GenSize generator sizing

What size generator runs a furnace, refrigerator, and lights?

Pre-computed outage scenario using the same GenSize formula.

Formula

Running watts + largest startup delta, then 25% headroom.

2.2k4.5k7.5-8.5k9.5-12.5k

Use the about 4,500 W inverter class as the starting estimate for Furnace blower, 1/2 HP, Refrigerator, LED lights x10.

Running 1,600 W + largest start delta 1,400 W = 3,000 W peak. Add 25% headroom: 3,750 W.

Sizing estimate only - confirm with a licensed electrician; never backfeed a panel.

Loads used

LoadRunning WStarting WNote
Furnace blower, 1/2 HP8001600estimated 2x blower start
Refrigerator7002100estimated 3x motor start
LED lights x1010010010 bulbs at 10 W each

Models in this class

Westinghouse iGen4500

3,700 running / 4,500 starting W. gasoline.

official manual lists 3700 running watts and 4500 peak watts

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Champion 100302

3,500 running / 4,000 starting W. gasoline.

official manual lists model 100302 with 3500 running watts and 4000 starting watts

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Generac GP3600

3,600 running / 4,500 starting W. gasoline.

official Generac page lists 3600 running watts and 4500 starting watts

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Why the estimate can change

Use nameplate watts when available. Pumps, compressors, altitude, fuel type, and extension-cord voltage drop can all change the practical generator size.