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Power Conditioning Equipment for Grid-Connected Systems

A photo of an inverter, which is basically a small, beige metal box with a display window on the front and vents on the side.

Inverters condition electricity so that it matches the requirements of the load.
Photo credit: Trudy Forsyth

Root Mean Square (RMS) Values

Most electrical appliances and equipment in the United States run on alternating current (AC) electricity, which is also referred to as single-phase, 120-volt AC service in homes, offices and some manufacturing facilities. If you could look at the electricity flow coming from a wall outlet in your home, you would see a sine wave (Figure 1) that oscillates between -170 volts and +170 volts. The rate of oscillation for the sine wave is 60 cycles per second and the AC term is used for the voltage, current and power.

The value of AC voltage is continually changing from zero up to the positive peak, through zero to the negative peak and back to zero again (Figure 1). For most of the time, the value of the voltage is less than the peak voltage and is not a good measure of its actual value. Instead we use the root mean square voltage (VRMS) which is 0.707 (or about 71%) of the peak voltage (Vpeak). The Root Mean Square (RMS) value is the effective value of a varying voltage or current. It is the equivalent of steady direct current (DC) constant value giving the same effect.

Graph of the sine waveform for AC Voltage. The y-axis shows the voltage and ranges from negative 200 to positive 200. The x-axis shows time. The sine wave starts at zero and peaks to 170 before passing down through zero and then to a negative peak of negative 170 and back up to zero. The sine wave then repeats. A horizontal line at 120 is labeled RMS Value=0.707 Peak. This line shows that the root mean square voltage is around 71% of the peak voltage.

Figure 1. Sine wave waveform for AC Voltage, Current or Power

Many available renewable energy technologies, such as photovoltiacs (PV), produce direct current (DC) electricity. To run many standard AC appliances, the DC electricity must first be converted to AC electricity using inverters and related equipment. Note the RMS value of an AC sine wave is the equivalent DC power if you are using a resistive load like a toaster.

There are four basic elements to an inverter:

  • Conversion—of constant DC power to oscillating AC power
  • Frequency of the AC cycles—should be 60 cycles per second
  • Voltage consistency—extent to which the RMS output voltage fluctuates
  • Quality of the AC sine curve—whether the shape of the AC wave is smooth as shown or jagged (with switching noise or distortion).

Simple electric devices, such as hair dryers and incandescent light bulbs, can run on fairly low-quality electricity. A consistent voltage and smooth sine curve are more important for sensitive electronic equipment, such as computers or microwave ovens, which cannot tolerate much power distortion.

Inverters condition electricity so that it matches the requirements of the load. If you plan to tie your system to the electricity grid, you will need to purchase conditioning equipment that can match the voltage, phase, frequency, and sine wave profile of the electricity produced by your system to that flowing through the grid.

A series of safety requirements for inverters have been developed by Underwriters Laboratories, a leading safety-testing and certification organization. These requirements, referred to as UL 1741, apply to power-producing stand-alone and grid-connected renewable energy systems. Either you or your installer should contact your power provider to see which models they accept for grid-connection; most simply require a grid-interactive inverter listed by an organization such as Underwriters Laboratories.

These factors affect the cost of inverters:

  • Application (utility-interconnected, stand-alone, or both)
  • Quality of the electricity it needs to produce for stand-alone
  • Voltage of the incoming current
  • AC wattage required by your loads (for stand-alone systems only)
  • Power required for the starting surge of some equipment
  • Additional inverter features such as meters and indicator lights.

When you size your inverter, be sure to plan for any future additional loads you might have. In the case of a grid-tied system in which you want to enlarge your renewable energy system, it is often cheaper to purchase an inverter with a larger input and output rating than you currently need than to replace it with a larger one later. Please note you cannot just replace an inverter with a larger unit later on. You would need to change wiring, disconnects, fuses, and other aspects of the system, which can be a difficult situation.