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. (This section
is under construction)
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.
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Some Basics First...
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Watts
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 pulls 10 amps at 12 volts, or 1 amp at 120 volts,
it is still 120 watts. A watt is defined as a bunch of Joules per second, so saying watts
per hour is like saying "miles per hour per day".
Watt-hours
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).
Amps
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 so many electrons per second).
Amp-Hours
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.
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Watts - Or what size do I need?
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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 refrigerators (compressors).
- 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 later).
- 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 so.
- 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.
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Sine wave, modified sine, square wave - say what???
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Sine Wave
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Modified (Quasi) Sine wave
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Square Wave
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Sine wave, Modified Sine wave, and Square wave.
There are 3 major types of inverters - sine wave (or "true" 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 (quasi-sine)
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.
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- 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 off.
- Square Wave
Very few but the very cheapest inverters any more are square wave. A square wave inverter
will run simple things like tools with universal motors with no problem - but not much else.
These are seldom seen any more except in the very cheap or very old ones.
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