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.
Generator 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.
Generator 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 HP||1,500 – 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 Saw||200 – 800W|
|Paint Sprayer||800 – 1,300W|
|Portable Oil Heater||900 – 1,000W|
|Router||900 – 1,000W|
|Sander, 4″ Belt||700 – 1,500W|
|Soldering Iron||100 – 300W|
|10 Amp Battery Charger||300 – 400W|
|Electric Motors*||Required Wattage|
|1/6 HP, 460 Watt||340 – 850W|
|1/4 HP, 725 Watt||450 – 1,050W|
|1/3 HP, 800 Watt||560 – 1,300W|
|1/2 HP, 970 Watt||760 – 1,800W|
|3/4 HP, 1,340 Watt||1,080 – 2,600W|
|1 HP, 1,700 Watt||1,250 – 3,000W|
|1-1/2 HP, 2,300 Watt||1,600 – 4,200W|
|Household Uses||Required Wattage|
|Air Conditioner, 10,000 BTU||2,000 – 3,000W|
|Coffee Pot||1,000 – 1,500W|
|Electric Heater||1,000 – 2,000W|
|Electric Stove (one element)||750 – 1,800W|
|Gas Furnace||300 – 1,500W|
|Hair Dryer||800 – 1,500W|
|Iron||1,000 – 1,500W|
|Microwave||500 – 1,500W|
|Oil Furnace||400 – 2,000W|
|Radio||30 – 100W|
|Refrigerator / Freezer||600 – 2,500W|
|Sump Pump||800 – 3,000W|
|Television||100 – 350W|
|Toaster||1,100 – 1,700W|
|Water Pump||1,000 – 3,000W|
*Electric motors require at least three times more wattage when first starting than when running.
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.