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Generator Sizing Calculator - Running & Starting Watts

Size a Portable or Standby Generator for Your Home, Shop, or Job Site

Screen generator capacity by adding known loads and separating running watts from starting watts. The output is a planning estimate that must be checked against equipment nameplates, locked-rotor data, manufacturer fuel curves, altitude and temperature derates, and the adopted electrical code.

Motors, compressors, and pumps can draw far more current during startup than during steady operation. This tool uses local editable wattage fixtures and a single-highest-surge method, then labels source gaps for simultaneous motor starts, load management, fuel supply, transfer equipment, carbon monoxide, and qualified review.

Use it to organize generator shopping or outage planning notes before a licensed electrician, installer, utility, supplier, manufacturer, or AHJ reviews the actual setup.

Pro Tip: Use the generator continuous rating, not advertising peak watts, and replace preset wattage and fuel-rate rows with nameplate and manufacturer data before purchase or installation decisions.

Size the feeder wire from your generator to the transfer switch or panel

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Quick check whether your existing circuit can safely handle a specific appliance

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Run a panel load study to see which loads are critical vs. optional for backup

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Screen generator fuel burn and emissions before permit or reporting method review

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Generator Sizing Calculator
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Plan the Review

Open a source-aware checklist with tool, material, safety, permit, and qualified-review prompts.

Open Generator Transfer Review Checklist →

How It Works

  1. Add Loads

    Enter loads from equipment nameplates or use the local presets as placeholders that still need verification. Include only loads expected to run at the same time.

  2. Set Running Watts

    Enter running wattage from the appliance nameplate, owner manual, measured data, or manufacturer specification. Local preset rows are not source-verified standards.

  3. Set Starting Watts

    Enter starting watts or calculate them from locked-rotor amps and voltage when available. The app does not validate a universal motor-start multiplier.

  4. Review Active Load Estimate

    The screen totals running watts and adds the highest single starting surge to the running load of the other active loads. Simultaneous motor starts need separate load-management review.

  5. Check Source Boundaries

    Use the recommendation as a screening number only. Transfer equipment, grounding, bonding, conductors, inlet, fuel supply, CO safety, and code compliance need qualified review.

Built For

  • Homeowners shopping for emergency backup generators before storm season
  • Construction job sites needing portable power for saws, drills, and compressors
  • RV and camping setups requiring a properly sized portable generator
  • Well pump backup power planning for rural properties without municipal water
  • Whole-house standby generator sizing for automatic transfer switch installations
  • Portable generator comparison shopping to match specific wattage requirements
  • Food truck and mobile vendor power planning for cooking and refrigeration equipment

Features & Capabilities

Running vs Starting Watt Separation

Clearly separates continuous running demand from peak starting surge. Generators must handle both: running watts determine fuel consumption and sustained capacity, while starting watts determine if the generator can actually start your largest motor loads.

Starting-Watt Planning

Separates steady running watts from user-entered starting watts so motor and compressor loads are visible before generator comparison.

Local Appliance Presets

Includes editable planning rows for common appliances and tools. Replace them with nameplate, manual, or measured values for decision use.

kW Planning Output

Shows watt and kW planning numbers for comparison against manufacturer continuous ratings. It does not determine kVA, power factor, voltage regulation, or transient acceptance.

Recommended Screening Number

Uses the larger of the single-highest-surge method and a 20% running-watt buffer. Manufacturer derates and load-management sequencing remain source gaps.

Load Priority Planning

Helps you identify which loads are critical (well pump, refrigerator, furnace blower) versus optional (clothes dryer, oven) so you can size your generator for must-have loads and add optional loads only if capacity allows.

Comparison

Appliance / Load Running Watts Starting Watts Notes
Refrigerator / Freezer 150-400W 800-1,200W Compressor surge on startup
Sump Pump (1/2 HP) 800W 1,300-2,200W Critical during storms
Well Pump (1/2 HP) 1,000W 2,000-4,000W High surge, essential load
Window AC (10,000 BTU) 1,200W 3,600W 3x starting surge typical
Central AC (3 ton) 3,500W 7,000-10,500W Requires large standby unit
Furnace Blower 500-800W 1,000-1,600W Must-have for heating season
Microwave (1,000W) 1,000W 1,000W Resistive load, no surge
Circular Saw (7-1/4") 1,400W 2,300W Job site essential
Air Compressor (1 HP) 1,600W 4,500W Very high surge ratio

Assumptions

  • Starting watts are user-entered or preset planning values and are not validated universal motor-start multipliers
  • Running watts are local placeholders until replaced with nameplate, manual, or measured data
  • Generator watts are treated as continuous rated watts, not peak advertising watts
  • Altitude and temperature derating are not applied unless the user adjusts inputs externally
  • Fuel consumption uses a local planning curve and is not a manufacturer fuel map
  • Only one large motor is assumed to start at a time unless the user enters a combined simultaneous surge manually

Limitations

  • Does not account for variable frequency drive (VFD) starting characteristics which reduce surge requirements
  • Fuel consumption varies significantly with altitude, temperature, load profile, and maintenance condition
  • Does not size transfer switch, feeder conductors, or overcurrent protection
  • Three-phase generator sizing does not account for load imbalance across phases
  • UPS and sensitive electronic loads may require tighter voltage and frequency regulation than standard generators provide
  • Does not calculate generator noise levels or exhaust emissions compliance

References

  • NFPA-70-2026-NEC-SOURCE - NEC generator and optional standby source pointer
  • NFPA-110-2025-EPSS-SOURCE - emergency and standby power systems source pointer
  • UL-2200-2025-GENERATOR-ASSEMBLIES-SOURCE - stationary engine generator assembly source pointer
  • OSHA-PORTABLE-GENERATOR-SAFETY-2020 - portable-generator safety source pointer
  • CPSC-PORTABLE-GENERATOR-CO-SAFETY-2026 - carbon monoxide and portable-generator safety pointer
  • NEMA-MOTORS-GENERATORS-SOURCE - motor and generator standards source pointer

Frequently Asked Questions

Running watts (also called rated watts) represent the continuous power an appliance needs to operate. Starting watts (also called surge watts or peak watts) represent the brief spike of power needed to start motors and compressors. This surge lasts only a fraction of a second but your generator must be able to deliver it or the motor will not start. Always size your generator based on the highest starting watt demand, not just the total running watts.
Headroom depends on the generator manufacturer, fuel, altitude, temperature, load type, duty cycle, and voltage/frequency limits. This tool uses a 20% running-watt buffer for screening only; use the manufacturer derate and installation data for final selection.
Portable generators typically range from 2,000 to 12,000 watts, run on gasoline, and require manual setup and connection during an outage. Standby generators are permanently installed, connected through an automatic transfer switch, run on natural gas or propane, and start automatically when power goes out. Standby units range from 7,500 to 150,000+ watts and require professional installation by a licensed electrician.
Maybe. Use the unit nameplate, locked-rotor data, soft-start equipment data, manufacturer guidance, and transfer/load-management design. The preset central AC row is only a planning placeholder.
Fuel choice depends on generator model, utility or supplier availability, storage, refill logistics, local code, exhaust location, fire safety, cost, and outage duration. Natural gas utility supply is not the same as stored fuel and can still be limited by pressure and service conditions.
There is no reliable house-size shortcut. Add the actual loads you need, then verify the result against nameplates, manufacturer generator data, transfer equipment, load-management controls, and local electrical requirements.
Maybe. A well pump can have high starting demand and long cable runs. Use pump nameplate or locked-rotor data, voltage, control type, soft-start status, simultaneous loads, and manufacturer guidance before deciding.
A standard home refrigerator draws 100-400 running watts depending on size and age, but needs 800-1,200 watts of starting surge when the compressor kicks on. A chest freezer is similar. The starting surge only lasts a fraction of a second but your generator must be able to deliver it. Always plan for the starting watts, not just the running watts, when sizing your generator for refrigeration.
Disclaimer: Generator sizing estimates are preliminary planning values based on editable local fixtures. Actual requirements depend on equipment nameplates, locked-rotor amps, generator continuous and surge ratings, fuel system, altitude, temperature, transfer equipment, grounding, bonding, conductors, CO/exhaust placement, adopted code edition, AHJ, and qualified electrical review.

Learn More

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