Solar Inverter

What is an inverter?

A power inverter or invert is a device that converts Direct Current (or DC) to Alternating Current (or AC) using transformers, switching and control circuits. And this is the basic function of any inverter: be it solar or regular inverter. A home UPS or home inverter take the DC power from the batteries and convert it to AC power used by appliances. A solar inverter also does the same if it is an "Off Grid" solar power system. In case of "Grid Connected" the DC power is from the solar panels and AC power is given to the grid.

Internal view of a solar inverter. Note the many large capacitors (blue cylinders), used to store energy briefly and improve the output waveform.

A solar inverter, or converter or PV inverter, converts the variable direct current (DC) output of a photovoltaic (PV) solar panel into a utility frequency alternating current (AC) that can be fed into a commercial electrical grid or used by a local, off-grid electrical network. It is a critical balance of system (BOS)-component in a photovoltaic system, allowing the use of ordinary AC-powered equipment. Solar power inverters have special functions adapted for use with photovoltaic arrays, including maximum power point tracking and anti-islanding protection.

Solar inverters use maximum power point tracking (MPPT) to get the maximum possible power from the PV array. Solar cells have a complex relationship between solar irradiation, temperature and total resistance that produces a non-linear output efficiency known as the I-V curve. It is the purpose of the MPPT system to sample the output of the cells and determine a resistance (load) to obtain maximum power for any given environmental conditions.

Advanced solar pumping inverters convert DC voltage from the solar array into AC voltage to drive submersible pumps directly without the need for batteries or other energy storage devices. By utilizing MPPT (maximum power point tracking), solar pumping inverters regulate output frequency to control the speed of the pumps in order to save the pump motor from damage.

Solar pumping inverters usually have multiple ports to allow the input of DC current generated by PV arrays, one port to allow the output of AC voltage, and a further port for input from a water-level sensor.

What is the difference between solar inverter and regular power inverter?

Power inverters or home UPS have been in use in India since quite some time. Thanks to frequent power cuts, home UPS or power inverter industry has done well in India. But with increasing interest in solar power and solar panels many people are thinking of using Solar Power to charge the batteries to save electricity drawn from the grid. Having heard of solar inverter, many people wonder if their existing inverter system is useful or not. In past, we at Bijli Bachao have got this question many times: what is the difference between solar inverter and regular inverter? With this article we would try to answer the question.

Working of a Home UPS/Inverter

A regular home UPS/inverter system has a system of inverter and batteries that is connected to the home power connection. When the power is coming from the grid, the UPS/inverter system charges the batteries using the power coming from the grid. When the power is off, the inverter takes the DC power from the batteries and converts it into AC used by appliances. There is an automated switch in the system that senses if the grid is not supplying power and switches the UPS into battery mode.

Working of a Off Grid Solar Powered system

An off grid solar powered system has an assembly of solar panels, charge controller, batteries and inverter as shown in the image below. The batteries are charged using solar panels and the inverter has the same function of converting DC to AC. It can also have an automated switch to sense if grid is not supplying power that can change the power source of the connected home from grid to batteries. Depending on the need the system can be designed to: 1) To completely rely on solar power 2) Charge the batteries from solar power when sun is available and from the grid when it is not.

An existing home UPS or Inverter system can be converted to be used with an off grid powered system by just adding a switch which can control the change source: solar or grid (based on availability).
Having said that, to answer the question posed in the title of this article: there is not much difference between a solar inverter used in off grid system and a regular inverter.

Working of a Grid Connected Solar Power System

A grid connected solar power system additionally supplies excess electricity to the grid. People go for this kind of system when they feel that they have enough space to install solar powered system and they can sell the extra electricity generated back to the electricity distribution company. The structure of a grid-connected system is shown below:

The purpose of the inverter in this kind of system is also to convert DC (generated from Solar Panel) to AC. But it additionally needs a MPPT (Maximum Power Point Tracking) feature to be built in it. Because the voltage generated by Solar Panel varies depending on the temperature and availability of sun (more on this on our article: Solar Glossary – photovoltaic, panels, modules, cells, voltage, watt and current), MPPT enables solar inverters t draw maximum power from the solar panels. The solar inverter used for grid-connected solution also includes an anti-islanding functionality to prevent risks to grid in case there is power failure in the grid.