A Backwoods Home Anthology
SOLAR POWER
How an inverter fits into your solar electric system
By Joel Davidson
Early on in PV history, the use of
inverters was downplayed and most
people used direct current. PV systems
were costly and small-sized. Inverters
consume power, and when PV was
two to three times its present cost, that
power was too expensive to waste.
PV pioneers were willing to put up
with DC systems. They either hunted
up DC appliances, modified existing
AC appliances, made gadgets from
scratch or did without. It was a time of
DC lights, fans and radios salvaged
from cars and trucks. Much time was
spent scrounging through catalogs and
shops looking for military surplus DC
motors and other goodies. Some very
creative solutions resulted and a few
DC PV businesses flourished for a time.
Even today the DC tradition has
Inverters change a battery s direct current to alternating current so you
been passed on as part of the credo of
can use the electricity with everyday household appliances.
energy conservation. A guideline for
in the DC mode. A voltmeter put across turers were accommodating. In one
designing small PV systems is, if you
the secondary windings of the input Automotive catalog, five models of
can power something directly with
transformer would often tell the story. DC vacuum cleaners were listed. I
DC, do it.
Reading the voltage and making tried them all. To my disappointment,
One idea that has grown less popular
modifications meant opening up the they were hardly more than toys
through changes in technology is
device. Electrical hazard warnings and (though some cost more than small
record turntable modification. Since
threats of voided warranties notwith- home uprights). They worked, but
better belt-driven turntables use DC
standing, it was strange to open up a were designed for small tasks, not
motors, it seemed logical to bypass the
case which says  No Serviceable Parts cleaning a house in the country where
input transformer and go straight to
Inside or  To Be Opened Only By A dirt is always getting tracked in.
the motor with 12 volts or whatever
Qualified Service Technician and There are other examples of DC
the DC requirement was.
find 12 volts DC just waiting to be devices that couldn t quite do the job.
For the non-technical: If you trace
powered by PV. There were blenders that couldn t
the power cord from the plug into
Amateur radio operators and com- crush ice, soldering irons that couldn t
some devices, the input transformer is
the first thing you may find. The pur- puter tinkerers were already familiar solder large wire, drills that broke
with these exploratory operations. after a few hours, bug zappers that
pose of these coils with a magnet is to
They knew that most solid-state elec- missed the big ones. Needless to say,
take the 120-volt AC utility power
tronics were low voltage, usually 12 there was room for improvement, and
coming into the transformer primary
volts or less. Thus, ham radios and things have improved. Nowadays, we
coil and return reduced voltage from
the secondary coil. That reduced volt- computers were among the first high can find better DC appliances though
tech equipment to be PV-powered. at a premium price.
age is then rectified to DC for use in
In the early 1980s two trends Fortunately, another trend was
the low-voltage DC circuitry of the
evolved. Tracking the growth of the occurring. People were beginning to
solid-state electronics found in most
everything nowadays. For the PV pio- recreational vehicle industry, all kinds experiment with inverters. Users were
of DC appliances began to appear. willing to sacrifice the AC appliances
neer, much time was spent discussing
RVers like gadgets, and the manufac- they had stored away and to experi-
which devices could be used directly
The Third Year 31
A Backwoods Home Anthology
ment with early square-wave inverters.
SQUARE WAVE STEPPED OR SINE WAVE
Some people used old-style motor
MODIFIED
inverters and got satisfactory service
SQUARE WAVE
120 -
but. low efficiency. Some even bought
expensive sine-wave inverters costing
as much as their entire PV system.
The more technical PV users at that
0 -
time were building their own. From
early designs and testing came a new
generation of inverters which was to
120 -
change the nature of PV use and make
the old DC bias obsolete.
The wave forms of various inverters from the most primitive simple
What is an inverter? square wave to pure sine wave. For most applications, the stepped
wave will work well and reduce inverter costs.
An inverter is a device that changes
and very little about the cost of large back on smooth and reliable PV
direct current to alternating current.
wire used for comparable DC loads. power again. (When I m on the road, I
(Converters, sometimes called recti-
We won t mention wire loss ineffi- also avoid unregulated grid power
fiers, change AC to DC). For our pur-
ciencies because of undersized con- from ruining my computer. A second
poses, we are speaking of 12-, 24-, or
ductors. That kind of penny-pinching battery in my car and a 300watt
48-volt DC power inverted to 120 or
is, in fact, just throwing good money inverter are my portable office power
240 volts AC. (See Figure 1)
away. Sometimes those losses and supply and Uninterruptible Power
Don t let other approximate AC
costs can be as much as the difference System. If I have to stay on location a
voltages confuse you. You will hear
in the cost of using an inverter. few days, a portable PV array pro-
110 volts or 115 volts or even 117
Factored into cost somehow should be vides all the power my office on the
volts. The range is 110 to 120.
the time spent making do with DC, road needs.)
However, actual readings maybe
but that is not often the case. More costly inverters put out a mod-
greater or less. There are two conven-
Inverters come in all sizes, shapes ified, or stepped, square wave which
tions, 110 and 120. Old timers use
and price ranges offering a vast array more closely matches grid power.
110, but the catalogs typically spec
of options. Some inverters produce the Some test equipment needs the exact
electrical equipment at 120 and 240
simple square wave suitable for most type of power produced by the utility
for utility voltage. Unless your equip-
loads. While there is reason for con- company (60-cycle reference sine
ment specifically requires something
cern about the quality of square-wave wave) to operate. In that case, a costly
other than utility power, 220, 230, and
AC, it will do the job. Surprisingly, sine-wave inverter is necessary. The
240 volts are also interchangeable.
most computers will operate on square early sine-wave inverters were actual-
The simple test is if you can plug it
wave. Some computers have power ly square-wave devices with ferro-
into grid power, call it what you will -
conditioning equipment built into their resistant transformers to smooth out
it s 120 or 240 volts AC.
power supplies which allows for the flip-flop square wave. Thus, effi-
Europe has different voltages, gener-
almost any quality of electricity. This ciency was sacrificed in the trans-
ally 230 volts 50 Hertz (named in
is done because grid power is so vari- former to produce pure sine-wave
honor of an early experimenter in
able. I powered my Apple H Plus and inversion.
electricity). The difference is the fre-
two disk drives and Epson printer with Now there is a new generation of
quency at which the current alternates
a very simple 550-watt square-wave inverters: relatively lower cost, very
(thus, alternating current or AC. In the
inverter satisfactorily. high-efficiency digital sine-wave
U.S. we use AC at 60 cycles per sec-
Recently, my office was moved to inverters. They can power anything
onds or 60 Hz.
the front of the building which is ser- just like grid power-and they are
While the purist may want to oper-
viced by the utility company, while changing the way PV will be used.
ate his PV-powered home entirely on
my PV system remained connected to So far we have been discussing
DC, there are some limitations.
the old office. One morning I turned solid-state, or electronic inverters. An
Besides limited availability of appli-
on my computer and a surge or spike old standby is the rotary or motor
ances, larger wire is required to carry
zapped the computer power supply inverter. This device is a motor which
the same power load. For example, a
resulting in costly repairs. When I got drives an alternator. The DC power
200 watt load at 12 volts needs wire
the computer back from the shop, I runs the motor. A common shaft ties
large enough to carry 16.6 amperes,
ran a 120 volt AC line from my old the motor and alternator together, and
whereas the same load at 120 volts is
PV system to the front office, specifi- the output of the alternator is sine-
only 1.66 amperes.
cally dedicated for my computer. wave AC.
Too often in PV discussions we hear
Needless to say, I am relieved to be
a lot about inverter efficiency losses
32 The Third Year
A Backwoods Home Anthology
If rotary inverters put out a nice sine power is applied. Without a load, it
Selecting an inverter
wave, why aren t they used more often waits. In the on and no-load state your
There are a few basic guidelines to
in PV systems? The simple fact is that inverter should use little power. If your
use in selecting an inverter for your
they are not very efficient. At less than inverter does not have, a low no-load
PV system. First, you need to know
65% efficiency, and as low as 30% mode, be sure to turn it off when not
your power requirements. We always
efficiency with loads 20% of the rated in use. Do not waste your PV power.
come back to this. The power require-
output, rotary inverters are not partic- What can you power with an inverter?
ments identify the loads and how long
ularly suited to PV. This is primarily Basically anything that operates from
they will be used. Your power require-
due to the amount of energy required the grid can be operated with the
ments list will give you an idea on
to move the motor and alternator. PV modem solid-state inverter. Resistive
what will be operating on AC and
electricity is too precious to waste on loads like coffee makers, toasters, and
what AC loads will be operating at the
spinning parts. But for the occasional, hair dryers will operate with no prob-
same time. The inverter must be able
short-duration sine-wave load, a rotary lem, as well as incandescent lights.
to handle the combined AC loads
inverter may be just the thing. Fluorescents may not work well if
which will be operated simultaneous-
Also, the rotary, inverter output volt- your inverter does not interface prop-
ly. In fact, the inverter must be able to
age varies in direct proportion to the erly with the fluorescent ballasts.
handle the surge of these loads, too.
DC input voltage. This means that as Some induction or brushless AC
Note the type of loads to be powered
the battery discharges, the output volt- motors have very high starting surge
by the inverter. Are they motor loads
age falls with it. Even though the out- requirements. Well pumps, garbage
with high starting surges? Is complex
put is a sine wave, you can see the disposals, dishwashers, refrigerators,
electronic equipment to be powered?
voltages varying from as low as 90 air conditioners, and washing
How about kitchen loads? Will you be
volts to upward of 140 volts AC. machines all have high starting surges.
running the garbage disposal and dish-
One nice thing about rotary inverters These surge requirements can be as
washer at the same time? If so, don t
is that they can really take a beating. much as five times the normal operat-
forget to include the well pump as you
They handle motor-starting well. I ing power requirement. Thus, the 300
will be using water, too. And finally,
know of cases where rotary inverters watt motor load of a refrigerator may
don t forget the refrigerator. It can
were used until they began to heat and not even start if your inverter is rated
come on while all these other loads
smoke. After cooling off a while, they less than 1500 watts.
are running.
were called back into service and per- Some motors do not work well with
Total your automatic loads and
formed without any problem. You some inverters. Why is this? Is it the
surges. Then add up demand loads,
can t beat that for durability. fault of the motor or the inverter? The
such as dishwasher and washing
On the other hand, a good solid-state answer is both. Motors are made as
machine, and your convenience loads,
inverter should have enough protec- cheaply as possible nowadays. Most
such as blenders and toasters, sepa-
tive circuitry to do the same. Of manufacturers leave out what is com-
rately and in possible combinations of
course, it shouldn t have smoke com- monly called the motor-run capacitor.
simultaneous operation. After you
ing out of it, but if it can t take motor The job of this capacitor is to smooth
have done that, you may find that you
surges and occasional overloading, out the interaction between the load
will have to monitor your combined
then it is under- designed. (motor) and the power source. Grid
loads to keep the size and cost of your
I mention under-design because power with its pure sine wave does
inverter to within reason.
inevitably an inverter is called on to not create as much bad feedback
There are a couple of other reasons
handle loads beyond its rated capacity. (inductive reactance) with such loads,
to look at your power requirements
People have a tendency to pinch pen- but some inverters do.
when considering an inverter. When
nies when buying an inverter, opting If you find that motors are not run-
you size your system and allocate
for the smallest size they can get away ning up to normal speed with your
your budget, the inverter should be
with. Then they push it to the maxi- inverter, try putting a 3- or 4-micro-
considered in your first purchase.
mum. Inverter manufactures know farad 400-volt electrolytic capacitor
Although inverters come hi a variety
this. For that reason, wise inverter across the motor windings. Such a
of sizes, it is false economy to buy a
builders factor in extra capacity to capacitor maybe purchased for under
small inverter you will soon outgrow.
insure long life. But don t rely on the $10 at most motor stores. Be sure to
Buy the inverter that will suit your
 fudge factor. Keep the use of your note polarity when installing the
future needs now.
inverter within its factory ratings. capacitor. If you don t know what you
There s a good reason for up-sizing.
Solid-state inverters may also have are doing, ask questions. Also, be sure
When starting out, you may have lots
an automatic demand on/off. This to put the capacitor on the motor side
of construction or remodeling work to
means that you can turn the inverter of the switch so that it does not load
do. A bigger inverter will help you
on and leave it on, using relatively, your inverter when the motor is off. If
through this period with ease. It sure
little power, until a load comes on. you use an inverter designed for reac-
is nice to have a quiet inverter instead
When it senses a load, the needed tive loads, this should not be a problem.
The Third Year 33
A Backwoods Home Anthology
of a noisy generator when you are dle big loads. The stack will be called boast a two-year warranty and so far
doing carpentry. And besides, what if on only if needed. there have been few warranty repairs.
your generator breaks down? Then Should you ever have a problem All units are delivered by UPS,
you are faced with a repair job before with one of your cascading inverters, meaning cheaper warranty repair and
you can do the construction work. you will still have the others. Not a delivery price. These units have no
Once you are settled in, the over- bad idea. For planning purposes, cas- adjustments to fail. The inverter is
capacity of a larger inverter bought for cading inverters make sense, too. self-monitoring and modifies its inter-
construction purposes still has merit. Let s say you are just getting started, nal workings and corresponding parts
Operating inverters at or near capacity and want to power your home with specifications as the temperature
may give high efficiency, but also may PV. Your eventual full complement of changes. These inverters have very
lead to shortened inverter life. Some, loads will be large. However, in the high surge capabilities and will run
over-capacity is good insurance. beginning you may be operating on a large loads for the short periods of
On the other hand, a small inverter limited budget or requiring a- limited time that are needed in a remote site
may be all you can afford, especially amount of power. By using the first of home. The basic 2000-watt 12 volt
if you are installing PV in stages. Be a cascading inverter, you can squeeze model or the 2500-watt 24 volt model
1
sure to plan each  additional part of by. Later you can add on as needed. will run the right /3 HP deep well
your wiring though, or you will have A word of warning: Never put two pump and the right washing machine
imbalanced circuits. If you use a cou- AC inputs on the same circuit or at the same time. Turn on voltage, turn
ple of smaller inverters for your loads, something will burn. That means off voltage, and charging ampere are
you have insurance in the form of never feed your generator or grid all user selectable. The best part of all
redundancy. If one inverter should power into a circuit being powered by is that they are much cheaper than
fail, you ll still have the other one. an inverter at the same time. Wire any of the competition.
However, if the bigger one fails, will your home so that this can never hap- Disadvantages
the smaller one adequately operate pen. Never feed generator or inverter With all this talk about the wonders
important loads? power into the utility grid without the of inverters and their benefits, what
This brings us to cascading invert- power company s permission. In the are the disadvantages? We will not
ers. While no inverters can be paral- first place, it is illegal. More impor- even consider poor quality, low-effi-
leled on the same circuit without one tantly, it endangers power line workers. ciency inverters. Of the good inverters,
burning up the other, some inverters Buying an inverter there will inevitably be problems, usu-
can be tandemed for increased capacity. When you buy an inverter, get one ally in the first few month, as they
Heart Interface was the first compa- that can be repaired. And hope that the  burn-in or get adjusted to regular
ny to address the needs of the inverter company that manufactured it and the use. Other problems occur because of
market and was the most popular person who sold it to you will stay in bad installation, undersized DC input
inverter for remote home use. Their business. Buying close-out inverters wires, rough handling, and overloading.
first inverters reached peak efficiency is unwise. Where will you get techni- A disadvantage already mentioned is
at a very low load level, one or two cal support when you need it? your reliance on the inverter to power
lights. Their surge capabilities were It is always a good idea to ask your all your loads. Should your inverter
good, and they ran motors efficiently. seller if you can call for technical fail even if fully warranted and all the
Among their deficiencies were relia- information after the inverter is in repairs or replacement are cost-free,
bility, changing internal adjustment place. If you have hired someone to you will be without all or some of
and slow warranty repair. Many of install your system, get a service con- your load-carrying capacity. If every-
these questions were met by Heart tract that includes replacement. This thing electrical is AC, then you are out
Interface. In 1985, Heart Interface may cost extra but it may be worth it of luck -and power. Let?s hope. you at
began marketing inverters which because should the inverter fail you least planned in DC emergency fights
could be tandemed for bigger won t be without power. and water to carry you through any
loads.They did this to improve effi- All inverters must be tested when waiting periods for parts and repairs.
ciency and to eliminate the costly installed. Some may have to be adjust- A less important disadvantage of
5000-watt inverter. Unless your loads ed for the specific installation. Be sure inverters is efficiency. This has
are always big, a 5000-watt inverter is to ask  What if .. ? If you don t like become, a secondary consideration
not such a good idea. It just doesn't~t the answer, shop elsewhere. A rep- since the advent of 90% plus efficiency
make sense to use a 5000-watt invert- utable inverter sales outlet should sat- field effect transistor (FET) inverter
er to power a 100-watt load. On the isfy your every need. technology. But it still should be con-
other hand, if you need an inverter to Probably the most popular inverter sidered. If your system is small and your
handle a 5 -watt, you must use one. today is the Trace. The Trace budget limited, perhaps an inverter is
Cascading or tandem inverters get Engineering Co. is a spin-off of Heart, not for you. A 10% loss can be costly
around this mix. By stacking up to and the company solved many of the if you are operating on a shoestring.
four 2500watt inverters, you can han- problems in the Heart units. They
34 The Third Year
A Backwoods Home Anthology
Conversely, an inverter makes on these systems one for buying the array while monitoring the power
wiring a lot cheaper. You can use power and one for selling. Parity pric- output. Therefore, the array s highest
readily available low-cost standard ing for alternative power systems is wattage regardless of a standardized
wiring and hardware. Switches and pretty much a thing of the past. voltage is what the electric meter sees.
breakers and all the other goodies So the development of grid-inter- Do not confuse power tracking with
used in AC homes are relatively inex- connect windchargers proceeded physically tracking the sun. Power
pensive. The savings on the wiring smoothly with intertie equipment cost- tracking is electronic peak power-
along with the convenience and sav- ing about as much as a battery bank. seeking circuitry built into the power-
ings in using standard appliances Parity pricing and the elimination of conditioning equipment. In some DC
makes shoestring DC almost a thing battery maintenance were important PV water pumping systems, peak
of the past. selling points. power tracking is used to match array
output to optimum motor operation.
The PV/grid connection
It is possible to install a medium to
large-sized residential PV array and
remain hooked up to the utility power
grid. The advantage of this type of
installation is that the battery storage
system is eliminated. When the sun is
PV ARRAY
not producing enough power to run
the home s electrical appliances, or
the peak load is greater than produc-
MAIN
tion, the grid-connect PV home gets
MOUNT
SYNCHRONOUS
INVERTER
its power from the utility company.
BRANCH
When PV production exceeds con-
CIRCUITS
sumption, the home is credited or
actually paid for the power it produces DISTRIBUTION PANEL
and puts into the utility grid.
Although there are a few thousand
The synchronous inverter interconnects the solar array to the home and the utility
grid-connect wind power homes, there
company. At the main distribution panel, power can flow in two directions.
are far fewer grid-connect PV homes.
During the day when an excess of PV power is being produced, it is sold to the
The two main reasons for this are eco-
utility. At night or when loads exceed PV production, power is bought by the utility.
nomics and a lack of marketing. It has
A few Gemini inverters were also The installation of synchronous
been difficult to present an economic
used on PV systems. Today we see inverters is generally beyond the
argument for PV/grid homes because
wind and hydro grid-interconnect sys- scope of the average homeowner. An
of their cost compared to buying a
tems using a variety of inverters simi- electrician is needed to do the job. In
monthly electric bill. But like so many
lar to . the Gemini. addition, utility company engineers
things that are not  cost-effective,
Synchronous inverters are line-com- will want to be involved to insure that
PV/grid homes could have been mar-
mutated, line-feeding inverters which the system is safe and producing
keted to the affluent for other than
change DC power to AC at standard power equal in quality to grid power.
economic reasons. Unfortunately, they
line voltages and frequency. In opera- The belief has been that when PV
were not. On the other hand,
tion, all the available DC power is prices drop, more grid- interconnect
Windworks, maker of the Gemini
converted to AC. If more power is systems would be installed. However,
Synchronous Inverter, did an excellent
available from the DC source than is when PV modules were selling at half
job building a reliable grid-connect
required by the home, the excess price (as they effectively, were when
inverter for wind systems and did a
flows into the AC grid where it is used the tax credits were available), there
good job selling it. In fact, they looked
by others. If less power is produced was no rush to install these systems.
so good that a Wisconsin utility com-
than is being used, the difference is Another factor that has limited the
pany bought the company.
provided by the AC grid. installation of PV grid interconnect
In addition, Windworks was able to
The inverters have circuitry capable systems is that utility companies con-
get parity pricing for their customers.
of handling unregulated DC power sidering grid interconnect of power
They did all the system engineering
input. For PV arrays, where the maxi- fields have the advantage of economy
with the approval of the utilities, and,
mum power output is not a function of of scale. Why would someone inter-
in exchange, customers usually got
a single variable, automatic tracking ested in PV and able to afford grid
one meter installed, which ran back-
circuitry seeks the highest output by interconnect system do so if their
wards when the system produced power.
incrementally varying the loading of utility company was going to do the
Nowadays we generally see two meters
The Third Year 35
A Backwoods Home Anthology
same thing on a larger scale at a lower Steve Strong, architect and designer, his  Impact 2000 PV-powered home
price per watt? Logically, by paying had done some excellent work on has been on Boston Edison, the utility
your monthly electric bill, you could grid-interconnect homes. All have that commissioned -the job, for they
buy utility solar electricity and support been costly, but they have paved the are now able to see that PV can work
a much grander step toward a PV society. way for general acceptance of the con- in their locale. It is also providing
As it turns out, few utilities are will- cept. Steve s first grid-interconnect valuable design guidelines for other
ing to put in PV power fields. Those PV home (built in Massachusetts) cost architects willing to follow in Steve s
utilities that have, publicize their about the same as non-PV homes in steps. "
(This article was reprinted with permission
efforts out of proportion and give the the same neighborhood. His work has
from Joel Davidson s book,  The New Solar
impression that they are  going solar been featured on the PBS series  The
Electric Home, which is available for $20.95
for their customers. Thus, rate payers All New This Old House. Perhaps
from aatec publications, P.O. Box 7119, Ann
are discouraged from the PV/grid one of the most important impacts of Arbor, MI 48107.)
connection because they mistakenly
think their utility company has
A BHM Writer s Profile
done it for them.
But even with short-term economics
Martin P. Waterman is a rural based
against them, some true pioneers have
writer whose work has appeared in
put in PV/grid-interconnect systems.
numerous publications in Canada and
Not only did they have to convince
the United States. He also writes a syndi-
themselves of the merit of PV, they
cated gardening column and is often a
also had to convince their utility com-
lecturer at horticultural symposiums.
pany. Now.that a few people have
Mr. Waterman is also involved
made the grid interconnection, we are
in agricultural research and is a rec-
seeing more cooperation from the util-
ognized fruit breeder striving to
ities. In fact, some of the most publi-
develop new hardier varieties for
cized grid interconnections have been
colder climates.
those done by utilities.
A BHM
Writer s Profile
Vernon Hopkins gets a kiss from
his great great granddaughter,
Devyn, on his 80th birthday.
Vernon is a retired trapper of 40
years, and he has witched wells for
the past 30 years with a success
rate of about 95%.
We profiled his extraordinary life
in Issue No. 2 and he subsequently
became a writer for the Backwoods
Home Magazine. Vernon brings his
extensive knowledge as a naturalist
and observer of the natural world
Getting electricity from the sun
to BHM.
36 The Third Year