Starlight Tools

Generator Size Calculator: kW, kVA, Running Watts, and Starting Watts

Estimate what size generator you need from running watts, starting watts, power factor, voltage, phase, headroom, and site derating. Use the manual inputs or build an appliance and equipment worksheet for home outage essentials, portable generators, standby systems, jobsite tools, server rooms, and small commercial loads.

Outputs running watts, required starting watts, kW, kVA, amps, derating, and a common generator size class. Private by design.

Inputs

Optional appliance load worksheet

Item Qty Running W Starting W Starts together Remove

Worksheet rows are optional. If rows are present, they replace the manual running and starting watt inputs.

Advanced sizing controls

Results

Total running watts—
Largest added starting load—
Required starting watts—
Recommended rated running watts—
Recommended surge watts—
Minimum calculated kVA—
Estimated running amps—
Estimated starting amps—
Derating and reserve—
Enter loads to calculate a generator size.
    Generators are marketed with running and surge ratings. Continuous running capacity is the sustained limit; surge capacity is temporary and should only cover startup events.

    Methodology and assumptions

    This calculator treats running watts as sustained real power and starting watts as the temporary peak needed when motors, compressors, pumps, and transformers start. The worksheet sums all running loads and adds the largest simultaneous starting increment: starting increment = starting watts - running watts. Rows marked "starts together" are included in the simultaneous starting event.

    Apparent power is estimated with kVA = kW / PF. Current uses the selected voltage and phase: single-phase amps = watts / volts, and three-phase amps = watts / (sqrt(3) x volts x PF). Site derating is estimated from fuel type, altitude, ambient temperature, duty rating, and additional reserve. Vendor data, nameplates, locked-rotor values, transfer-switch rules, fuel supply, and local electrical codes should confirm final selection.

    Last reviewed: June 29, 2026. The calculator runs locally in your browser.

    Formula

    Total running watts: sum(quantity x running watts)

    Required starting watts: running watts + largest simultaneous starting increment

    Recommended running watts: running watts x (1 + headroom + reserve + derating)

    Recommended surge watts: starting watts x (1 + reserve + derating)

    Generator kVA: recommended running kW / PF

    Practical sizing tables

    TopicTypical rangeUse in sizing
    Power factor: resistive heat and incandescent lighting0.95 to 1.00kVA stays close to kW.
    Power factor: motors, pumps, compressors0.75 to 0.90Use the nameplate PF when available; otherwise size conservatively.
    Power factor: UPS, server racks, non-linear electronics0.80 to 0.95Check harmonic and UPS generator compatibility, not only watts.
    Motor starting multipliers2x to 6x running wattsUse actual locked-rotor or starting watts when available.
    Headroom10%, 20%, or 30%+Use 10% for known steady loads, 20% for mixed loads, and 30%+ for motors, growth, heat, or critical systems.
    Common appliance rangesLights 300-1,500 W; refrigerator 600-800 running / 1,800-2,200 starting; sump pump 800-1,200 / 2,000-4,000; central AC 3,500-6,000 / 8,000-15,000Use nameplate data where possible; presets are planning values.

    Worked examples

    Small home outage essentials

    3,600 running W, 5,600 starting W, 20% headroom, PF 0.90.

    Recommended: 4.3 kW running, 5.6 kW surge, 4.8 kVA. Shop around a 5 to 6 kW portable generator.

    Portable jobsite tools

    5,000 running W, 8,000 starting W, 20% headroom, PF 0.85.

    Recommended: 6.0 kW running, 8.0 kW surge, 7.1 kVA. Shop around an 8 to 10 kW class generator.

    Whole-home standby

    14,000 running W, 22,000 starting W, 25% headroom, PF 0.90.

    Recommended: 17.5 kW running, 22.0 kW surge, 19.4 kVA. Shop around a 22 to 24 kW standby generator.

    Server and network closet

    4,800 running W, 5,200 starting W, 30% headroom, PF 0.95.

    Recommended: 6.2 kW running, 5.2 kW surge, 6.6 kVA. Shop around a 7.5 to 8 kW class generator and verify UPS compatibility.

    Three-phase pump or HVAC load

    25,000 running W, 48,000 starting W, 20% headroom, PF 0.88.

    Recommended: 30.0 kW running, 48.0 kW surge, 34.1 kVA. Shop around a 50 kVA class if the starting event governs.

    FAQs

    What size generator do I need for a refrigerator and furnace?

    Add the refrigerator and furnace blower running watts, then add the largest starting increment that may occur while other loads are already running. Many essential-load setups fall near 3 to 5 kW, but pumps, microwaves, and freezers can push the required surge higher.

    Should I size by running watts or starting watts?

    Use both. The running rating must cover sustained demand. The surge rating must cover short startup demand from motors and compressors.

    How much headroom should I add?

    Use 10% for measured, stable loads; 20% for mixed home, office, or jobsite loads; and 30% or more for future expansion, hard motor starts, hot sites, or critical uptime.

    Why is my kVA higher than kW?

    kVA includes power factor effects. If PF is 0.80, a 20 kW load needs about 25 kVA before headroom and derating.

    Does altitude reduce generator output?

    Yes. Engine output generally drops at higher altitude and hot ambient temperature. The advanced controls estimate this allowance, but vendor derating charts should govern final selection.

    Can one generator run central AC?

    Often, but central AC compressor starting demand can be much higher than running watts. Use the unit nameplate, locked-rotor data, or a soft-start specification before buying.

    What is the difference between standby, prime, and continuous ratings?

    Standby ratings are for emergency backup, prime ratings support variable longer-duration use, and continuous ratings are for sustained constant loads. Continuous and prime selections usually need more conservative sizing.

    When should I ask an electrician or generator vendor?

    Ask for help with transfer switches, whole-home standby systems, three-phase equipment, large motors, medical or life-safety loads, parallel systems, fuel supply, and code compliance.

    Is this private?

    Yes. All calculations run locally in your browser.

    Disclaimer

    Generator sizing is a planning estimate. Confirm with vendor data, site conditions, nameplate values, and local electrical codes before purchase or installation.

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