Portable Generator Sizing: How to Choose a Portable Generator

Your running wattage and starting wattage requirements dictate whether you need a small, medium or large unit.

When electrical connections are limited or nonexistent at a jobsite, a portable generator keeps power tools, lights and appliances running. Portable generators also are handy for emergency or standby power.

How do portable generators work?

A portable generator consists of an internal combustion engine, an alternator, starter, fuel tank and external outlets. The engine is started either by pulling a recoil rope or pushing an electric start button. The engine spins the alternator and induces an electric current. The external outlets provide AC current to plugged-in devices or extension cords.

What size portable generator do I need?

Portable generators come in a variety of sizes. If you’re asking yourself “how big of a portable generator do I need?”, know that small (2.0-2.4kW) generators are perfect for light-duty jobs or use as an emergency power source. Medium generators (3.0-3.4kW or 5.0-5.4kW) are ideal for powering tools such as compressors, grinders and fusion welders.

Large generators (typically 6-9kW or more, such as this 10-11.9kW model) can power light towers and multiple tools or appliances. Large generators have more available outlet configurations.

How to size a portable generator

To determine the size generator you need, add up the running watts (rated watts) of all the tools or appliances you plan to operate from the generator at the same time. Running wattage, as the name implies, is the amount of power a device needs to run. If you’ll be using a drill and two belt sanders simultaneously, you’ll need enough watts to power all three.

Also factor in the surge watts, aka starting watts, if relevant. Many appliances and tools with a motor or compressor require much more power in the first two to three seconds. This extra power is known as starting wattage or surge wattage. Generators provide this extra wattage as a brief surge. Check the starting watts of the all the tools or appliances you’ll be using. You may have to consult the operating manual or contact the manufacturer to get this number if it’s not listed on the item.

Follow the formula below to calculate your total power requirements and the generator size you need. If you prefer, use a portable generator size calculator to help you find the tool and appliance watts and do the math.

  1. Determine the number of running watts a tool uses. If the running watt rating isn’t listed on the item, multiply the amps by the volts (W=A x V). You can find the amps and volts used by the tool or appliance on the label or in the operating manual.
  2. Choose the highest starting watts of all the tools you plan to use. Since most tools won’t start at the exact same time, include only the starting watts of the tool with the highest number.
  3. Add the running watts for all tools and the starting watts of the item with the highest number. That’s the size generator you need in watts. Note that some portable generators provide both the running watts and starting watts in the specs, but many provide only one number. If that’s the case, it’s usually the starting watts, which means the running watts will be slightly lower (roughly 10%). Err on the side of a slightly larger generator if your needs are on the high end of the range provided.
  4. Divide by 1,000 to get the size in kilowatts. Generators are sized in kilowatts. If you need 6,000 watts, that’s 6 kilowatts, so you need a 5.0-6.5kW portable generator.

One last power consideration: Make sure the generator has enough outlets to plug in all of the tools or appliances you want to operate at the same time.

Running watts and starting watts examples

Below are approximate running watts and starting watts for common power tools. Check your tool for the exact numbers.

  • Circular saw (1,400 watts/2,300 starting watts)
  • Table saw (1,800 /4,500)
  • Bench grinder (1,400 /2,500)
  • Electric drill (600 /900)
  • Hammer drill (1,000 /3,000)
  • Orbital sander (1,200 /2,600)
  • Air compressor, ½ HP (1,000 /2,000)
  • Air compressor, 1 HP (1,400 /3,600)

Other considerations when choosing a portable generator

Portable generator sizing isn’t the only variable that matters. It’s also important to consider the fuel type, noise rating and features you may need or want.

Fuel type

Portable generators on construction sites generally run on gasoline, diesel or propane. There are advantages and disadvantages to each. Consider the cost and availability of the fuel, as well as the size of the fuel tank. More frequent refills equal more downtime on the job.

  • Gasoline is readily available, but storing it in large quantities can be dangerous because it’s highly flammable. If there’s a widespread power outage, you’ll have trouble getting the fuel you need, since fuel pumps require electricity to operate. Gasoline engines aren’t as clean burning as diesel and may require more frequent servicing.
  • Diesel is less flammable than gasoline or propane and is also the most fuel-efficient and least expensive choice. Diesel generators generally run longer without maintenance than gasoline-powered generators. But like gasoline, diesel fuel may be difficult to obtain if the power goes out, and diesel generators are often louder than gas or propane models.
  • Propane is the cleanest burning but least efficient fuel. Propane generators are generally the quietest, but they may need more frequent servicing than gasoline-powered generators. Propane fuel is easier to store in large tanks, which means less refilling, but the fuel is highly flammable if the tanks are punctured.

Some portable generators are now available in dual fuel (gasoline and propane) or even tri-fuel (propane gas, natural gas and gasoline) options.

Decibel ratings

Portable generators can be very loud. Look for a unit with a low decibel rating, particularly if you’ll be using it near people. It’s especially important to choose a quiet generator if it will be used in a confined space. Some generators at the lower end of the wattage spectrum are rated as low as 57 decibels at 100% load.

Quieter than conventional generators are silenced generators, which are enclosed in an acoustic cabinet. Quieter still — but more expensive —are inverter generators, which adjust the speed of the engine to demand instead of running at a constant speed, making them more efficient. These convert AC power to DC power before digitally “inverting” it back to AC power.

Other features to look for

Features that help you get the job done faster with less effort may be worth the investment. Here are a few to consider.

  • Professional grade: Look for a unit that’s built for everyday duty. Portable generators that are ideal for a temporary power outage may not stand up to the demands of a jobsite. Thick-gauge steel construction and a large fuel tank can be indications that the generator is meant for heavy-duty use.
  • Electric start: If convenience matters, choose a generator with an electric start button. Be aware that you’ll need to keep the battery charged. And, models with an electric start may cost more than pull-cord models.
  • Wheels and handles: If you’ll be moving the generator around the site, no-flat wheels and collapsible transporting handles are useful features.
  • Tier 4 engine:If you’re planning to rent a diesel generator, look for one that meets the EPA’s Tier 4 emissions standards. These diesel generators reduce the amount of nitrous oxide, particulate matter and hydrocarbons released into the atmosphere.

Other handy features to look for include idle control, covered outlets and full-panel GFCI protection.

Visit our online marketplace to browse our selection of portable generators.

Freelance writer Mary Lou Jay writes about business and technical developments in a variety of industries. She has been covering residential and commercial construction for more than 25 years.


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