The Inverter is the center of most renewable
energy (RE) power systems. It can perform many functions, depending
on the model, but its basic one is to transform DC
electricity, which comes from solar panels and is stored in batteries,
into AC electricity, which is used in
most American households and feeds the regular wall sockets we all know.
INVERTER, CONVERTER, TRANSFORMER: What's
the difference?
While Inverters turn DC electricity into AC electricity,
Converters
change the voltage of DC electricity (up or down), and
Transformers change the voltage of AC electricity (up or down).
Comparing Inverter Features:
1) WAVE FORM:
This refers to the shape of the electrical AC wave that is produced
by the inverter. The ideal form of AC power is the pure, smooth sine wave;
it is what the electrical grid is supposed to provide. Different
inverters have different wave forms that range from rough approximations
of a sine wave to pure sine waves that are smoother, "cleaner,"
than that of the grid.
Modified Sine Wave is the wave form
most inverters produce. It is more accurately called Mod square wave,
since it more closely resembles a square
wave than a sine). Since this wave form isn't fluid in shape and
has large "corners," it sometimes causes problems with
certain appliances.
Ultramodern washing machines and a few other appliances
with electronically controlled, variable-speed motors won't work at all
on mod-sine power, and some people in the RE field come
down hard on mod-sine inverters because of this.
However, mod-sine isn't as bad as all this might make it seem.
Most people experience
no problems whatsoever with their mod-sine system: We
have found mod-sine inverters to be great for low cost, off-grid homes
and grid backup systems. Despite the claims, mod-sine inverters work perfectly
well with the vast majority of appliances including most stereos and computers.
We ran our entire shop off a Trace DR 2424 for years, and many of our
more affordable systems still use DR inverters.
The best part is: they're the most reliable.
In our years of RE work, we haven't had a single DR fail in the field.
The next step up from mod-sine is the wave form generated by the Xantrex/Trace
SW and SW+ series. Trace calls it a sine wave, but we call it a stepped-sine
wave, since it is composed of 34 to 52 little "steps", depending
upon the amount of load placed on the inverter at the time. The stepped
sine is utility grade and can be fed (sold) to the grid, although it may
not be clean enough for some critical applications. Stepped-sine inverters
can cause the same problems as mod-sine inverters, though less pronounced.
(see side bar of Mod sine problems).
The OutBack inverters also produce a stepped sine wave, though they too
are billed as sine wave inverters. Their wave-form is much smoother than
a Xantrex SW or SW+, having around five times as many steps. For many
applications the OutBacks seem have the best of both worlds: a very clean
wave-form, with surge capacity and the features of a general use inverter,
such as battery charger and transfer switch.
Pure sine wave
inverters are made by a few manufacturers. As mentioned above, the power
provided by them can be "cleaner" than grid power. Exeltechs
are the cleanest of the clean, being the preferred inverter for off-grid
recording studios. High-end recording studios with grid power are sometimes
set up with pure sine inverters for the musical equipment and recording
gear, so as to guard against noise and voltage dips and spikes from the
grid. Sometimes people will dedicate a small pure sine inverter for their
stereo, computer, or other critical load, while using a larger, less expensive
mod-sine inverter for all the other house loads.
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2) GRID-INTERTIE vs. OFF-GRID:
Inverters for off-grid battery-based systems are designed for
stand-alone power systems that can operate independently of the electric
grid. Many of these inverters have AC inputs
for grid power as well; so, if grid power is
available to the site, it can be used for back-up power and emergency
battery charging the way one would use a generator. In this way, an "off-grid"
inverter can have the option of grid back-up. This is different
than what is called grid-tie.
Grid-tie
systems can be of two types: batteryless and battery-based.
Batteryless
grid-tie systems don't use batteries to store electricity,instead
they use the electric grid for that purpose: electricity is drawn (bought)
from the grid when energy use is higher than system
production, and electricity is fed (sold) to the grid when more energy
is produced than is used.
As utility companies become more accepting of grid-tied RE systems with
buyback or net metering
capabilities, they are becoming more popular. Having a batteryless system
not only does away with the expense of buying and periodically replacing
batteries; also avoided are the hassles of maintenance, as well as the
need to monitor the system to make sure that the energy demands placed
on it balance the energy production, lest the batteries become too drained.
Since excess demand will simply be handled by the grid, the result is
a very self-regulating system that needs little attention.
A batteryless grid-tie inverter, like the SMA SunnyBoy, is able to accept
a wide range of Solar module power that can be much higher than the voltages
standard with battery-based inverters (up to 600V with the SunnyBoy 2500).
Wiring a solar array for a higher voltage saves money on wire and reduces
the power losses associated with lower voltages.
Battery-based grid-tie
systems combine the best of both worlds: surplus power can be sold to
the utility company, and power can be bought from the utility's electric
grid when needed. Plus, if grid power fails, the system does not shut
down with it. Programming is still being developed
to keep up with all the choices possible with this arrangement. When to
sell, when to charge the battery, etc.
Note:
The rest of this guide does not apply to inverters without batteries.
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3) PRICE & CAPACITY vs. NEED:
Price
is obviously a big consideration. Buying too much inverter is a waste
of money: getting an inverter with more power or more features than needed,
or getting a pure-sine wave inverter when modified-sine wave power would
run all your loads perfectly well. But not buying enough inverter can
also be a waste: getting a cheapo that will be a door stop in a year,
a quality one that is smaller than you need or might soon need, or a mod-sine
to power sensitive electronics. The very first step in selecting an
inverter is to look critically at the loads it will need to run. The cost
of your inverter, and of your power system as a whole, is directly related
to the amount of electricity you use.
4)
SIZING THE INVERTER:
To do this, you must first figure out and add
together the wattages of all the continuous
loads that will likely run simultaneously, in
a highest electrical use scenario. (Relax, absolute precision isn't important;
it's best to overestimate a bit). These include lights, home entertainment,
computer, microwave, etc. (To help you with this, see the Solar
Sizer for a more complete listing of household loads.) This total
must be the same or less than the continuous wattage rating of the inverter
you buy.
Next, to determine the surge capacity of the
inverter you need, add up the surge watts of all the loads that might
start at the same time. Then add this total to the continuous wattage
total you calculated above. Think of the water pump and the washing machine
turning on at the same time as the refrigerator compressor (three surge
loads) while you microwave dinner with the TV and half the lights in the
house turned on (continuous loads).
If your inverter is undersized, a combination of loads such as
this will trip the inverter's output breaker; this is known as nuisance
tripping, and it truly is a nuisance.
Inverter Power Rating to start an AC well pump
(with no additional loads)
An inverter sized by these minimum guidelines will dip its voltage during
the starting surge. This is not harmful, but it will cause lights to dim.
Fluorescents may blink off, and computers are likely to crash. To eliminate
voltage dips, oversize the inverter by an additional 50% minimum plus
the watts capacity required to handle other household loads at the same
time.
Inverter Sizing Chart Copyright ©2002 by Dankoff Solar Products,
Inc
Link to Complete Dankoff Article
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5) CHEAPO vs. QUALITY:
There is a
very important distinction to be made between the many cheapo inverters,
which are sold in import tool catalogs and big box stores, and the high
quality inverters made specifically for independent
and renewable power applications. They're both called inverters,
but they're very different products, each with its own purpose. The cheapo
inverters are meant to be hooked up to a car's battery for occasional
use: to run a power tool or tv out in the field for example. The inverters
we sell are designed to run an independent power system 24 hours a day,
7 days a week. The cheapo inverters simply are not designed for continual
use or made of components that can hold up to it.
From time to
time we encounter home-brew-boys who insist that money spent on a
quality inverter is money
wasted. We also run into some of these same guys who decide to invest in
some reliable equipment, after burning through a couple of the cheapo inverters.
It took them some real world experience to realize that the purchase
price is the least important of all the differences between cheapo inverters
and the quality ones. In the long run they spent more money than it
would've cost to do it right the first time.
Let's take
a closer look at a couple of mod-sine inverters with comparable wattage
ratings:
*
The above chart
is misleading in so far as the "cost per year" is based only on
the inverter's price, which, in the case of the offbrand 2500, does not
include a battery charger or transfer switch. So, for the comparison
to be truly equal, both of these should be added
to the cost of the Brand X.
A 75 amp, 12 volt battery charger would cost upwards of $500
(the DR 2412 has a 120 amp charger), and a 30 amp transfer switch
would cost around $100. Now, since the charger and transfer switch are already
incorporated into the DR, labor and materials to wire the cheapo inverter,
transfer switch, and charger together should
also be factored into the equation. Thus, the true price of the cheapo inverter
setup would be $1050, plus wire, conduit and labor--more money for a lower
quality system that will last for a fraction of the time that the DR will!
But you could
also just grab that Brand X 2500 (13.5 lbs.) with one hand and sling a Xantrex
DR 2412 (45 lbs.) under the other arm, and you'll feel another big difference--the
DR and other quality inverters are transformer based, whereas the Aims,
and other such inverters, use high-frequency switching (transistor based),
which is much more prone to burn out.
A word
to the thrifty: when price shopping, make sure you compare apples to apples.
And get some advice from a professional before you buy. If you are planning
on living with this equipment and want it to last, don't skimp; in the
long run, buying quality is the best deal.
Next:
Inverter Comparison Charts.
Link to
Catalog for Current Listings.
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