In a perfect world there would be temporary power where and when you needed it, but in reality, many contractors must rely on portable generators some or all of the time. As a framing contractor, I've had to use them myself, so I understand how important it is to get a dependable machine that can provide the required power. In this article I will explain how to size a portable generator and what to look for in terms of features.

To make it easier for you to find an appropriate generator, I have included a list of gasoline models (the most common type) with built-in GFCI protection (to ensure OSHA compliance) and 3,500 to 7,000 watts of rated output (see specification chart on page 3). The manufacturers and equipment-rental folks I interviewed said the typical construction crew needs between 4,000 and 6,000 watts of rated power. I broadened that range, because it seems overly restrictive and I've often seen units smaller and larger than that on job sites.

Output


Two of the more important specifications for a generator are rated and maximum output. Rated output is easy to understand: It's the number of watts the machine can produce for an indefinite period of time. As one manufacturer put it, the generator can produce that amount of power as long as there is fuel in the tank and oil in the crankcase.

Maximum output is more complicated, because with no industry-wide standard for what, exactly, this term refers to, manufacturers are free to measure it however they like. Most treat it as the highest number of watts the generator can produce, even if it can only produce that wattage for a fraction of a second. The length of time that the generator can sustain this output is up to the manufacturer; it could be anywhere from less than 1 second to 10 seconds or more – long enough to provide the surge of power it takes to start a tool.

A few manufacturers carry an intermediate number that falls between rated and surge. For example, GMC does not test its machines for surge; for this company, maximum output is the highest output the generator can sustain for two hours. Honda carries three numbers: rated, maximum, and "iAVR." Maximum is what the machine can sustain for at least 30 minutes, and iAVR is akin to what others call surge or starting output.

When I refer to output in this story, I will use the terms maximum, surge, and starting interchangeably, because that's what most manufacturers do.

Sizing a Generator


The trickiest part of buying a generator is deciding how much capacity you need. If you undersize the unit, it will strain to meet demand – and both it and your tools will work harder and wear out more quickly. In extreme cases, the panel breaker will trip and the generator will stop delivering electricity.

An oversize generator will work just fine – but it costs more, consumes more fuel, and is harder to haul around.

Types of loads. The first step in sizing a generator is determining which tools you will be using together and how much power they require. Resistive loads like lights and electric heaters need the amount indicated on their labels: A 100-watt bulb uses 100 watts and a 1,000-watt heater uses 1,000 watts. It's not so simple with power tools. They use more power at startup than they do when running, and you'll need to get both numbers.

The starting and running wattages of common tools can be found in the generator manufacturers' sizing guides. If the guide does not contain data for a tool you plan to use, you'll have to get it from the tool's manufacturer or make an estimate based on the amp rating on the tool's nameplate. Since generators are rated in watts, you'll need to convert the amp rating to watts. The conversion formula is volts x amps = watts, so if a tool draws 15 amps of current, it will require 1,800 watts (120 volts x 15 amps) of power.

Most handheld and benchtop tools have universal motors, which use about twice as much power at startup as they do when running. If you can't find the running amps for a tool that has a universal motor, it's safe to use the nameplate amp rating instead. Bear in mind that this will overstate the running amps, because when a tool is running but not under load it will draw somewhat less current than the nameplate amount.

If the machine has an induction motor, you will need to find the amp rating and motor code for that device. The amp rating will be on the nameplate; if the motor code is not there too, you'll have to get it from the manufacturer. A contractor-style table saw or brick saw might have a Code G motor, meaning that it starts with no load and load is gradually applied once it comes to full speed. This type of motor will draw about 3-1/2 times its rated power at startup. Tools that start under load – such as air compressors – typically have Code L or M motors. These motors draw about six times their rated power at startup.

Load management. Generator manufacturers assume that you'll be smart enough not to start all of your tools at once, and most recommend buying a machine capable of providing the number of watts it takes to start your largest tool while your other tools are running, plus 10 percent (as a safety factor). This means carrying starting watts for your largest tool and running watts for the rest.

In most cases, the biggest tool will be a table saw, miter saw, demo hammer, or compressor. A compressor may require so much power at startup that it's impractical to run it off a generator. For this reason, contractors who regularly run big compressors will buy gas-powered models so that their generators need to power only smaller tools.

The adjoining chart shows how to size a generator for a framing crew that is using a circular saw, a 1/2-inch drill, and a miter saw:

You'll need 5,830 watts for the brief period of time that your miter saw is starting while your drill and circular saw are running. Most of the time you'll need less – 4,180 watts when all three tools are up and running. Based on these assumptions, you should buy a generator with a rated output of about 4,180 watts and a maximum (surge) output of about 5,830 watts.

The high-end generator shown at top has full-panel GFCI protection, a voltage selector, and auto throttle (idle) control. The voltage selector allows the user to maximize power to the 120-volt receptacles by switching off the 240-volt circuit. Idle control cuts fuel consumption and noise by reducing the speed of the motor when there is no demand for power. On the modestly equipped machine above, the duplex receptacles are GFCI-protected but the twist-lock receptacles are not; an hour meter allows the owner to track usage so that he knows when it's time to change oil.
The high-end generator shown at top has full-panel GFCI protection, a voltage selector, and auto throttle (idle) control. The voltage selector allows the user to maximize power to the 120-volt receptacles by switching off the 240-volt circuit. Idle control cuts fuel consumption and noise by reducing the speed of the motor when there is no demand for power. On the modestly equipped machine above, the duplex receptacles are GFCI-protected but the twist-lock receptacles are not; an hour meter allows the owner to track usage so that he knows when it's time to change oil.

These are conservative numbers that reflect the fact that manufacturers want you to have enough capacity.

I would personally have no problem omitting the recommended 10 percent safety factor – and even downsizing a bit more – because it's very unlikely that every other tool will be running during that fraction of a second when the biggest tool starts.

Clean Power


If your list of job-site necessities includes a smart phone, a computer, or any other equipment (including power tools) that contains microprocessors, you need to understand the term "total harmonic distortion" (THD). Harmonic distortion is an inconsistency in the wave form of electrical voltage and current. Power with a high level of THD is "dirty," and power with a low level of THD is "clean."

A decade ago, the THD from any given generator might be in the 30 percent to 40 percent range. Nowadays the best generators produce electricity with 5 percent to 10 percent THD, which is comparable to regular household current.

Although harmonic distortion is not a concern when powering resistive loads or motors, it can wreak havoc on microprocessors. Charging the battery of your smart phone from a dirty power source can shorten the battery's useful life or even damage the embedded firmware. Powering your laptop or computerized testing equipment with high-THD power can do permanent damage to the microprocessors in those devices as well.

If you think you might need to use your generator to power a home during a power outage, keep in mind that harmonic distortion can damage microprocessor-controlled thermostats and household appliances. The standby generators designed to power homes produce power with 10 percent or less THD.

Automatic Voltage Regulator


Many generators contain an automatic voltage regulator (AVR) to help the machine maintain consistent voltage output under varying loads. AVR is designed to prevent voltage spikes and brownouts. In some cases it incorporates a capacitor that can release a burst of stored power during brief moments of particularly high demand.

GFCI Protection


Ground fault circuit interrupters (GFCIs) are expensive, and some manufacturers skip them altogether in order to produce a less expensive generator. However, OSHA requires GFCI protection for job-site power sources, so you'll have to provide it somehow. While it's possible to use a GFCI-protected spider box, I think relying on an outside device is a poor idea because someone on the crew might plug in without using it. You could buy a whole lot of generators for the cost of an injury, or of defending yourself at an OSHA hearing.

Check to see what kind of protection the generator has; on many models, only the 120-volt duplex receptacles are protected. The best machines have full panel protection so that every receptacle is protected (including 120- and 240-volt twist-lock receptacles).

Runtime


Another quality to consider is how long the unit will run on a tank of fuel. Runtimes are typically based on a generator running at half capacity. How many hours you get per fill-up depends on the size of the gas tank and how hard you run the machine. If the tank runs dry in the middle of the workday, you lose valuable time while everyone waits for you to refuel.

Small generators (top) have recoil start – you pull a cord to start them. Some larger models have battery-powered electric starters, which are backed up by recoil in case the battery goes dead. Most electric-start generators rely on sealed lead acid batteries (above), but DeWalt uses its own 18-volt XRP cordless tool batteries (left). When the battery dies you can pop it out and snap in a spare.
Small generators (top) have recoil start – you pull a cord to start them. Some larger models have battery-powered electric starters, which are backed up by recoil in case the battery goes dead. Most electric-start generators rely on sealed lead acid batteries (above), but DeWalt uses its own 18-volt XRP cordless tool batteries (left). When the battery dies you can pop it out and snap in a spare.

Automatic Idle Control


For a generator to produce power at the correct voltage and hertz, it has to run at 3,600 rpm. But there is no point in running it that fast when you don't have to, so spend a little extra to get a model with automatic idle control. Auto idle control lowers the motor's rpm when there is no current draw, which reduces noise, fuel consumption, and wear and tear on the machine. When there is a demand for power, the motor cranks back up to 3,600 rpm.

Receptacles


Most pro-grade generators have one 240-volt twist receptacle, four 120-volt duplex receptacles, and one or two 120-volt twist receptacles. Some have voltage selector switches, which allow you to turn off the 240-volt receptacle when you are not using it so that the total generator output is available to the 120-volt receptacles.

Recoil or Electric Start


While most portable generators in the 3,500-watt to 7,000-watt range use a standard recoil (pull) starter, a few come with electric starters. For the most part, the experts I spoke with felt that electric starters were just one more thing that could break or cause problems – with one exception. On units with larger engines, my sources said, powered starters can be handy, because the greater the engine displacement, the harder you must pull to turn it over. Most electric-start generators have a recoil backup so that you can still start them if the battery is dead.

Decibel Rating


Unless you wear hearing protection, continued exposure to anything over 85 decibels can cause permanent hearing damage. If you're concerned about your hearing (and you should be), look for a unit with a decibel rating of 75 or less. This number is typically measured from 7 yards away while the generator is running at 50 percent load.

Portability


In my opinion, there are two kinds of portable generators: ones that include wheel kits and ones that come with potential back injuries. A 6,000-watt generator can weigh more than 250 pounds. Personally I wouldn't consider buying a unit that didn't roll.

Tires can be solid, semi-solid, or pneumatic. Pneumatic tires have the best bounce and will get you over bumps with the least effort, but they can be punctured and go flat. Solid tires are good on smooth surfaces only. Semisolid wheels are a reasonable compromise; they have some bounce and they won't go flat.

If you plan to hoist the generator with a boom or forklift, look for a model with a lifting hook.

Wheel kits are worth the extra expense because rolling a generator is a lot easier than carrying it. Four wheels are fine for use on smooth, flat surfaces, but two wheels (right) are better on rough terrain. The four-wheel model at left has folding handles that make it possible to lift one end of the unit and roll it like a wheelbarrow.
Wheel kits are worth the extra expense because rolling a generator is a lot easier than carrying it. Four wheels are fine for use on smooth, flat surfaces, but two wheels (right) are better on rough terrain. The four-wheel model at left has folding handles that make it possible to lift one end of the unit and roll it like a wheelbarrow.

Durability and Maintenance


If something can break, it will – so look for a generator with a beefy wraparound metal frame. Some of the better units could probably fall off the back of a truck without being damaged. Others have minimal frames that provide less protection. You can tell by looking which manufacturers are the most serious about protecting the machine.

I'd recommend getting a metal gas tank. Plastic tanks are cheaper, but they can become brittle in extreme cold. If I'm choosing between comparable models and only one has a steel tank, I'll choose the steel tank every time.

A heavy metal roll cage and shielding protect this generator's engine, alternator, and control panel from the beating they'll receive on the job site. The piece across the top is a lifting hook.
A heavy metal roll cage and shielding protect this generator's engine, alternator, and control panel from the beating they'll receive on the job site. The piece across the top is a lifting hook.

To make it easier to keep track of scheduled maintenance, some generators are equipped with hour meters.

If the unit does not have this feature and you track your crew's hours, figure the machine runs about 80 percent of the time your crew is working. If you have to change oil every 100 hours and you're working 40-hour weeks, you'd probably be safe changing the oil every three weeks. Should you ever lose track, dump it out. A few quarts of oil are a whole lot cheaper than a new engine.

California Certified


If you work in California, your choices will be limited to models that have been emissions-certified by the California Air Resources Board (CARB). These units tend to be more expensive.

Contributing editor Michael Davis owns Framing Square in Conifer, Colo.

Interactive Specification Charts


Below are two versions of our portable generator specifications. Chart 1 uses Scribd and allows for "zoomable" viewing of a PDF version. Chart 2 uses Google Spreadsheet technology, allowing you click links and to scroll up, down, left, and right through the data.

Scribd Chart


Google Spreadsheet


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