The energy we receive off of the 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 current. There 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 are:

• 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 off-the-grid inverters.

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.

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 evaporated.

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 will find:

• 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 inverter itself.

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 requires.

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.