Invertors

An Inverter is used to change Direct Current (DC) from the PV modules or battery banks to Alternating Current (AC) which is the form most modern appliances need to operate.

Inverters are classified as either Stand-Alone invertors which are used in Stand Alone systems independent of the utility grids, Utility interactive or Grid Interactive invertors used for Grid Interactive PV Systems which operate in conjunction with the utility mains synchronizing the output phase, frequency and voltage with the utility or Bi-Modal inverters that can act as both stand alone or grid interactive.

Another classification for inverters is the type of waveform they product. The three most common waveforms include the following:

• Square Wave (Basic and least efficient)
• Modified Sine Wave (Better than Square Wave)
• Sine Wave (Best possible)

A square wave AC current switches between positive and negative values every half period and is the most basic form AC current produced by an Inverter. They often have difficulty starting motors, are least efficient and produce a lot of distortion that can sometime be heard as a buzzing sound. Sine Wave inverters are the most expensive of the inverters but produce the purest AC current similar to the AC current in the grid, have the highest efficiency and the least distortion. Modified sine Wave inverters are in between and are used in PV systems that do not operate sensitive equipment.

Square wave inverters provide little output voltage control, limited surge capability and considerable harmonic distortion. They are appropriate only for small resistive heating loads, small appliances and incandescent lights. They can actually burn up motors in certain equipment.

Modified square wave inverters are appropriate for operating a wide variety of loads, including motors, lights and standard electronic equipment like TVs and stereos. Certain devices may pick up inverter noise and clocks and microwave ovens that run on digital time will run either fast or slow.

Sine-Wave inverters can be used to operate sensitive electronic hardware that require a high quality waveform and have many advantages over the over inverters including high surge capacity. Sine Wave inverters can also feed electricity back to the grid.

A system designer should know the optimal features of an inverter when choosing one and should make sure it can support the current design as well as expansion of PV capacity in the future if required. Some features that must be considered when choosing an inverter include good efficiency rates (more than 80%), low standby losses, a good surge capacity to support motors, good quality of AC waveform, low distortion or noise reliability and cost. The inverter should be sized to supply expected continuous load demand and a surge rating capable of starting motor loads and other loads with surge requirements as calculated in the design of the system. Another important aspect to take into account when choosing an Inverter is the future goal of the system - if you intend to increase the power producing capacity of the system in the future it is a good idea to buy an inverter with a larger capacity to start with and add PV modules as you go forward.

Grid Interactive Inverters have a KEY function built into them called Islanding that prevents PV generated electricity to flow into the power lines when the electricity Grid is down. This is very important as electric company employees may be working on down power lines and could be seriously injured if PV generated electricity is not prevented from entering the grid.

Another note on Inverters, most inverters are designed to be in a stand-by or sleep mode and wait for a certain wattage before they will turn on and some small loads may be smaller than the "turn on point" of the inverter. The inverter will not turn on and a larger load may need to be applied to activate the inverter. Certain computers and electronic devices do not present a load until a line voltage is available which could be a problem as the inverter is waiting for a load and the load is waiting for the inverter.

Typical Inverter Specifications are listed below: