Calculating Generator Output Main Image

Calculating the generator's output is important for deciding what size generator is right for you. Doing this is very simple and will save you some headaches in the long run.

Watts = Volts x Amps

Generators can only put out a finite amount of power. Companies use watts to rate a generator's output. The wattage is calculated by multiplying the voltage by the electrical device's load capacity in amperage (Watts = Volts x Amps). For example, a generator may be listed as 1,500 watts delivering 120 volts.

Amps = Watts / Volts

Now you can find the amperage that it can output at 120 volts by dividing the watts by the volts (Amps = Watts / Volts). So a 1,500 watt generator delivering 120 volts can output 12.5 amps.

Dual voltage

Some generators are dual voltage and also output 240 volts. Find the amps available at the higher voltage. Now the 1,500 watt generator is delivering 6.25 amps at 240 volts. Just to note, some generators are not be able to deliver 120V and 240V at the same time, so check the specs.

What are you powering?

Whether it's a few things around the house or your camping equipment -- the total load from the devices that you are powering cannot exceed the generator's output. Take a look at the electrical spec label or owners manual for the devices that you want to power with the portable generator. Then add the watts up in order to figure out what portable generator that you need. Generators are usually listed with a constant/continuous load specification, as well. This is the amount of power that the generator can safely put out for an extended period of time. Some devices also require a large amount of starting wattage compared to their running wattage. For example, a washing machine may require 750W while running, but 2,300W while starting. You may want to differentiate what will be constantly running as well as the maximum amount of power that you'll need.

How many watts?

Air Compressors, 1/2 HP1,500 - 3,000W
Circular Saw, 7-1/4"1,000 - 2,500W
Electric Chainsaw, 14"800 - 1,500W
Electric Drill, 1/4" & 3/8"300 - 600W
Electric Drill, 1/2"350 - 1,200W
Grinders, 6"1,000 - 2,600W
Jig Saw200 - 800W
Paint Sprayer800 - 1,300W
Portable Oil Heater900 - 1,000W
Router900 - 1,000W
Sander, 4" Belt700 - 1,500W
Soldering Iron100 - 300W
10 Amp Battery Charger300 - 400W
Electric Motors*Required Wattage
1/6 HP, 460 Watt340 - 850W
1/4 HP, 725 Watt450 - 1,050W
1/3 HP, 800 Watt560 - 1,300W
1/2 HP, 970 Watt760 - 1,800W
3/4 HP, 1,340 Watt1,080 - 2,600W
1 HP, 1,700 Watt1,250 - 3,000W
1-1/2 HP, 2,300 Watt1,600 - 4,200W
Household UsesRequired Wattage
Air Conditioner, 10,000 BTU2,000 - 3,000W
Coffee Pot1,000 - 1,500W
Electric Heater1,000 - 2,000W
Electric Stove (one element)750 - 1,800W
Gas Furnace300 - 1,500W
Hair Dryer800 - 1,500W
Iron1,000 - 1,500W
Microwave500 - 1,500W
Oil Furnace400 - 2,000W
Radio30 - 100W
Refrigerator / Freezer600 - 2,500W
Sump Pump800 - 3,000W
Television100 - 350W
Toaster1,100 - 1,700W
Water Pump1,000 - 3,000W

*Electric motors require at least three times more wattage when first starting than when running.

More Resources

You can get more electrical device load calculations here.

When choosing a generator, it's a good idea to oversize it. If your load is going to be 1,500W then it may be best to look a generator that can deliver 2,500W.

WARNING: NEVER back feed your home's electrical system with an extension cord and a portable generator. This may cause severe damage to all electrical appliances and it creates an electrical hazard to all persons.