ParrotNotes

Generator Wattage Calculator

Add up the running and starting watts of the appliances you want to power to size a portable or home standby generator, with a margin. Enter how many of each appliance must run at the same time; the running and extra starting watts are filled in from a manufacturer's chart and you can change any of them. You get the running and starting watts the generator needs, in watts and kW. An estimate from typical figures: the appliance nameplates, the generator's instructions and a licensed electrician decide.

Your numbers

%

Added to both totals so the generator is not run at its limit. 20% is common.

Around the house

Refrigerator or freezer

700 W running, +2,200 W to start

Sump pump, 1/2 HP

1,050 W running, +2,150 W to start

Well pump, 1/2 HP

1,000 W running, +2,100 W to start, often 240 V

Furnace fan blower, 1/2 HP

875 W running, +2,300 W to start

Window AC, 12,000 BTU

3,250 W running, +3,950 W to start

Central AC, 24,000 BTU

3,800 W running, +4,950 W to start, often 240 V

Heat pump

4,700 W running, +4,500 W to start, often 240 V

Space heater

1,800 W running

Microwave, 1,000 W

1,000 W running

Computer with 17 in monitor

800 W running

Television, 27 in

500 W running

Radio

100 W running

Phone charger

25 W running

On the jobsite

Air compressor, 1 HP

1,600 W running, +4,500 W to start

Air compressor, 1/2 HP

1,000 W running, +2,000 W to start

Circular saw, 7-1/4 in

1,400 W running, +2,300 W to start

Miter saw, 10 in

1,800 W running, +1,800 W to start

Table saw, 10 in

2,000 W running, +2,000 W to start

Hand drill, 1/2 in

600 W running, +900 W to start

Halogen work light

1,000 W running

Reciprocating saw

960 W running

RV

RV air conditioner, 13,500 BTU

1,500 W running, +700 W to start

Your own loads

Your load 1

Running and extra starting watts from the nameplate

W
W
Your load 2

Running and extra starting watts from the nameplate

W
W
Your load 3

Running and extra starting watts from the nameplate

W
W

Extra starting watts are on top of the running watts. If a nameplate gives only volts and amps, multiply them: 5 A × 120 V = 600 W.

Generator running watts needed

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Starting (surge) watts needed
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Total running watts
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Largest extra starting load
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Running plus largest start
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Margin added
20%
Size class
–

An estimate from typical figures: the appliance nameplates, the generator's instructions and a licensed electrician decide.

Connecting a generator to house wiring needs a transfer switch or other listed transfer equipment (NEC 702.5), installed by a licensed electrician. Never plug a generator into a wall outlet.

The calculation runs in your browser and ParrotNotes does not store your numbers. They sit in the page address so you can bookmark or share the result.

How the generator size is calculated

This is the worksheet method generator makers print in their brochures. Add the running watts of everything that will run at the same time, multiplied by how many of each. Find the single largest additional starting load and add it once: motors are started one at a time, so only one surge has to be covered at the moment it happens. Then add a margin, 20% unless you change it, so the generator is not run flat out.

Worked example: a refrigerator 700 W, a 1/2 HP sump pump 1,050 W, a furnace fan blower 875 W, a microwave 1,000 W and a TV 500 W. Running total 4,125 W. The largest extra starting load is the furnace fan at 2,300 W, so 4,125 + 2,300 = 6,425 W at the moment it starts. With a 20% margin, look for at least 4,950 running watts and 7,710 starting watts.

The default figures come from a manufacturer's chart and are approximate. Every one is editable: check the nameplate of your own appliance. If it lists only volts and amps, multiply them: 5 A at 120 V is 600 W (energy.gov).

120 V and 240 V loads

Most plug-in appliances run on 120 V. Central air conditioners, heat pumps, many well pumps, electric ranges, dryers and water heaters run on 240 V, so the generator needs a 120/240 V output and they have to be connected through a transfer switch, not an extension cord.

A 120/240 V generator splits its output across two 120 V halves. Spread large 120 V loads across both halves so neither is overloaded while the other is light.

Portable or home standby

A portable generator powers a short list of essentials through extension cords or a transfer switch. The calculator shows the portable range up to 10,000 running watts, the largest portable in the manufacturer brochure it cites. Above that, or when you want the house to switch over by itself, a permanently installed home standby generator with an automatic transfer switch is the usual choice.

A standby system is an optional standby system under NEC Article 702. It needs transfer equipment (702.5), and a licensed electrician sizes it with a load calculation under Article 220 and the local permit, not from an appliance list. Use the number here to start that conversation.

For electricians, HVAC and home-improvement sales on an in-home visit

The load list comes from the homeowner, out loud and in pieces: the fridge and the chest freezer, the sump pump in the basement, the well, a window unit for the bedroom, and the CPAP. Half of it gets remembered later, after the quote.

Tick the loads here as they are named, show the customer the running and starting totals, and quote the generator and transfer switch for the list you agreed on.

An estimate from typical figures: the appliance nameplates, the generator's instructions and a licensed electrician decide. Run a portable generator outdoors, at least 20 feet from windows, doors and attached garages, and never plug it into a wall outlet.

Sources, read on 2026-10-24:

Want the running cost? The electricity cost calculator turns watts into dollars, and the wire size calculator sizes the circuit to a transfer switch or inlet.

More for home-services and electrical trades: Home improvement sales, HVAC sales, Solar sales, Electrical distributor sales.

The load list gets said in the kitchen. Keep it.

On an in-home visit the homeowner names what has to stay on: the fridge, the sump pump, the well, a window unit for the bedroom. Record the visit in ParrotNotes and get a structured note, a summary and the action items, so the quote sizes the generator for the loads they listed. Free for 100 minutes of recording a month.

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Generator wattage calculator FAQ