The energy we receive off
electronic grid that feeds our
home is of a type known as Alternating Current or (AC). Normally the
alternation cycle is 60 times per second or what is known as 60 hertz.
Most of the appliances in our home are designed to operate using AC
are significant advantages for the electrical grid in using AC
current. One of the most significant advantages is that the
energy can travel a greater distance without needing to be boosted.
Unfortunately renewable energy generating systems such as solar panels
and wind turbines produce Direct Current
(DC). Consequently there is a mismatch between the power the
solar panels provide and the type of electricity our homes are
designed to use. This is where inverters come in.
Inverters are electrical components that will convert DC current
into AC current. Inverters are an essential component of a
residential or business photovoltaic system whether your system is
designed to be grid-tied or off-the-grid. When used with a
grid-tied solar PV system the inverter converts the DC power coming
from the solar panels and passes the AC power either to the existing
load center (circuit breakers) where it is used by your home's AC
appliances, or if not needed, it sends the electricity out to the grid
which is also based on AC current. When an inverter is used as
part of an off-the-grid home the inverter takes the DC current either
from the solar panels or from the batteries which also use DC current.
Types of Inverters
Though both grid-tied and off-the-grid systems require inverters,
the type of inverters that they need are very different.
Generally inverters can be classified into three types. These
- Grid-Tied Inverters - In a
grid-tied implementation the solar panels are usually wired in series
to produce very high voltages, usually between 200 and 600 volts.
By using high voltage smaller wiring can be used to connect the panels
together. This makes the panels easier to work with and reduces
wiring costs, which recently have become a significant expense due to
the rise in the price of copper. Grid-tied inverters are designed to
take as input the high DC voltages that are common in grid-tied
systems. They are also designed with certain safety features so
that if the grid-goes down for any reason the inverter will turn off
the current coming from the solar panels. This is done for
safety so that an electrical utility worker doesn't get shocked by the
current coming from your solar panels.
- Stand-alone Inverters - Stand-alone inverters are
designed for off-the-grid systems. They are designed to receive DC
power both from the solar panels and from the battery backup system.
Off-the-grid inverters usually have a number of features designed to
optimize the performance of your battery system. In addition they
are designed to work with lower voltages given that the batteries
are usually operating at between 12 to 48 volts. This is a much
lower voltage than what is used with a grid-tied inverter.
- Grid-tied Inverters with Battery Backup - These types of
inverters are used in hybrid systems which are grid-tied but also
have small battery banks as backup should the grid ever go down.
They tend to have features which are found in both grid-tied and
The Evolution of the Inverter
Inverters have evolved significantly in the last 25 years.
Early inverters were a bit unreliable but modern inverters now have
extremely good reliability records thanks to the stability of digital
circuitry. They have also improved in efficiency when it comes to
converting DC current into AC current. Early inverters often
lost a significant amount of energy in the conversion process but
modern inverters suffer only about a 5% loss with 95% of the DC
current being converted to AC current.
Probably the biggest changes in inverters has to do with the
quality of the energy they produce. The AC current we get off
of the electric grid is generated in the form
of a sine wave. If you are going to convert DC electricity into
AC electricity then you want to get the current to be as close to a
sine wave as possible to avoid interference. Early inverters
used a transistor to quickly switch the polarity of the direct current
from positive to negative at close to 60 times per second. This
approach creates a type of current which has what is known as a square
wave signal. Once the square wave current was created it was passed through a
transformer to increase the voltage up to either the 120 volts or 240
volts typically used in the home. Unfortunately, simple square wave
signals are subject to a type of interference called harmonic
distortion which tends to degrade the signal and can make them less
than compatible with the AC coming off-the-grid. In grid-tied
systems this represented a significant barrier to the acceptance of
solar panel generated energy by the utility companies.
The diagram below shows a picture of a square wave, a modified square
wave and a sine wave.
Modified Square Wave Sine Wave
In order to circumvent the problems associated with square wave
current inverters evolved and began using a more refined form of square wave called a modified square wave.
These inverters used field effect transistors (FET) or
silicon-controlled rectifiers (SCR) to add additional steps to the
wave. The result had less harmonic distortion and could handle
large surges in current more easily. While an improvement there
were still some devices in the home (digital clocks for example) which
did not perform as well using the modified square wave current. Today nearly all inverters generate high quality AC sine waves.
This is done through the use of sophisticated digital circuitry. Once
sine wave quality inverters became available most of the resistance
from the utility companies to grid-tying solar energy systems
Additional Inverter Features
Beyond just the basics of converting DC current to AC current, most
inverters today have a number of additional features, many of which
are quite useful. Here are some of the more common ones you
- Maximum Power Point Tracking (MPPT) - To understand this
feature it helps to remember that the total number of watts of
electricity our solar panels put out is a function of volts times
amps. For any given solar panel there is a point at which the
optimal combination of volts and amps will produce the most
electricity. This is called the maximum power point. This
power point changes throughout the day. Modern inverters will
account for this and adjust the voltage so that it is always at the
maximum power point. This will ensure that we get the greatest
amount of electricity possible from the solar panels.
- Ground-fault Protection (GFP) - The National Electric
Code (NEC) requires that inverters have protection from electrical
shocks and so have a type of circuit built into them that cuts the
current in microseconds should it detect a ground-fault or loss of
current. This makes the system safer.
- AC/DC Disconnects - Many inverters provide built-in
disconnects for either the DC current coming into the unit or for
the AC current coming out. These disconnects are needed if you
are going to safely work on the solar panels or should you ever need
to replace the inverter.
- Weatherproof Enclosures - In some installations it
may make the most sense to install the inverter on the outside of
the building. Some inverters come with a weatherproof box
which allows the unit to be safely installed outside.
- LCD Displays - Many inverters now come with very
informative digital LCD displays. Most will show you the
current amount of power being provided by the solar panels, the
daily and cumulative energy production, PV array voltage and current
and utility voltage and frequency. One new feature we have seen
popping up on some inverters is a calculation of the amount of
carbon dioxide you are off-setting by generating your own
electricity instead of getting it off of the grid. Its a fun
way to see first hand the impact you are personally having on
preventing global warming!
- Communication Ports - Because inverters now provide
digital readouts it is possible to pass this information via
communication ports either to a laptop or to a remote display.
Some of the newer inverters use wireless displays which means that
you can place a remote display unit in a convenient location in your
home and see how the system is performing without having to go down
into your basement or wiring closet to read the data off of the
Special Features found in Stand-alone Inverters
As discussed earlier, inverters which are designed for use off of
the grid usually come with an additional set of features. Here
are some of the more important ones:
- High Surge Capacity - When using an inverter off of the
grid all the power for using an appliance has to be generated by
your internal system. Certain types of appliances,
particularly those with large motors in them like washing machines
or refrigerators often need a strong surge of current in order to
start. An off-the-grid inverter is designed to handle strong
surges in current demand.
- Automatic Low Battery Shutoff - Some off-the-grid
inverters have the ability to sense when your battery bank is
getting too drained. If your batteries are drained much below
25% they can be permanently damaged so the inverters can be set to
cut off electric demand before this happens or may sound a warning.
This function is often referred to as a Low Voltage Disconnect (LVD).
- Generator Auto Start - Some inverters can go a step
further and automatically turn on a generator if it senses that the
battery bank has gotten too low. Once the batteries are
sufficiently charged it will then shut off the generator. This
feature can be very nice for off-the-grid homes when the owner is
away and not able to monitor the system.
- Battery Charging - Some off-the-grid inverters have a
battery charger built right into the inverter so a generator can
charge the batteries directly through the inverter. In this
case the inverter actually does conversion going the other direction
by changing the high voltage AC power coming from the generator back
into lower voltage DC power which can be used to charge the battery.
Buying an Inverter
When putting in a complete PV system the solar contractor will have
matched the inverter to fit the requirements of your solar system.
Usually the first step is to determine the total output of your solar
panels in watts and then match the inverter to it. The more solar
panels you have the more watts of power you will be generating.
The size of an inverter is measured by its maximum continuous output
in watts. Inverters usually range in size from 700 to 7000 watts
in capacity. Most homes tend to fall into the 3000-4000 watt
range. It is generally a good idea to get an inverter that will
handle more power than what your system initially outputs. That
way, should you ever decide to add more solar panels, you can do this
without having to replace the inverter.
For a grid-tied inverters it is important to match the voltage
coming from your solar arrays so that it does not exceed the input
range for the inverter. Most grid-tied inverters are designed to
accept DC current at between 75 and 600 volts. The solar panels
can be wired such that this range is not exceeded. One factor that has
to be taken into consideration in doing this is temperature. On
the high side the contractor has to account for the fact that on very
cold days the solar panels will actually put out more than their rated
voltage. So if the maximum voltage was 600 volts the panels have
to be wired such that they do not exceed 10% less than this in order
to account for the temperature effect. Going the other way,
voltage from your solar panels will drop when the temperatures are
very high so if the bottom end of the input range is 75 volts, then
the panels should normally put out more than this otherwise the
inverter will cut off because it does not have the minimum power it
Inverter prices are, for the most part, based upon the total
wattage AC output of the inverter. Some of the major
manufacturers of inverters are SMA America which produces a popular
inverter series called Sunny Boys, Fronius, Xantrex, Outback
and Magnum. A 3000 watt grid-tied inverter runs between $2500
and $3500 retail. 5000 watt inverters run between $4200 and $5200.
Given the advances in inverters it is a little hard to say how long
they will last because most of the newer models have not been around
very long. For purposes of financial planning it is probably
safe to estimate that a properly sized inverter will last at least 5
years though it could go much longer.