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Electrical

Generator Sizing Calculator

Generator size from running load and motor starting surge.

Your measurementsEvery assumption editable01

Add up the continuous draw of everything you intend to run at once.

The single biggest motor load — fridge, well pump, air conditioner.

Motors draw several times their running watts for a moment at startup.

Running a generator at its limit continuously shortens its life.

Generator sizeCalculated live02

Result

7.4kW

7400 W — starting peak 7400 W

Running load
5000 W
Largest motor
1200 W running
Starting surge3× starting multiplier
+2400 W
Peak demandeverything running, largest motor starting
7400 W
Running load with headroom+20%
6000 W
Generator size7.4 kW
7400 W

Only one motor starts at a time in the usual case, so the surge allowance applies to the largest motor rather than to all of them at once. If two large motors could realistically start together, add the second surge as well.

Generators quote both running and starting watts, and the starting figure is a brief peak rather than a capability. Size on the running figure with headroom and check the starting figure covers your surge.

Anything connected to a house's wiring needs a transfer switch or interlock. Back-feeding through an outlet energises the utility drop and can kill a lineman working on what they believe is a dead circuit.

Add up what you need to run, identify the largest motor, and this works out both the running requirement and the starting peak. Only one motor starts at a time in the usual case, which is why the surge applies to the largest rather than to all of them.

Why use this tool?What it does differently03

Why use this tool?

Surge handled correctly

Applied to the largest single motor rather than summed across everything, which is how it actually behaves.

Headroom included

Running a generator at its limit continuously shortens its life.

Transfer switch warning

Back-feeding through an outlet energises the utility drop and can kill a lineman.

Adjustable multiplier

From 2× for soft-start equipment to 5× for a hard-starting compressor.

How this worksThe method04

How this generator sizing calculator works

Peak demand is the total running load plus the additional surge the largest motor draws while starting — that motor's running watts multiplied by the starting factor, minus what it was already drawing.

The surge applies to one motor because motors rarely start simultaneously. If two large motors could realistically start together, add the second surge manually.

Separately, the running load is increased by a headroom percentage, because a generator run continuously at its rated output wears quickly and has no margin. The recommended size is whichever of the two figures is larger.

How to use itStep by step05

How to use it

  1. Step 1: Total the running watts

    Everything you intend to run at once, at its continuous draw.

  2. Step 2: Identify the largest motor

    Fridge, well pump, air conditioner — whichever has the biggest running watts.

  3. Step 3: Pick a starting multiplier

    2× for soft-start, 3× typical induction, 4–5× for compressors under load.

  4. Step 4: Install a transfer switch

    Anything connected to house wiring needs one. This is not optional.

Example usageWorked figures06

Example usage

Essential circuits
5,000 W running with a 1,200 W fridge at 3× surge gives a 7,400 W peak against 6,000 W running with headroom — a 7.5 kW generator.
Soft-start equipment
The same load with a soft-start kit at 2× drops the peak to 6,200 W, which is why those kits exist.
A hard-starting compressor
The same 5,000 W running load with a 2,500 W air conditioner at 5× surge peaks at 15,000 W — triple the running figure.
Frequently asked questionsCommon questions07

Frequently asked questions

What size generator do I need?

Total your running watts, add the starting surge of the largest motor, and allow headroom. A typical essential-circuits setup lands between 7 and 10 kW.

What is the difference between running and starting watts?

Running watts is continuous draw; starting watts is the brief peak a motor pulls at startup, often three to five times higher. Generators quote both.

Do I add starting watts for every motor?

Normally no — motors rarely start simultaneously, so the surge applies to the largest. Add a second surge only if two large motors could realistically start together.

Why do I need a transfer switch?

Back-feeding a generator through an outlet energises the utility service drop, which can kill a lineman working on what they believe is a dead circuit. A transfer switch or interlock prevents it.

Can I run a generator at full capacity?

Not continuously. Sustained operation at rated output shortens engine life and leaves no margin for an unexpected load, which is why headroom is built into the result.

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