Inverter

A standard offgrid system with an inverter. An inverter is connected directly to the battery bank through an overcurrent protection device.

PV modules and energy storage systems function with direct current (DC), yet due to the advantages of alternating current (AC) the majority of the appliances produced in the world are built to function with an AC input source. This means that it is common to incorporate an inverter, which can convert from DC to AC, into any any system that is intended to function with more than just basic appliances like lighting, cell phones and radios. There term inverter covers many different products with different functionality and cost, thus it is important to understand the different factors that go into choosing an inverter to determine the right one for your application.

Inverters are the most electronically complex component and thus are a likely failure point, meaning that investing in a quality inverter is a wise choice. If the inverter in a system that is built around AC fails, then the system will stop functioning completely. For this reason, many system designers of small offgrid systems choose to incorporate DC lighting or a DC-based refrigerator into a system to offer the a more robust system that will continue to provide these basic functions even in the event of an inverter failure.

Waveforms

A comparison of the different inverter output wave forms. 1. Pure sine wave 2. Modified sine wave 3. Square wave

The most important characteristic of an inverter - that helps to define its functionality and quality - is the waveform of its alternating current output. The AC that the grid supplies comes in a pure sine wave, which is what all AC appliances are designed to use as their input. A smooth variation between directions of current flow that is operating is necessary for the proper functioning of various complex appliances, but for other simpler appliances it doesn't matter. The three types of output wave forms that are available in the market are the following:

• Pure sine wave (PSW):An inverter that outputs AC in a sine wave that is indistinguishable from that supplied by the electricity grid. The creation of a pure sine wave requires a more complex inverter design that costs more, but the additional cost of a pure sine wave inverter often bring additional efficiency and quality. It is recommended that any system that relies on AC continously to supply loads incorporate a PSW inverter.
• Modified sine wave (MSW):An inverter that outputs AC in a waveform that is more rough than a pure sine wave, but that is indistinguishable for most appliances. MSW are a more economical option for small systems that require AC, but that are only going to supply simple loads (cell phones, radios, lights etc.) Should not be considered if a system is intended to be used with certain types of loads - motors, laser printers, battery chargers, washing machines, high-end music equipment - as it cause them to work improperly or damage them. Motors will consume roughly 25% more energy with a MSW inverter compared to a PSW inverter and the life of the motor will be shortened as that extra energy will be converted into heat. If a system doesn't rely on AC continously, but only periodically for smaller loads, then it is an option that should be considered.
• Square wave: The simplest and cheapest form of inverter. Current direction switches very rapidly and can damage certain appliances. Will work fine with simple loads like cell phones and lighting, but not recommended for use in a PV system. Frequently poorly designed and manufactured. A modified sine wave or pure sine wave inverter will not cost very much more.