency curves calibrated to 25%, 50%, 75%, and 100% load points.
- Solar calculations use NREL PVWatts methodology for peak sun hour estimates by US region.
- All generator recommendations include the 20–25% safety margin mandated by NEC Article 220 for continuous loads.
- Content reviewed and updated when manufacturer specs change, new standards are published, or real-world data warrants correction.
- Affiliate relationships do not influence methodology or recommendations — see our affiliate disclosure.
Data Sources & Standards
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Questions or Corrections?
If you find an error in our wattage data, a calculation that doesn't match your electrician's estimate, or a generator model we should cover, we want to hear about it.
Contact UsImportant 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 before installing a transfer switch or standby generator system. Never rely solely on an online calculator for decisions involving life-safety equipment, medical devices, or code-compliance determinations.