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Inverters: How To Choose An Inverter For An Independent
Power System
The inverter is one of the most important and most complex
components in an independent energy system. To choose an inverter,
you don't have to understand its inner workings, but you should
know some basic functions, capabilities, and limitations.
This article gives you some of the information you'll need
to choose the right inverter and use it wisely.
WHY YOU NEED AN INVERTER
Independent electric energy systems are untethered from the
electrical utility grid. They vary in size from tiny yard
lights to remote homes, villages, parks, and medical and military
facilities. They also include mobile, portable, and emergency
backup systems. Their common bond is the storage battery,
which absorbs and releases energy in the form of direct current
(DC) electricity
In contrast, the utility grid supplies you with alternating
current (AC) electricity. AC is the standard form of electricity
for anything that "plugs in" to utility power. DC flows in
a single direction. AC alternates its direction many times
per second. AC is used for grid service because it is more
practical for long distance transmission.
An inverter converts DC to AC, and also changes the voltage.
In other words, it is a power adapter. It allows a battery-based
system to run conventional appliances through conventional
home wiring. There are ways to use DC directly, but for a
modern lifestyle, you will need an inverter for the vast majority,
if not all of your loads (loads are devices that use energy).
Incidentally, there is another type of inverter called grid-interactive.
It is used to feed solar (or other renewable) energy into
a grid-connected home and to feed excess energy back into
the utility grid. If such a system does not use batteries
for backup storage, it is not independent from the grid, and
is not within the scope of this article.
NOT A SIMPLE DEVICE
Outwardly, an inverter looks like a box with one or two switches
on it, but inside there is a small universe of dynamic activity.
A modern home inverter must cope with a wide range of loads,
from a single night light to the big surge required to start
a well pump or a power tool. The battery voltage of a solar
or wind system can vary as much as 35 percent (with varying
state of charge and activity).
Through all of this, the inverter must regulate the quality
of its output within narrow constraints, with a minimum of
power loss. This is no simple task. Additionally, some inverters
provide battery backup charging, and can even feed excess
power into the grid.
DEFINE YOUR NEEDS
To choose an inverter, you should first define your needs.
Then you need to learn about the inverters that are available.
Inverter manufacturers print everything you need to know on
their specification sheets (commonly called "spec sheets").
Here is a list of the factors that you should consider.
APPLICATION ENVIRONMENT
Where is the inverter to be used? Inverters are available
for use in buildings (including homes), for recreational vehicles,
boats, and portable applications. Will it be connected to
the utility grid in some way? Electrical conventions and safety
standards differ for various applications, so don't improvise.
ELECTRICAL STANDARDS
The DC input voltage must conform to that of the electrical
system and battery bank. 12 volts is no longer the dominant
standard for home energy systems, except for very small, simple
systems. 24 and 48 volts are the common standards now. A higher
voltage system carries less current, which makes system wiring
cheaper and easier.
The inverter's AC output must conform to the conventional
power in the region in order to run locally available appliances.
The standard for AC utility service in North America is 115
and 230 volts at a frequency of 60 Hertz (cycles per second).
In Europe, South America, and most other places, it's 220
volts at 50 Hertz.
Safety Certification An inverter should be certified by an
independent testing laboratory such as UL, ETL, CSA, etc.,
and be stamped accordingly. This is your assurance that it
will be safe, will meet the manufacturer's specifications,
and will be approved in an electrical inspection. There are
different design and rating standards for various application
environments (buildings, vehicles, boats, etc.). These also
vary from one country to another.
POWER CAPACITY
How much load can an inverter handle? Its power output is
rated in watts (watts = amps x volts). There are three levels
of power rating-a continuous rating, a limited-time rating,
and a surge rating. Continuous means the amount of power the
inverter can handle for an indefinite period of hours. When
an inverter is rated at a certain number of watts, that number
generally refers to its continuous rating.
The limited-time rating is a higher number of watts that
it can handle for a defined period of time, typically 10 or
20 minutes. The inverter specifications should define these
ratings in relation to ambient temperature (the temperature
of the surrounding atmosphere). When the inverter gets too
hot, it will shut off. This will happen more quickly in a
hot atmosphere. The third level of power rating, surge capacity,
is critical to its ability to start motors, and is discussed
below.
Some inverters are designed to be interconnected or expanded
in a modular fashion, in order to increase their capacity.
The most common scheme is to "stack" two inverters. A cable
connects the two inverters to synchronize them so they perform
as one unit.
POWER QUALITY -- SINE WAVE vs. "MODIFIED
SINE WAVE"
Some inverters produce "cleaner" power than others. Simply
stated, "sine wave" is clean; anything else is dirty. A sine
wave has a naturally smooth geometry, like the track of a
swinging pendulum. It is the ideal form of AC power. The utility
grid produces sine wave power in its generators and (normally)
delivers it to the customer relatively free of distortion.
A sine wave inverter can deliver cleaner, more stable power
than most grid connections.
How clean is a "sine wave"? The manufacturer may use the
terms "pure" or "true" to imply a low degree of distortion.
The facts are included in the inverter's specifications. Total
harmonic distortion (THD) lower than 6 percent should satisfy
normal home requirements. Look for less than 3 percent if
you have unusually critical electronics, as in a recording
studio for example.
Other specs are important too. RMS voltage regulation keeps
your lights steady. It should be plus or minus 5 percent or
less. Peak voltage (Vp) regulation needs to be plus or minus
10 percent or less.
A "modified sine wave" inverter is less expensive, but it
produces a distorted square waveform that resembles the track
of a pendulum being slammed back and forth by hammers. In
truth, it isn't a sine wave at all. The misleading term "modified
sine wave" was invented by advertising people. Engineers prefer
to call it "modified square wave."
The "modified sine wave" has detrimental effects on many
electrical loads. It reduces the energy efficiency of motors
and transformers by 10 to 20 percent. The wasted energy causes
abnormal heat which reduces the reliability and longevity
of motors and transformers and other devices, including some
appliances and computers. The choppy waveform confuses some
digital timing devices.
About 5 percent of household appliances simply won't work
on modified sine wave power at all. A buzz will be heard from
the speakers of nearly every audio device. An annoying buzz
will also be emitted by some fluorescent lights, ceiling fans,
and transformers. Some microwave ovens buzz or produce less
heat. TVs and computers often show rolling lines on the screen.
Surge protectors may overheat and should not be used.
Modified sine wave inverters were tolerated in the 1980s,
but since then, true sine wave inverters have become more
efficient and more affordable. Some people compromise by using
a modified wave inverter to run their larger power tools or
other occasional heavy loads, and a small sine wave inverter
to run their smaller, more frequent, and more sensitive loads.
Modified wave inverters in renewable energy systems have started
fading into history.
EFFICIENCY
It is not possible to convert power without losing some of
it (it's like friction). Power is lost in the form of heat.
Efficiency is the ratio of power out to power in, expressed
as a percentage. If the efficiency is 90 percent, 10 percent
of the power is lost in the inverter. The efficiency of an
inverter varies with the load. Typically, it will be highest
at about two thirds of the inverter's capacity. This is called
its "peak efficiency." The inverter requires some power just
to run itself, so the efficiency of a large inverter will
be low when running very small loads.
In a typical home, there are many hours of the day when the
electrical load is very low. Under these conditions, an inverter's
efficiency may be around 50 percent or less. The full story
is told by a graph of efficiency vs. load, as published by
the inverter manufacturer. This is called the "efficiency
curve." Read these curves carefully. Some manufacturers cheat
by starting the curve at 100 watts or so, not at zero!
Because the efficiency varies with load, don't assume that
an inverter with 93 percent peak efficiency is better than
one with 85 percent peak efficiency. If the 85 percent efficient
unit is more efficient at low power levels, it may waste less
energy through the course of a typical day.
INTERNAL PROTECTION
An inverter's sensitive components must be well protected
against surges from nearby lightning and static, and from
surges that bounce back from motors under overload conditions.
It must also be protected from overloads. Overloads can be
caused by a faulty appliance, a wiring fault, or simply too
much load running at one time.
An inverter must include several sensing circuits to shut
itself off if it cannot properly serve the load. It also needs
to shut off if the DC supply voltage is too low, due to a
low battery state-of-charge or other weakness in the supply
circuit. This protects the batteries from over-discharge damage,
as well as protecting the inverter and the loads. These protective
measures are all standard on inverters that are certified
for use in buildings.
INDUCTIVE LOADS and SURGE CAPACITY
Some loads absorb the AC wave's energy with a time delay
(like towing a car with a rubber strap). These are called
inductive loads. Motors are the most severely inductive loads.
They are found in well pumps, washing machines, refrigerators,
power tools, etc. TVs and microwave ovens are also inductive
loads. Like motors, they draw a surge of power when they start.
If an inverter cannot efficiently feed an inductive load,
it may simply shut down instead of starting the device. If
the inverter's surge capacity is marginal, its output voltage
will dip during the surge. This can cause a dimming of the
lights in the house, and will sometimes crash a computer.
Any weakness in the battery and cabling to the inverter will
further limit its ability to start a motor. A battery bank
that is undersized, in poor condition, or has corroded connections,
can be a weak link in the power chain. The inverter cables
and the battery interconnect cables must be big, and I mean
REALLY big, perhaps the size of a large thumb! The spike of
DC current through these cables is many hundreds of amps at
the instant of motor starting. Follow the inverter's instruction
manual when sizing the cables, or you'll cheat yourself. Coat
battery connections with a protective coating to reduce corrosion.
IDLE POWER
Idle power is the consumption of the inverter when it is
on, but no loads are running. It is "wasted" power, so if
you expect the inverter to be on for many hours during which
there is very little load (as in most residential situations),
you want this to be as low as possible. Typical idle power
ranges from 15 watts to 50 watts for a home-size inverter.
An inverter's spec sheet may describe the inverter's "idle
current" in amps. To get watts, just multiply the amps times
the DC voltage of the system.
LOW SWITCHING FREQUENCY vs. HIGH SWITCHING
FREQUENCY
There are two ways to build an inverter. Without diving into
theory, I'll simply say that there are differences in weight,
cost, surge capacity, idle power, and noise.
A low switching frequency inverter is big and heavy (generally
about 20 pounds (10 kg) per kilowatt), and more expensive.
It has the high surge capacity (four to eight times the continuous
capacity) needed to start large motors. Beware of the acoustical
buzz that low switching frequency inverters make. If you install
one near a living space, you may be unhappy with the noise.
A high switching frequency inverter is much smaller and lighter
(generally about 5 pounds (2.5 kg) per kilowatt), and also
less expensive. It has less surge capacity, typically about
two times the continuous capacity. It produces little or no
audible noise. The idle power is generally higher. If the
inverter is oversized for motor starting, its idle power will
be higher yet, and may be prohibitive. Most homes that have
a well pump or other motors greater than 1 HP will find a
low switching frequency inverter to be more economical.
Both types of inverter have their virtues. Some people "divide
and conquer" by splitting their loads and using two inverters.
This adds a measure of redundancy. If one ever fails, the
other one can serve as backup.
AUTOMATIC ON/OFF
Inverter idling can be a substantial load on a small power
system. Most inverters made for home power systems have automatic
load-sensing. The inverter puts out a brief pulse of power
about every second (more or less). When you switch on an AC
load, it senses the current draw and turns itself on. Manufacturers
have various names for this feature, including "load demand,"
"sleep mode," "power saver," "autostart," and "standby."
Automatic on/off can make life awkward because a tiny load
may not trigger the inverter to turn on or stay on. For example,
a washing machine may pause between cycles, with only the
timer running. The timer draws less than 10 watts. The inverter's
turn-on "threshold" may be 10 or 15 watts. The inverter shuts
off and doesn't come back on until it sees an additional load
from some other appliance. You may have to leave a light on
while running the washer.
Some people can't adapt to such situations. Therefore, inverters
with automatic on/off also have an always-on setting. With
it, you can run your low-power night lights, your clocks,
fax, answering machine and other tiny loads, without losing
continuity. In that case, a good system designer will add
the inverter's idle power into the load calculation (24 hours
a day). The cost of the power system will be higher, but it
will meet the expectations of modern living.
PHANTOM LOADS and IDLING LOADS
High tech consumers (most of us Americans) are stuck with
gadgets that draw power whenever they are plugged in. Some
of them use power to do nothing at all. An example is a TV
with a remote control. Its electric eye system is on day and
night, watching for your signal to turn the screen on. Every
appliance with an external wall-plug transformer uses power
even when the appliance is turned off. These little demons
are called "phantom loads" because their power draw is unexpected,
unseen, and easily forgotten.
A similar concern is "idling loads." These are devices that
must be on all the time in order to function when needed.
These include smoke detectors, alarm systems, motion detector
lights, fax machines, and answering machines. Central heating
systems have a transformer in their thermostat circuit that
stays on all the time. Cordless (rechargeable) appliances
draw power even after their batteries reach a full charge.
If in doubt, feel the device. If it's warm, that indicates
wasted energy. How many phantom or idling loads do you have?
There are several ways to cope with phantom and idling loads:
* You may be able to avoid them (in a small cabin or simple-living
situation).
* You can minimize their use and disconnect them when not
needed, using external switches (such as switched plug-in
strips or receptacles).
* You can work around them by modifying certain equipment
to shut off completely (central heating thermostat circuits,
for example).
* You can use some DC appliances.
* You can pay the additional cost for a large enough power
system to handle the extra loads plus the inverter's idle
current.
Be careful and honest if you contemplate avoiding all phantom
and idling loads. You cannot always anticipate future needs
or human behavior.
POWERING A WATER SUPPLY PUMP
At a remote site, a water well or pressure pump often places
the greatest demand on the inverter. It warrants special consideration.
Most pumps draw a very high surge of current during startup.
The inverter must have sufficient surge capacity to handle
it while running any other loads that may be on. It is important
to size an inverter sufficiently, especially to handle the
starting surge. Oversize it still further if you want it to
start the pump without causing lights to dim or blink. Ask
your supplier for help doing this because inverter manufacturers
have not been supplying sufficient data for sizing in relation
to pumps.
If you do not already have a pump installed, you can get
a 220 volt pump if you don't need more than 1/2 HP. A water
pump contractor will often supply a higher power pump than
is needed for a resource-conserving household. You can request
a smaller pump, or it may be feasible (and economical) to
replace an existing pump with a smaller one. You can also
consider one of a growing number of high-effiency DC pumps
that are available, to eliminate the load from your inverter.
BATTERY CHARGING FEATURES
Backup battery charging is essential to most renewable energy
systems because there are likely to be occasions when the
natural energy supply is insufficient. Some inverters have
a built-in battery charger that will recharge the battery
bank whenever power is applied from an AC generator or from
the utility grid (if the batteries are not already charged).
This also means that an inverter can be a complete emergency
backup system for on-grid power needs (just add batteries).
A backup battery charger doesn't have to be built into the
inverter. Separate chargers are, in some cases, superior to
those built into inverters. This is especially true in the
case of low switching frequency inverters, which tend to require
an oversized generator to produce the full rated charge current.
The specifications that relate to battery charging systems
include maximum charging rate (amps) and AC input power requirements.
The best chargers have two or three-stage charge control,
accommodation of different battery types (flooded or sealed),
temperature compensation, and other refinements.
Be careful when sizing a generator to meet the requirements
of an inverter/charger. Some inverters require that the generator
be oversized (because of low power factor, which is beyond
the scope of this article). Be sure to get experienced advice
on this, or you may be disappointed by the results.
QUALITY PAYS
A good inverter is an industrial quality device that is proven
reliable, certified for safety, and can last for decades.
A cheap inverter may soon end up in the junk pile, and can
even be a fire hazard. Consider your inverter to be a foundation
component. Buy a good one that allows for future expansion
of your needs.
YOUR FINAL CHOICE
Choosing an inverter is not a difficult task. Define where
it is to be used. Define what type of loads (appliances) you
will be powering. Determine the maximum power the inverter
will need to handle. Is the quality of the power critical?
Does size and weight matter? The inverter selection table
will help you to determine what type of inverter is best for
you.
Your next step is to learn what inverters are available on
the market. Study advertisements and catalogs, or ask your
favorite dealer. It is best to listen to professional advice,
and to purchase your equipment from a trained and experienced
dealer/installer. We hope this article helps you make the
right choice.
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