Generator Size Calculator

How the size is worked out

A generator has two ratings. The running (rated) watts are what it can supply all day. The starting (peak) watts last a few seconds, enough to start a motor. Your list needs both, so the calculator makes two sums.

The first sum is the running total: each item’s quantity times its running watts. The second is the peak. Motors draw an extra burst when they start, but rarely in the same instant, so the makers say to add only the single highest starting figure to the running total. Briggs & Stratton, Champion, Generac and the Westinghouse manual all give the same instruction, and the Westinghouse example (2,410 running plus 1,600 starting is 4,010) is one of this page’s test cases.

Add the starting figure as printed, not the difference between starting and running watts. That motor’s running watts are already in the running total, so subtracting first would leave the generator too small.

Headroom and the running need

The running need is the running total plus the headroom you choose. The default is 20%, which puts the running load at about 83% of the generator’s rating, inside the band Generac gives for best performance. The headroom goes on the running watts only; the start is a short burst, so the peak stays as it is.

Every figure that tells you how big to buy is rounded up: watts to the next 10 W, kW to one decimal place. The amps beside them show the running load at 120 V and 240 V.

The three presets and everything else

The item list has three motor presets, because a motor’s start is the number most people cannot read off a label: a refrigerator or freezer, a 1/3 hp sump pump and a 1/2 hp furnace blower. Each sits at the top of the makers’ range, shown in the hint under the row; you can change either number.

Everything else is typed. Heaters, lights, TVs and most electronics print their watts on the label and have no extra start, so type the watts and leave the starting box blank. If a label gives amps, use the amps × volts boxes on a typed item.

There is no preset for central air, a well pump or a microwave: makers’ numbers for them differ by a factor of two or three.

Makers disagree on the start of a 3-ton unit: 5,000 to 6,000 W (Champion) against 15,000 to 18,000 W (Generac). The outdoor unit’s nameplate settles it with the locked-rotor amps (LRA); spec tables list 77 A for one Carrier 3-ton and 83.3 A for one Goodman. Type LRA × volts as the starting watts of a typed item for a high estimate, or ask the installer about a soft start.

Gasoline, propane and natural gas

On propane a generator delivers less than its gasoline rating, so Propane shows the gasoline rating to look for: both needs divided by the propane factor. Natural gas varies more by maker, so use your model’s natural-gas rating.

How long a tank lasts

Open “How long will a tank last?” under the headroom buttons. Runtime is the fuel on hand divided by the fuel use per hour, in the same unit: 4.7 gallons at 1.2 gallons an hour runs 3.9 hours, rounded down.

Take the fuel use from your generator’s spec sheet at the load nearest your running watts, because it rises with load. If the sheet prints hours on a full tank, type both numbers into the helper boxes.

A gallon of propane weighs 4.24 pounds, so a 20 lb cylinder holds about 4.7 gallons. Tanks fill to 80% of their size, so a 500 gallon tank holds 400 gallons. Natural gas comes through the meter, so it has no tank to run out.

Wiring it to the house

A portable generator connects to the house only through a transfer switch installed by a licensed electrician. Never back-feed a circuit through an outlet. The calculator does not size the transfer switch, cords or circuits.

What this estimate is

The result is an estimate from physics and published maker figures, with every step shown. It counts only the items you list. Altitude, heat and a worn engine lower what a generator delivers, and the page does not model them. When the manual or label disagrees, the manual wins.

Worked example

1. Inputs: 1 × Refrigerator or freezer: 700 W running, 2,200 W starting; 1 × Sump pump 1/3 hp: 800 W running, 1,600 W starting; 1 × Furnace blower 1/2 hp: 800 W running, 2,350 W starting; 1 × Lights and TV: 420 W running, no starting burst; Headroom on running watts: 20%; Fuel: gasoline.
2. Running total = 1 × 700 W + 1 × 800 W + 1 × 800 W + 1 × 420 W = 2,720 W
3. Highest single start = 2,350 W (Furnace blower 1/2 hp)
4. Peak = 2,720 W + 2,350 W = 5,070 W
5. Running need = 2,720 W × (1 + 20%) = 2,720 W × 1.20 = 3,264 W, rounded up to 3,270 W
6. Load at 120 V = 2,720 W ÷ 120 V = 22.67 A
7. Load at 240 V = 2,720 W ÷ 240 V = 11.33 A

Questions people ask

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