Clean Power

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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.

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.

Automatic Idle Control

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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.

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.