Solar Power Inverters

What is an inverter? What kind of inverter is used for solar home power?

In America and many parts of the world, homes and appliances are designed for alternating current or AC current.  The electricity that flows from the utility grid is alternating current.  Because the electricity that flows from solar panels is direct current, or DC current, the energy must be converted to alternating current that will operate appliances in your solar home.  If you are grid-intertied, the energy must match the electricity that the grid produces.  This is the primary function of the solar inverter: to convert DC energy to compatible AC energy.  Solar inverters today are sophisticated pieces of equipment with many other functions also.

The power inverter is a critical component in a solar electric system and is typically one of the most costly parts in a renewable energy system. But this is changing as overall demand increases.  The solar inverter market used to be dominated by only a few large companies but this has changed and many companies are now in the inverter business.  Technological and manufacturing advances make pricing more competitive.  And now there are options which give you choices which are less costly.

Most larger inverters function as battery chargers as well as current converters.  Transfer switches can also be incorporated into these inverter/chargers so that AC loads can be powered directly from a gas or propane generator when the battery charger is operating.

Choosing an inverter

Inverters differ by several factors:

• Wattage rating, or how much power in watts the inverter can deliver.
• Surge power.  How much power and for how long it can deliver the power needed to start motors or other loads.
• Efficiency, or how efficient the inverter is at low, medium and high power draws, as well as how much power is used when on idle.
• Quality or shape of the electrical wave, whether pure sine wave or modified sine wave.
• Functionality.  From simply converting DC to AC to the highly sophisticated, multifunctional gridtie inverters which accept and convert energy from several differrent sources and route excess into the grid in a compatible form and in a safe way.

Off-grid or stand-alone inverters

Off-grid inverters are available in three power waveforms:  square wave, modified square wave (or modified sine wave) and pure sine wave.  Square wave inverters are the lowest efficiency and lowest cost.  The prices of better quality inverters are low enough to make square wave inverters much less attractive.   Modified sine wave inverters are an economical choice where waveform is not critical for performance of special electronics.  Their high surge capacity allows them to start motors with high loads, and their high efficiency makes them economical with power when running small loads like stereos or lights or small fans.

Unfortunately, modified sine wave inverters can negatively affect, and even ruin, some low cost rechargeable flashlights, and appliances.  Their waveform is not compatible with many appliances, including laser printers, copiers, sewing machines, and washing machines.  Some televisions and audio equipment will have a background buzz or line that may be annoying.

Gridtie inverters

All gridtie inverters produce the pure sine wave that is compatible with utility electricity.  Sine wave inverters are a little more expensive, but they are compatible with almost any equipment or appliances that can be operated on grid power.    These multifunction inverters allow you to sell excess power to your utility company and also maintain a back-up battery bank for emergency power in the event of a utility power failure.

Note this major difference between off-grid power and gridtie power:  With off-grid power, as long as you do not use more electricity than you make, you will have power.  With gridtie power, if the grid goes down, your system goes down, too, UNLESS you have had the foresight to add a battery backup system.

Choosing a solar inverter can be challenging and you should not buy one just based on cost. Take your time. Talk with experts, because a 10% difference in power conversion efficiency could cost you thousands extra in more solar panels. Buy one that's big enough to expand so that you don't have to replace it as your electrical needs grow.

Micro-inverters are improving cost and efficiency of solar electric systems.

Micro-inverters for grid-tied applications are the newest advancement in the inverter industry. Micro-inverters allow each solar panel in a solar array series to function as an independent solar power system, which is advantageous for several reasons.

It is possible to produce useable electricity with only one panel and one micro-inverter, so you can start small.  As your electrical needs and budget grow, you can add one more panel and one more micro-inverter if you like.   Micro-inverters also improve overall output of a larger array because shading or malfunction with any panel do not affect the output of the rest of the array.   Conversely, with larger solar inverters, which are sized for the whole array of panels, if one solar panel performs poorly, then the whole array would perform negatively.  Using micro-inverters, each panel operates to its capacity, unaffected by the performance of the other solar panels.

Other inverters must be sized according to the overall wattage of your home's solar array.  If you have undersized your system you may have to buy another, larger inverter before expanding.  It's better to choose an inverter that is initially larger than you need, so that you can expand later.

Sizing your Solar Inverter

The size inverter you need depends on the total number of watts (or amps) of the appliances and equipment you want to run.  You have to do your homework here, even if you are going to ask for help from a sales person or electrician.  Nobody knows how much power you are going to use but YOU.  The information you may need for this exercise can be found on your current electrical bills and by doing load calculations based on the power consumption figures labeled on the backs of your appliances and tools.

This may sound like a daunting task, but with the installation of a solar power system, you become your own utility company.  You need to know if you will be using more energy than you make!

Most inverter manufacturers provide sizing charts, and many solar sales technicians and electricians can help you size your system.