Generator users want dependability and power. The generator is
the only power source out there, and it needs to work or the
job comes to a halt. Often, workers beat up on this critical
tool like it owes them a gambling debt. Then they expect the
machine to start on one pull and provide gobs of power for circ
and recip saws, miter boxes, even compressors. The good news is
that they can usually get away with that treatment for a long
time because many of today's generators are tough enough to
This article will help you determine the right-sized generator
for your needs -- and one you can afford -- with features
that'll last through the job-site demolition derby.
Each job site is different, of course, but any crew running
off mobile power needs enough electricity to stay moving. You
can probably get away with a 3,500-watt machine on many sites,
but most generator manufacturers recommend 5,000 to 6,000 watts
to cover just about every power need -- even starting
compressors -- on a residential job site.
One generator-sizing method suggests that you list all the
tools you use on site simultaneously (for example, three circ
saws, a miter saw, and a recip for your framers, or two miter
saws and a 2-horse compressor for your trimmers). Then multiply
the tools' combined amps by 120 volts:
amps x volts = watts needed
That equation, theoretically, tells you how many watts your
generator should have.
The problem with that method is that once you've added up all
your tools and battery chargers, plus the radio, you usually
come to a number that exceeds 5,000 or 6,000 watts. The reality
is that to start, electric motors need about triple their amp
rating for a split second. But once the tool is running, a
motor draws about half that number. For instance, a circ saw
that's rated for 15 amps needs about 45 amps to start. That
same saw needs only 8 amps to run unless somebody binds the
blade. So the more realistic equation is:
amps x volts x .75 = watts needed
Surge capacity and sustained watts. Since electric
motors require about triple their amp rating to start, the
generator engine cranks way up for about a third of a second to
double or triple its rated watts. This is called surge
capacity. This statistic, some manufacturers say, is a tricky
number to define, and many don't even publish it. Most
generators have sufficient surge capacity to start your tools.
The number that matters is that of sustained watts. Sustained
watts measures what the generator can produce
Generator manufacturers suggest 4,500 to 6,000 sustained watts
for residential job sites. Using an underpowered machine can
starve your power tools for current, which means they'll start
slowly and generate unnecessary heat. That heat can slowly cook
the windings in your electric tools, shortening their useful
life, according to generator makers.
While it's easier to take a smaller 3,500-watt machine out of
the truck every day, and it may feel like it's got plenty of
power, manufacturers say that it may be damaging your tools.
Moving up to a 5,000-watt machine can keep your tools -- and
your generator -- alive longer.
A generator's performance is difficult to measure and quantify
on site. Unlike a miter saw, whose positive adjustments and
excess vibration you can feel and whose accuracy you can
measure, your generator produces invisible current. But even
it's probably 50 to 100 feet away when you're using it and
you're used to its background noise, there are things you can
see -- and hear -- that make a difference in performance.
Control panel. Better generators have a control panel
that's protected from impact yet easy to access. Having enough
places to plug in is important, too. Dividing the current from
your generator through multiple circuits and multiple cords
saves wear on both the generator and your tools. You probably
can't tell how many circuits your generator is wired with, but
you can count the receptacles. Look for a generator to have a
120/240 twist lock and as many GFCI receptacles as you can get.
Lots of high-quality generators come with only one GFCI
receptacle. DeWalt provides two, giving you four places to plug
in cords. Doing that disperses the current over several cords
and cuts down on heat.
A well-protected yet accessible control
panel is important. Also, you want as many receptacles to
in to as possible. Spreading the current over numerous cords
makes the generator's work easier.
The DeWalt DG6000 features a 120/240
twist lock and two GFCI receptacles.
Frames. Manufacturers have also improved the frames
that these machines ride in, making them tougher and better
positioned to take a beating from falling debris or a bumpy
truck ride. And a frame with a skyhook can make moving the
machine around on site much easier. If you have a forklift on
site, you can equip it with a chain and move the generator
where you need it.
Idle control. Idle control revs the generator down when
there's no current draw, and it's a must-have for some
generator users, especially those working in tight quarters. It
saves fuel, significantly cuts down on noise, and limits wear
and tear. The static on your job-site radio is loud enough
without having to listen to a generator crank out 3,600 rpm for
no reason. This feature also affects the price tag. Typically,
the units that have it are more expensive.
Low oil. There are plenty of stories of builders having
to rebuild or replace generators that a rookie set up on a
hill, trapping all the oil in the pan. Low-oil shutoff isn't
considered a feature anymore as much as it's thought of as
standard equipment. If enough oil doesn't reach the piston, the
generator shuts itself down instead of seizing.
Gas tank. A metal gas tank also might save you a trip
to the repair shop. Plastic gas tanks swell in the summer and
become brittle and breakable in the cold months. Metal tanks
are tougher. You'll typically find them on Honda, Kubota,
Makita, Mitsubishi, Robin, DeWalt, and Yamaha equipment.
Plastic gas tanks are usually found on lower- and moderately
priced models, but think about this: The gas tank you fill your
generator from is likely plastic. You can probably expect your
generator gas tank to last about as long.
A metal gas tank can take a beating. It
doesn't swell in the heat or get brittle in the cold like
If your generator will be used all the time, spring for a
wheel kit. A wheel kit not only makes moving the generator from
the truck and around the site easier, it also makes maintenance
easier. By getting the oil plug off the ground, it facilitates
access to it and the air filter.
Wheels. You'll find three types of wheels out there:
solid rubber, pneumatic, and semi-pneumatic. Solid rubber
wheels like you'll find on Honda's machines are typically about
2 inches wide and roll well on hard, dry surfaces. They're a
little trickier in ankle-deep mud. Pneumatic tires (about 4
inches wide), like the big 12-inchers you'll find on Generac
wheel kits or the smaller tires on Makita and Robin units, roll
great in most terrain --especially mud -- because they're
bigger, softer, and wider than the solid wheels. The downside
is that they can pop, leaving you with one flat tire and 175
pounds of generator to get in the truck. The semi-pneumatic
tires that are now available on some DeWalt and Porter-Cable
models are made from thick rubber with an air pocket inside for
cushioning. They're not as easy rolling as pneumatic tires, but
they're big and wide enough to ride over soft ground and they
won't pop if you run over a roofing nail.
A wheel kit makes everything from
mobility to maintenance easier. Look at the tire choices to see
what's right for you: pneumatic, semi-pneumatic, or
Footprint. Some generators -- like Honda's,
Mitsubishi's, Yamaha's, and Kubota's -- are compact. They're
snugged inside their low-profile frames. Others -- such as the
Campbell Hausfeld, Generac, and Porter-Cable machines -- take
up a bit more space. A larger machine is no big deal on site,
and these airy designs actually help keep the unit cool; the
size matters more in the back of your van or pickup. Compact
units that pack the punch you need usually also pack a higher
A tight, compact design makes sense for
a lot of users. It takes up less space in the truck and is
easier to move around. A model with a skyhook adds even more
options -- you can move it around the site with a
Electric start. The jury is split on this one. Some
guys just don't want to pull a cord. They want to turn a key
and hear their generator fire right up. Others say that an
electric start is just one more thing to break, and they'll go
for the cord every time. Still others just don't want to pay
for the extra feature.
Every manufacturer I spoke with laughed when I asked about
maintenance. To a person, they said that not maintaining the
generator at all is the biggest maintenance problem they see.
Maintenance can make a big difference. Yamaha reports that the
company has had the same generators active daily in rental
fleets for five to seven years and that they've logged
thousands of hours.
An oil change is due on a typical generator about every 100
hours. Some generators, such as Generac units, come with hour
meters so you know when you're supposed to change the oil --
or, more realistically, how far past the recommended time your
guys have taken the machine.
Largely, the meter gets ignored, however. Most users who
change the oil change it by date, not by hours run. Often when
a worker pulls the dipstick and sees low oil, he dumps in
another quart instead of taking the time to change it. As the
machine burns oil, workers keep adding it and still expect the
unit to start on one pull.
Changing the oil regularly will markedly improve the engine's
performance and longevity. You should use a multiweight oil,
such as 10W-30 or 10W-40. If you don't like doing the work
yourself, a good place to have your generator maintained is
your local rental shop. You should change or clean your air
filter (and oil filter if there is one) at the same time.
Another generator killer is fouled gas. Gas can become fouled
in two ways. It can go bad during a period of non-use, or it
can get polluted by sand, water, or rust from the inside of
your gas can. A good way to prevent pollution is to use a
multi-stage filter system to make sure the gas that fires in
the cylinders is pure.
What You Don't See
While you're up to your shoulders in generator boxes and
specs, trying to decide which machine is best for your present
and future needs, don't forget to consider the elements that
you don't see. A lot of that invisible stuff can greatly affect
the price you pay. Of the five main parts of a generator --
engine, alternator, frame, gas tank, and control panel -- only
the engine and the alternator produce electrical current.
Alternators. There are two kinds of alternators: brush
and brushless. They produce the same current, but a brush
alternator requires maintenance, because eventually the brushes
wear out. Manufacturers that put brushless alternators on their
machines say they've eliminated a wear part, which increases
generator life. Many manufacturers offer brushless alternators,
including Honda, Kubota, Mitsubishi, Porter-Cable, and
There are two kinds of alternators:
brush and brushless. Brushes wear out and need to be changed. A
brushless alternator needs no maintenance.
The alternator is the mechanism that converts mechanical
energy (the engine turning) into electrical current (zap). If
the engine runs at 3,600 rpm, the alternator transfers that
motion into 60 hertz, the requisite frequency to run an
electric motor. One measure of the current coming out of the
alternator is harmonic distortion, which measures the
"cleanliness" of the current. The current that comes out of the
wall in a house is 1% to 2% distorted and is considered
"clean." Some generators run at 14% distortion, which is
considered dirty. A high-end generator runs quite a bit cleaner
than that, at 5% to 6%. Now that you know this, how can you
look at it? Good luck. Your tool supplier might know, but
current cleanliness isn't listed in manufacturer specs.
Current. The problem with dirty current, according to
some manufacturers, is its byproduct: heat. Heat is bad for
your tools for the same reason it's bad for your car -- it
melts things. The cleaner the current, the longer your tools
will last. The cleanest current of all comes from the newest
generator technology, called inverter technology. Inverters
replace the alternator and turn the mechanical energy into
electrical energy by using sophisticated electronics.
Presently, Honda, Mitsubishi, and Yamaha have this technology
on smaller generators (1,000 to 3,000 watts) and are developing
it for larger units. Inverter generators use less fuel, weigh
about a third less, have super-clean current, and cost about
the same as a regular unit.
Automatic voltage regulation. AVR is another upgrade
feature that separates generators. It's sort of like electronic
feedback control (EFC) in your power tools. EFC draws more
power from its source, as needed, to keep rpm constant during
an application. For instance, when your router bit hits a knot,
it needs more power to keep bit speed constant. AVR provides
even current on demand so your circ saw doesn't bog out during
a cut when the compressor kicks on. Manufacturers say it puts
less strain on your tools.
Many builders buy either the lowest-cost tool or the brand
they've been using for years. Others go for the high end and
big power, assuming that more expensive is better. Typically,
when it comes to generators, the more expensive tools start
easier, last longer, are smaller, and have more
well-thought-out features, like protected control panels, wheel
kits, AVR, clean current, and brushless alternators. Lower- and
moderately priced machines mix up the features: To save money a
company might put a plastic gas tank and an okay muffler on a
unit with AVR, a brushless alternator, and semi-pneumatic
tires. At the low end -- units that lack GFCIs, run loud and
hot, and have plastic gas tanks -- you can expect to pay $600
to $900. In the middle of the road, you'll find units with a
mix of features -- good engines and solid design features but
not as compact -- for $1,000 to $1,500. Once you reach the
higher end of the price spectrum, you'll find all the details
and bases covered: good sustained power, easy-starting engines,
compact design, ample plug-in choices, idle control, and quiet
operation. These units will set you back $2,000 to
Mark Clement is senior editor of Tools of the Trade