First off, insure that your inverter is a safe power module. This is crucial, your power inverter
should be certified by an independent
testing laboratory such as UL, ETL, CSA, etc., and
stamped accordingly. This is your assurance that it
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.
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.
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, sine wave inverters have
become more efficient and more affordable. Some
people compromise by using a modified sine 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 sine wave inverters in renewable
energy systems have started fading into history.
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.
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.
Stacking two inverters together allows them to produce double their rated capacity as well as 3-wire, 220 VAC power. This enables you to operate large power tools, deep well pumps and any other load that requires 220 VAC power.
Some loads absorb the AC wave’s energy in a regimented fashion, other loads will take all available power until they reach sufficient start up speed. This produces a time delay within the system and can create shocks, like towing a car with a rubber strap. These loads, if not sufficiently fed, create lags within the system, and can have big effects of the surrounding electrical system. If you have a central A/C and your lights dim, that is the result of a motor or other inductive load drawing all available power, starving the lights momentarily. 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 slowly starting the device, as in the central a/c example above. 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 can crash a computer, ruin LCD TV screens, Plasma TVs and other sensitive electronics.
Any weakness in the battery and cabling to the
inverter will further limit its ability to start a motor as the increased amperage draw will result in further voltage deterioration. See wire voltage drop here
A battery bank that is undersized, has corroded
connections, or is in poor condition, 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! See next section 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.
The simple reason for loss of current in wire is resistance, this loss is transformed to heat by friction. Your local electrical ordinances govern the maximum voltage loss, and your electrician is initmately familiar with the problem, but you should be aware. If you try to build your own system, DO NOT CHEAT YOUR EFFICIENCY WITH SMALL WIRES.
Let's forget about that heat for a moment, and the resulting fire considerations, and concentrate on the loss. To illustrate the loss point we have run some equations on a series of loads at 12 volts. We varied the amperage from 1 (one) to 350 (three hundred fifty) amps, and kept the voltage the same on the test end. (SEVERAL OF THESE WOULD HAVE STARTED A FIRE IF INSTALLED - CONSULT LOCAL CODE BEFORE INSTALLING!!!) This first graph shows the loss of voltage at the business end of the wire.
This loss is dramatic on the 6 guage wire, but to truly get a feeling of the loss you need to see the resulting wattage. Using ohms law, volts x amps = watts, we can calculate the wattage at the business end of the wire. With this graph you can see that the 6 guage wire tops out under 1.1 KW no matter the amperage.
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
an inverter sized for a home system. An inverter’s spec
sheet may describe the inverter’s “idle current” in amps.
To get watts,just multiply this times the DC voltage of
the system.
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.
Another 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.
A good inverter is an industrial quality device
proven reliable, certified for safety, and can last
decades. A cheap inverter may soon end up
junkpile, and can even be a fire hazard. Consider
inverter to be a foundation component. Buy a good
that allows for future expansion of your needs.
Portions of this page can be found on Home Power Magazine's Home Power #82 • April / May 2001, Reprinted with permission from Home Power The remainder is the express copyright and posession of Broomfield Designers and BD Batteries
BD Batteries - A Broomfield-Designers Company
Serving the Local Colorado Community 9-5 Mountain & Emergencies P. 303-800-4725 F. 303-600-9726
Serving the Buffalo and the Great Lakes Communities 9-5 Eastern & Emergencies P. 716-531-4875
Mailing Address: 5023 West 120th Ave. #138, Broomfield, CO 80020
OUR DENVER WAREHOUSE IS MOVING TO A NEW LOCATION AT THIS TIME.
WE WERE SCHEDULED TO REOPEN OUR BRICK AND MORTAR BATTERY WAREHOUSE IN EARLY 2011.
IN THE MEAN TIME OUR WEBSITE AND PHONES OPERATE FINE!
We do thank the public for the outreach of concern, and votes of confidence.
Do call Lorene prior to schedule used battery pickups, new battery dropoffs, etc. by appointment.
The information contain on this deep cycle AGM batteries website is deemed reliable, but not guaranteed. Please verify specific information on marine AGM batteries, rv AGM batteries or auto AGM batteries listed here by calling us direct. We are proud to offer the highest quality AGM batteries and hope you will consider using our deep cycle AGM batteries in the near future!
