For Americans in the 21st century, it's hard to imagine life
without electric power. We take for granted that our lights,
microwaves, air conditioners, and televisions will always work for
us at the flick of a switch. But should a hurricane hit the
community, we would instantly be plunged back into the 1800s
— or some post-apocalyptic future where people have to piece
their lives together from the remnants of modern civilization.
Tired, hot, and frustrated, citizens would wait in line for hours
to buy basic needs such as water, ice, food, and fuel. And the item
that stores would likely run out of first in such scenarios would
be portable generators.
Avoid Job-Site Generators
The small, gasoline-powered portable generator is the classic ad
hoc response to a major power outage. Yet it may be the homeowner's
worst possible choice. Frank Navetta, who runs PowerPro Service
Co., Inc., on Long Island (www.powerprogenerators.com), considers portable
generators "a self-defeating proposition" even for a household that
needs to run only a microwave and the refrigerator, and much more
so for the 8,000-square-foot mansion with wine cellar, hot tub, and
25 tons of air conditioning. Able to provide only enough output for
a few lights and a refrigerator, gas-powered generators need
frequent refueling and make a lot of noise. But the negatives
extend far beyond these practical limitations.
After major storms, portable generators often kill a few people,
either by carbon monoxide poisoning or by electrocution.
Generator-caused gasoline spills occasionally result in structure
fires and serious burns. And if the do-it-yourself user decides to
get creative and wire the generator directly into a home circuit, a
generator set can "back-feed" the utility by energizing the service
drop that leads back to the pole on the street. The transformer on
the pole, functioning in reverse, then steps the current up to a
lethal 600 volts and sends it into the transmission lines, where it
could kill a utility worker trying to restore power to the whole
neighborhood. Professionally installed backup power systems, in
contrast, are coupled to house power by a "double-throw" switch
that maintains safe grounding. The switch disconnects all
generator-powered circuits from the utility service before it
connects those loads to the generator.
Compared with a portable gasoline-fueled generator, permanently
wired and professionally installed generator sets fueled by natural
gas, liquid propane (LP), or diesel are a safer, more reliable way
to back up utility power.
Selecting a Standby System
Beyond portable generators, backup power systems come in a range of
sizes, usually categorized by output capacity:
From 7 kilowatts up to about 15 kW, systems have enough power for a
modest-sized home, depending on air conditioning needs. These
systems use air-cooled engines like those manufactured for chain
saws, lawnmowers, or small lawn tractors. These units can also be
converted to run on LP or natural gas.
Air-cooled machines are intended for occasional use. During an
extended power outage, someone will have to stop the unit every 8
to 10 hours, at a minimum, to check the oil and air filters.
Above 15 or 16 kW, the generator engines are conversions of
standard water-cooled General Motors or Ford auto and truck engine
blocks. An 8-liter-displacement V-8 motor can power a 125-kW
generator on LP, and up to a 150-kW generator on natural gas fuel.
Diesel engines from companies like John Deere are also used to
power generator sets in this midsize range.
Electric motors need more power to start up than they use once
they're running. Shown here are estimated operating watts and
starting watts for a few common household motors. Standby
generators must be sized to supply sufficient startup current for
all the connected loads.
Above 150 kW, generators jump to industrial packages, built by
companies such as Caterpillar, Cummins, or Waukesha. Top-end unit
are typically designed to run on natural gas and LP, though less
expensive diesels are usually an option, too.
Diesels have some special maintenance needs, however. Like all
generators, they have to be started up and test-run (some models
self-test on a weekly or monthly cycle). Diesels, in particular,
must be allowed to warm up and run at full load and normal
operating temperature for 20 minutes or more, each time they start.
Otherwise, unburned fuel will clog valves and the fouled machine
may not start or run when it's needed. In very cold winter
conditions, diesels may need a fuel adjustment: a 60/40 mix of
diesel fuel and kerosene won't gel at subzero temperatures the way
straight diesel fuel will. (All generators, whatever the fuel,
should have engine block heaters that warm the engine oil reservoir
when temperatures drop below 40°F. Standby generators don't
idle for 10 minutes at startup as a car might on a cold morning.
They are designed to come up to a full 3,600 rpm or 1,800 rpm in
about 10 seconds.)
An integrated double-pole, double-throw transfer switch safely
disconnects house circuits from the utility before connecting them
to the generator, while maintaining a continuous ground for both
power sources. The switch may be manual (above) or automatic
Natural gas and LP have their own set of drawbacks, though. In the
event of a power outage, natural gas pipeline pressures may drop
below the operating requirements of the unit. It's important to
make sure the gas flow through the meter is rated high enough to
supply the generator, as well as any other loads in the house for
cooking, heating, or clothes drying. Also, pay attention to the
length of the supply pipe and the number of bends, warns Jay
Blevins, of Kinsley Power Systems in East Granby, Conn. (www.kinsleypower.com). "Just because the unit has a
half-inch fitting doesn't mean that a half-inch pipe will work. If
it's too long or has too many elbows, it may not."
For extra backup, some backup power specialists can provide
dual-fuel generators that can automatically switch to bottled
propane if the natural gas supply falters — without dropping
When choosing a generator, the first question must be "How big?" At
the low end, the answer depends mostly on whether the homeowner
wants to power the entire house or just a few critical circuits.
"Typically, the hinge point is air conditioning," says Frank
Navetta. "A 15-kilowatt machine can do heat, refrigeration, a
handful of lights. You can do sump pumps. You can even get away
with occasionally using a window air conditioner. But if someone
has two or three 5-ton air conditioners, the inrush current to
start those compressors is so high that you end up needing 30 or 40
kilowatts. And once those air conditioners are started, now you
have all this extra current. You might as well use it to run the
Builder Paul Buske, of Kitty Hawk, N.C.-based Stormont & Co.
(www.stormontandcompany.com), builds primary residences
as well as second homes and vacation rental properties on North
Carolina's Outer Banks. Buske reports that his area's power
reliability has improved considerably since the 1980s: "We used to
have brief outages all the time, just because of salt corrosion and
arcing on transmission-line transformers. But our utility has moved
a lot of powerlines underground, and these days we have fewer
interruptions." Backup power nowadays is more about storm readiness
than about day-to-day convenience, notes Buske. Rental property
investors rarely opt for standby power systems, he adds.
"We have set a lot of year-round houses up with generators, but
nobody yet has decided that they want a generator big enough to run
the whole house," explains Buske. "So we go through the house with
them, while they pick and choose what they feel they will really
need during an outage; and we size the generator to fit that
Most of Stormont & Co.'s year-round houses are laid out with
main living space on the first-floor level, and extra bedrooms and
baths on upper floors. Each story is served by its own heat pump
and air conditioner, which makes it easy to split the power
supplies between floors. "Our generator normally runs the air
conditioner for the main level, a refrigerator and freezer, the
kitchen appliances, and enough light circuits so that the main
living level isn't in the dark," says Buske. Circuits that will run
off the generator are isolated onto a separate subpanel.
Some loads, like ordinary lighting, are easy to estimate. But loads
involving motors — well pumps, sump pumps, air conditioner
compressors, and so on — are trickier. Compressor, pump, and
fan motors — just like power tools — draw more power
starting up than they do when running. Generators have to be sized
to handle the startup draw of all the equipment they serve, not
just the equipment's operating requirements. Otherwise, big
appliances like air conditioners won't even start.
Smart switches. Temporary power requirements can be accommodated
with smart load-management circuitry. Many residential power
systems, including models from Briggs & Stratton (www.homegeneratorsystems.com), Generac (www.generac.com), and
Kohler (www.kohlerpower.com), include "smart" switches and
load-control circuitry that can prioritize between various current
Air conditioning manufacturer Rheem (www.rheemac.com),
which markets the Briggs & Stratton standby generators under
the Rheem and Ruud brands, offers the smart load-management systems
as an option. "All of our transfer switches come with an air
conditioner control module," explains Rheem product manager Art
Berg. "If the home is on generator power, and the air conditioning
system calls for cooling, the control module looks to see if it has
the wattage available to start the air conditioner." If the power's
not available, the switch waits. "Every 15 seconds it will keep
looking for the power," continues. "As soon as any appliance goes
off-line — if you turn off the oven or the clothes dryer
stops — it will find the power, start the air conditioner,
and go back into operating mode."
If only selected circuits are to be powered by a generator, a
separate subpanel is usually installed for just those circuits
behind the transfer switch.
For even finer control, systems may also include a load-control
center that has "load-shedding" capability. This feature allows you
to select up to six appliances that can go off-line when it's time
to start the air conditioner. So when the thermostat calls for air
conditioning, the smart box will first check to see if it has the
wattage to start the air conditioner. If it doesn't, it will start
shutting down the selected appliances just long enough to give the
air conditioner its starting wattage. When the air conditioner goes
into operating mode, the smart box brings the other loads back
Coupled with the smart controls, a standby generator can afford to
be as much as 10 kW smaller, according to Berg. "Using the
load-control center, a 12-kW unit will replace a 20-kW unit, and a
15 kW will replace a 25 kW, in most cases." These smaller 12- and
15-kW generator sets are significantly cheaper to buy.
Ordinary household loads like lighting or stoves aren't fussy
about power quality, but modern electronic equipment needs "clean
power" with a smooth, regular waveform and without excessive
harmonic distortion. Units with electronic governors and digital
voltage regulators, such as this one on a Generac unit, can
maintain fine control over power quality.
Power quality. Simple "linear loads" like incandescent lighting or
electric stove heating elements can tolerate current fluctuations,
but new devices, including fluorescent lighting, computers, plasma
televisions, and "uninterruptible power system" (UPS) equipment all
require "clean power."
Whether natural gas or liquid propane (LP) is the fuel, gas
meters and supply lines must be sized to supply sufficient fuel to
operate the generator as well as the home's other gas appliances
(such as heating equipment or cooking stoves). Advanced "dual-fuel"
systems can use natural gas as the main fuel source, with an LP
backup source available in case natural gas pipeline pressures drop
during a power outage.
Clean power means a smooth, 60-Hz (60 cycles per second) sine-wave
electrical signal. To produce this, generators have to spin at
exactly 3,600 rpm if they have a two-pole alternator, and at
exactly 1,800 rpm if they have a four-pole alternator. However, a
cheap, underpowered generator tends to run at a little faster under
light loads, slow down under heavier loads, and speed up again as a
mechanical governor kicks in to compensate. This fluctuation
creates harmonic distortion — a deformation of the sine wave
dubbed "dirty" power that can damage sophisticated solid-state
circuitry. In fact, UPS systems and some solid-state controls on
the newest furnaces and air conditioners may refuse to operate at
all under fluctuating generator power.
Top-of-the-line units are able to maintain near exact speeds by
means of electronically controlled governors and digital voltage
regulators that can keep engine rpms and power output within at
least 1.5% of the desired 60 Hz.
Once the right-sized generator is chosen, it needs to be wired into
the house. If only selected loads are to be generator-powered,
those circuits typically get their own subpanel. Upstream of the
subpanel (or upstream of the main circuit breaker panel if the
generator is intended to serve the whole house) is the key safety
device: a "double-pole, double-throw" switch that disconnects the
house circuits from the utility grid power feed before connecting
them to the generator. "We call it a ‘break-before-make'
switch," says Jay Blevins. "The switch breaks the connection to the
utility before it makes the connection to the generator."
A professional contractor who specializes in generators must be
part motorhead, part sparky, in order to best install and maintain
the equipment. Here, contractor Jay Hart of Hart Power Solutions
(www.hpsobx.com) in Kill Devil Hills, N.C., performs
routine motor maintenance on a generator set at a new home built by
Kitty Hawk, N.C. builder Stormont & Co. (www.stormontandcompany.com).
If it's a manual switch, the homeowner will have to physically
throw it during a power failure. If it's automatic, it will switch
over on its own and send a signal to the generator to start. But
either way, the important thing is that the house loads never be
connected to the generator and the utility at the same time —
and that the generator never, ever, be connected to the service
drop connecting the house to the pole. Besides the life and safety
risk created if the generator is able to back-feed the utility
powerlines, there's a risk of damage to the standby generator if
the generator is accidentally hooked into the grid when the utility
power is on.
Local codes determine the foundation requirements for a
generator set. A gravel pad or concrete slab is typically required
(above). In high-wind zones, however, generators may need to be
anchored, and in flood zones, they often have to be elevated
Team sport. If a builder is handling the hookup, it will involve
multiple trades. Notes Buske: "My electrician has to work hand in
hand with my generator guy. My heat and air guy has to be involved.
All these people have to be able to work together — cause
what I don't need at the end of this job is one guy pointing his
finger at the other saying, ‘You didn't do your job right.'
The builder's best bet is to go with a specialist, argues Frank
Navetta — a full-service generator set supplier with trained
maintenance crews, who can professionally install the unit and
offer the homeowner a service plan. A builder may save a few
hundred dollars by purchasing the generator set himself and paying
his electrician to install it, but if it doesn't work, it becomes
the builder's headache. ~
At hookup, it's critical to pay attention to how the unit is
Typically, a generator set's ground wire is connected to the home's
main ground wire at the main service panel, which leads to a copper
ground stake driven into the earth. That ground is hard-wired, not
switch-controlled: the generator, the circuits it powers, and the
other home circuits all stay grounded all the time (so throwing the
transfer switch transfers the connection for the live wires but not
the continuous ground wire). However, a generator set may be
provided with a means to ground the generator frame directly to its
own copper stake, and some supplier manuals say to make that
independent ground when the set is installed. Since double
grounding is usually a no-no, the issue can throw electricians off
What's the answer? Experts say that for residential applications,
tying the generator's ground to the main house ground is the way to
go — an additional separate ground at the generator set is
unnecessary, and doing so could even cause a hazard in the case of
a ground fault. (The separate grounding option on some generator
sets is provided in case the application is for what the electrical
code terms a "separately derived system" — not usual in a
residential setup.) But not all systems are the same, and grounding
is an issue for the local inspector to decide. Always have the
installation inspected, and let the local inspector make the