Selecting an Inveter - Solar and Backup
How to select an inverter for a solar system. Covers sine wave, modified sine
wave, grid tie, and backup power.
We carry many types, sizes, brands, and models of inverters. Various options
are also available. Choosing which one is best from such a long list can be
a chore. There is no "best" inverter for all purposes - what might be great
for an ambulance would not be suited for an RV. Power output is usually the
main factor, but there are many others.
There are many factors that go into selecting the best inverter (and options)
for your application, especially when you get into the higher power ranges (800
watts or more). This page should give you the information you need to get your
selection down to what will work best for you.
Some Basics First...
The poor watt is often misunderstood. Watts are basically just a measure of how
much power a device uses when turned on, or can supply. A watt is a watt - there
is no such thing as "watts per hour", or "watts per day". If a something uses
100 watts, that is simply the voltage times
the amps. If it draws 10 amps
at 12 volts, or 1 amp at 120 volts, it is still 120 watts. A watt is defined as
one Joule per second, so saying watts per hour is like saying "miles per
hour per day".
A watt-hour (or kilowatt hour, kWh) is simply how many watts times how many
hours that is used for. This is what most people mean when they say "watts per
day". If a light uses 100 watts, and it is on for 9 hours, that is 900
watt-hours. If a microwave uses 1500 watts, and runs for 10 minutes, that is
1/6th of an hour x 1500, or 250 WH. When you buy power from your friendly
utility (look at your last bill), they sell it to you at so much per kWh. A kWh
is a "kilowatt hour", or 1000 watts for one hour (or 1 watt for 1000 hours).
An amp is a measure of electrical current at the moment. (Amps do not come in
"amps per hour" or "amps per day" either). Amps are important because it
determines what wire size you need, especially on the DC (low voltage) side of
an inverter. All wire has resistance, and amps flowing through a wire makes
heat. If your wire is too small for the amps, you get hot wires. You can also
get voltage drops in the wire if it is too small. This is not usually a good
thing. An amp is defined as 1 Coulomb per second.
A Coulomb is the charge of 6.24 x 1018 electrons. Therefore, 1 Amp is equal to the charge of 6.24 x 1018 electrons passing a point in a circuit in 1 second.
Amp-hours (usually abbreviated as AH)
are what most people mean when they say "amps per hour" etc. Amps x time = AH.
AH are very important, as it is the main measure of battery capacity.
Since most inverters run from batteries, the AH capacity determines how long you
can run. See our battery
page for much more detailed information.
Watts - Or What Size do I Need?
Peak Power vs Typical or Average
An inverter needs to supply two needs - Peak,
or surge power, and the typical or usual power.
- Surge is
the maximum power that the inverter can supply, usually for only a short
time - a few seconds up to 15 minutes or so. Some appliances, particularly
those with electric motors, need a much higher startup surge than they do
when running. Pumps are the most common example - another common one is
- Typical is
what the inverter has to supply on a steady basis. This is the continuous rating.
This is usually much lower than the surge. For example, this would be what a
refrigerator pulls after the first few seconds it takes for the motor to
start up, or what it takes to run the microwave - or what all loads combined
will total up to. (see our note about appliance power and/or name tag
ratings at the end of this section).
- Average power
would usually be much less than typical or surge, and is not usually a
factor in choosing an inverter. If you run a pump for 20 minutes and a small
TV for 20 minutes during a one hour period, the average might be only 300
watts, even though the pump requires 2000. Average power is only useful in
estimating battery capacity needed. Inverters must be sized for the maximum
peak load, and for the typical continuous load.
Power Ratings of inverters
Inverters come in size ratings all the way from 50 watts up to 50,000 watts,
although units larger than 11,000 watts are very seldom used in household or
other PV systems. The first thing you have to know about your inverter is what
will be the maximum surge, and for how long. (More about 230 volts pumps etc
- Surge: All inverters have a
continuous rating and a surge rating. The surge rating is usually specified
at so many watts for so many seconds. This means that the inverter will
handle an overload of
that many watts for a short period of time. This surge capacity will vary
considerably between inverters, and different types of inverters, and even
within the same brand. It may range from as little as 20% to as much s 300%.
Generally, a 3 to 15 second surge rating is enough to cover 99% of all
appliances - the motor in a pump may actually surge for only 1/2 second or
- General Rules: The inverters with
the lowest surge ratings are the high speed electronic switching type (the
most common). These are typically from 25% to 50% maximum overload. This
includes most inverters made by Statpower, Exeltech, Power to Go, and
nearly all the inexpensive inverters in the 50 to 5000 watt range. The
highest surge ratings are the transformer based low-frequency switchers.
This includes most Xantrex, Magnum, and Outback Power. Surge ratings on
these can range up to 300% for short periods. While high frequency switching
allows a much smaller and lighter unit, due to the much smaller transformers
used it also reduces the surge or peak capacity.
- Pros and Cons: Although the high
frequency switching type don't have the surge capacity of the transformer
based, they do have some definite advantages. They are much lighter, usually
quite a bit smaller, and (especially in the lower power ranges) they are
much cheaper. However, if you are going to run something like a submersible
well pump, you will need either very high surge capacity or you will need to
oversize the inverter above it's typical usage, so that even at maximum
surge the inverter will not exceed it's surge rating.
Sine Wave, Modified Sine and Square Wave - Say What?
Modified Sine Wave
Sine Wave, Modified Sine Wave, and Square Wave.
There are 3 major types of inverters - sine wave (sometimes referred to as a "true"
or "pure" sine wave), modified
sine wave (actually a modified square wave), and square wave.
- Sine Wave
A sine wave is what
you get from your local utility company and (usually) from a generator.
This is because it is generated by rotating AC machinery and sine waves are
a natural product of rotating AC machinery. The major advantage of a sine wave
inverter is that all of the equipment which is sold on the market is
designed for a sine wave. This guarantees that the equipment will work
to its full specifications. Some appliances, such as motors and
microwave ovens will only produce full output with sine wave power.
A few appliances, such as bread makers, light dimmers, and some battery chargers require a sine wave to
work at all. Sine wave inverters are always more expensive - from 2 to 3 times as much.
- Modified Sine Wave
A modified sine wave inverter actually has a waveform more like a square
wave, but with an extra step or so. A modified sine wave inverter will work
fine with most equipment, although the efficiency or power will be reduced
with some. Motors, such as refrigerator motor,
pumps, fans etc will use more power from the inverter due to lower
efficiency. Most motors will use about 20% more power. This is because a
fair percentage of a modified sine wave is higher frequencies - that is,
not 60 Hz - so the motors cannot use it. Some fluorescent lights will not
operate quite as bright, and some may buzz or make annoying humming
noises. Appliances with electronic timers
and/or digital clocks will often not operate correctly. Many appliances
get their timing from the line power - basically, they take the 60 Hz
(cycles per second) and divide it down to 1 per second or whatever is
needed. Because the modified sine wave is noisier and rougher than a
pure sine wave, clocks and timers may run faster or not work at all.
They also have some parts of the wave that are not 60 Hz, which can make
clocks run fast. Items such as bread makers and light
dimmers may not work at all - in many cases appliances that use
electronic temperature controls will not control. The most common is on
such things as variable speed drills will only have two speeds - on and
- Square Wave
Very few but the
cheapest inverters are square wave. A square wave inverter
will run simple things like tools with universal motors without a problem,
but not much else. Square wave inverters are seldom seen any more.