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Generator Sizing Calculator - Running & Starting Watts

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

Calculate exactly how much generator capacity you need by adding up your loads and accounting for motor starting surge. This calculator separates running watts from starting watts so you know the true peak demand your generator must handle.

Motors, compressors, and pumps draw 2-6 times their running wattage during startup. If your generator cannot deliver that surge, the breaker trips or the generator stalls. This tool applies the correct surge multipliers automatically and recommends a generator size with appropriate headroom built in.

Whether you are shopping for a portable generator for storm backup, planning a standby system for your whole house, or sizing a unit for a construction job site, this calculator takes the guesswork out of the process.

Pro Tip: Never plan to run a generator at 100% capacity. Target 70-80% of rated watts for reliability and fuel efficiency. Overloading shortens generator life, increases fuel consumption, and can damage connected equipment.

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How It Works

  1. Add Your Loads

    Select appliances, tools, and equipment from the built-in database or enter custom loads. Include everything you want to run simultaneously during an outage or on the job site.

  2. Set Running Watts

    Enter or confirm the running wattage for each load. Check the appliance nameplate, owner's manual, or use the common appliance database built into the calculator.

  3. Mark Motor Loads

    Identify which loads have motors or compressors (refrigerators, AC units, sump pumps, power tools). The calculator applies the appropriate starting surge multiplier - typically 2-3x for refrigerators and up to 5-6x for large compressors.

  4. View Total Watts

    The calculator shows your total running watts (continuous demand) and peak starting watts (highest possible surge when the largest motor starts). Both numbers matter when choosing a generator.

  5. Get Generator Recommendation

    Based on your totals, the calculator recommends a generator size range with appropriate headroom factored in. You will see both kW and kVA ratings to match manufacturer specifications.

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.

Motor Surge Multipliers

Automatically applies correct starting surge multipliers for different motor types. A 1/2 HP well pump draws 1,000 running watts but may need 3,000-4,000 watts to start. The calculator handles these multipliers so you do not undersize.

Common Appliance Database

Includes a built-in database of typical wattage values for common appliances, tools, and equipment. Saves you from hunting down nameplate data for standard items like refrigerators, sump pumps, and window AC units.

kW and kVA Output

Displays results in both kilowatts (kW) and kilovolt-amperes (kVA) since generator manufacturers use both ratings. Helps you compare units across brands and avoid confusion between the two measurements.

Recommended Size Ranges

Instead of a single number, the calculator provides a recommended range that includes headroom for reliability, future load additions, and efficient operation at 70-80% of rated capacity.

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

  • Motor starting surge multipliers use industry-standard values (3× for capacitor-start, 6× for locked-rotor induction motors)
  • Running watts based on typical nameplate values for common residential and commercial appliances
  • Generator power factor assumed at 0.8 lagging for sizing calculations
  • Altitude derating not applied (sea-level performance assumed unless user adjusts)
  • Fuel consumption estimates based on manufacturer-published half-load and full-load rates
  • All loads assumed to start sequentially unless user specifies simultaneous start

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 110 — Standard for Emergency and Standby Power Systems
  • EGSA 100P — Performance Standard for Engine Driven Generators
  • NEC Article 702 — Optional Standby Systems
  • NEC Article 445 — Generators
  • IEEE Std 446 (Orange Book) — Recommended Practice for Emergency and Standby Power Systems
  • Generac, Kohler, and Cummins published sizing guides (cross-referenced for surge multiplier validation)

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.
Plan to use only 70-80% of the generator's rated capacity for continuous loads. This headroom improves fuel efficiency, reduces wear on the engine, keeps the generator cooler, and leaves room for unexpected loads. A 10,000-watt generator should carry no more than 7,000-8,000 watts of continuous load.
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.
Yes, but you need a large generator. A 3-ton central AC unit draws approximately 3,500 running watts and up to 10,500 starting watts. When you add other essential loads, you typically need at least a 14-20 kW standby generator. Most portable generators cannot handle central AC. Window units and mini-splits are more practical for portable generator backup.
Gasoline is cheapest upfront but has a limited shelf life (6 months without stabilizer), creates storage risks, and is hard to obtain during widespread outages. Propane stores indefinitely and is readily available but requires tank installation. Natural gas is the most convenient for standby generators since it flows continuously from the utility line, but it is not available in all areas. Diesel is popular for large commercial units due to fuel efficiency and longevity.
A typical 3-bedroom house needs 5,000-7,500 watts for essential loads (refrigerator, freezer, furnace blower, sump pump, lights, and a few outlets) or 15,000-22,000 watts to run everything including central AC and an electric range. Start by adding your must-have appliances using this calculator. Most homeowners find that a 7,500-watt portable generator covers essentials during an outage, while a 20-22 kW standby generator provides whole-house coverage.
Yes, but you need to size it carefully. A typical 1/2 HP well pump draws about 1,000 running watts but needs 2,000-4,000 starting watts to kick on. A 3,500-4,000 watt generator can usually start a 1/2 HP pump, but add your other loads too - the generator must handle the well pump starting surge on top of everything else that is already running. Use this calculator to add your well pump and other loads to see the total starting watts required.
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 based on typical appliance wattages and standard surge multipliers. Actual requirements vary by equipment manufacturer, age, and condition. Consult the nameplate data on your specific appliances and work with a licensed electrician for permanent standby generator installations.

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