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Generated on: July 20, 2026 at 09:33 PM

Generator Sizing Calculator

Inputs

Total running watts3,000 W
Largest surge watts2,000 W
Safety margin20 %

Estimates assume your appliance wattages are accurate and that only one large motor surges at a time. Hard-wired equipment, three-phase loads, and whole-home transfer-switch installations should be sized by a licensed electrician. Always follow the manufacturer’s ratings and local electrical code.

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Lifestyle

Generator Sizing Calculator

Size a portable or standby generator from your appliance load. Enter total running watts, the largest motor surge, and a safety margin to get the running and starting watts you need.

Inputs

Load & Margin

%
0 – 100 %

Results

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Generator Size

Details

A load of 3,000 W needs a generator rated about ... running, with at least ... of starting (surge) capacity to absorb the largest motor kicking in.

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Why Generator Sizing Matters

An undersized generator stalls, overheats, or trips the moment a fridge compressor kicks in; an oversized one wastes fuel and money idling far below its rating. The goal is to match the machine to two different numbers — the steady load you run continuously, and the brief spike when the biggest motor starts.

A typical outage kit — a fridge, a few lights, a sump pump, and a phone charger — draws roughly 1,300 watts running, but the sump pump alone can spike past 2,000 watts for a fraction of a second as its motor starts. Size only for the running figure and the generator chokes on that spike.

Running Watts vs. Starting Watts

Every motor-driven or compressor appliance has two power figures:

  • Running (rated) watts — the continuous draw once it is up to speed.
  • Starting (surge) watts — the momentary spike, often 2–3× the running figure, as the motor overcomes inertia.

Resistive loads — heaters, kettles, toasters, incandescent bulbs — have no surge: their running and starting watts are the same. Only one large motor usually starts at any given instant, so you size the surge requirement around the single biggest starter, not the sum of all of them.

The Formula

Running rating=Total running watts×(1+margin)\text{Running rating} = \text{Total running watts} \times (1 + \text{margin}) Starting rating=Total running watts+Largest surge watts\text{Starting rating} = \text{Total running watts} + \text{Largest surge watts}

The safety margin keeps the generator off its ceiling. Running a unit continuously above roughly 80 % of its rating runs it hot and leaves no room for an extra device, so a 20 % margin is a sensible default.

Worked Example

  • Total running watts: 3,000 W
  • Largest surge watts: 2,000 W (the sump pump)
  • Safety margin: 20 %
Running rating=3,000×1.20=3,600 W\text{Running rating} = 3{,}000 \times 1.20 = 3{,}600 \text{ W} Starting rating=3,000+2,000=5,000 W\text{Starting rating} = 3{,}000 + 2{,}000 = 5{,}000 \text{ W}

Look for a generator that provides at least 3,600 running watts and 5,000 starting watts. Many portable units are marketed by their surge figure (e.g. "5000 W peak / 4000 W running"), so always confirm both numbers on the spec sheet.

How to Total Your Load

  1. List every appliance you intend to run during an outage.
  2. Read the running watts from each nameplate. If only volts and amps are shown, multiply them — a 120 V, 6 A device is about 720 W.
  3. Add the running watts together for your total running load.
  4. Identify the single appliance with the largest startup surge and note its surge watts.

Build in a little slack: people almost always plug in more than they planned. A modest margin now is cheaper than a second trip to the store.

Common Loads at a Glance

ApplianceRunning wattsTypical surge
LED lights (per room)60–120none
Refrigerator/freezer150–400800–1,200
Sump pump (1/2 hp)1,0502,150
Window air conditioner1,000–1,5002,200–3,200
Microwave1,000–1,500none
Furnace blower (1/2 hp)8001,600

Figures vary by model; always size from your own appliances’ labels rather than a table.

Frequently Asked Questions (FAQ)

What is the difference between running and starting watts?

Running (or rated) watts is the continuous power an appliance draws once it is up and going. Starting (or surge) watts is the brief, larger spike a motor pulls in the first fraction of a second as it overcomes inertia. A generator lists both: its running rating must cover your continuous load, and its surge rating must cover the largest startup spike.

Where do I find each appliance’s wattage?

Look at the nameplate or label, usually near the power cord or on the back. If only volts and amps are shown, multiply them: watts ≈ volts × amps (e.g. 120 V × 6 A = 720 W). Resistive loads (heaters, kettles, incandescent bulbs) have no surge; motor-driven and compressor appliances do.

Why add a safety margin?

Running a generator at 100 % of its rating continuously runs it hot, wastes fuel, and leaves no room to plug in one more thing. Sizing for about 80 % load — a 20 % margin — keeps it in its efficient range and extends engine life. Raise the margin if you expect to add appliances later.

Does this work for inverter generators too?

Yes. The watt arithmetic is identical for inverter and conventional generators. Inverter models produce cleaner power (lower harmonic distortion), which matters for sensitive electronics, but you still size them by the same running and starting watts.

Disclaimer

Estimates assume your appliance wattages are accurate and that only one large motor surges at a time. Hard-wired equipment, three-phase loads, and whole-home transfer-switch installations should be sized by a licensed electrician. Always follow the manufacturer’s ratings and local electrical code.

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