Why are inverters crucial for PV systems? Without these devices it’s not possible to use solar energy for appliances, electronic devices or lamps. That’s why it is so important to know what an inverter is and what it does.

What is a photovoltaic inverter?

Inverter is a key element of the photovoltaic system: it is an electronic device which “invert” the direct current generated from the solar energy into alternating current, used in our houses and offices. It acts as energy “translator”, because it makes energy usable for self-consumption or for the grid.

Inverters are a sort of medium size metal boxes, containing an electric panel.

Inverter: what does it do?

In addition to its conversion function it is also used to avoid wasting energy generated from solar panels. It must:

• Minimize (2%) energy dispersion caused by conversion;
• Regulate frequency and voltage of electricity (respectively 50Hz and 220-239v, both for houses and industrial buildings), despite the voltage variations outcoming from the photovoltaic generator;
• Optimize the power of the system, finding and keeping the right operating point;
• Send a report in case of internal failure or irregularities;
• Monitor the grid and protect the plant against overvoltage, short circuit or blackout.
• Monitor and control the temperature of the inverter itself, which depends on where it is located. It is better to choose a cool spot.
• Notify performance and operating data through the display.

How to choose an inverter

Once you have understood the importance of an inverter in a PV system you must consider three aspects in order to make the right choice for you:

• Power
• How modules are connected
• Type of inverter

Let’s deepen each of them.

Power of the inverter

Power may vary depending on the type of plant: it is necessary to estimate the maximum consumption that your household or your business activity may reach. To do so, you can sum up the energy amount consumed by the devices which are connected at the same time. Generally speaking the power of a domestic system varies from 2kW to 5kW. Commercial systems range from 10 to 20kW, industrial systems even higher (a photovoltaic plant, for instance, ranges from 500-800kW to many MW)

Panel connection to the inverter

It is also important to consider how panels are connected to the inverter.

• In a domestic system panels are placed in strings, namely a line of photovoltaic modules in series, therefore a string inverter will be necessary; its power ranges from 250W to 15kW.
• If multiple strings of panels are connected you will need a multistring inverter, recommended for those systems where part of the surfaces is in shade.
• In case of centralized connection, a central inverter is required, to which each panel will be connected; its power ranges from 15 kW to 1 MW, and it is a recommended solution for big plants.
• Finally, some plants could use microinverters, which are small inverters installed behind every panel, whose power is less than 250W. They can promptly monitor the performances and they can maximise the performances of each panel, therefore of the whole system.

Type of inverter

The last aspect is the type of inverter. We distinguish three different features to be combined:

• Single-phase inverter, recommended for small plants vs three-phase inverter mostly used for bigger plants and replaceable by groups of multiple single-phase inverters;
• Inverters with transformer, if required by law or if the plant requires grounding; otherwise there are inverters without any transformer, smaller, less bulky and with a higher yield, however they don’t provide any electrical insulation;
• Modified sine wave inverters, they are cheaper, but generate more interferences in terms of noise and temperature, they are not recommended for devices equipped with engine, nor for photovoltaic plants requiring pure sine wave inverter, which are more expensive but able to connect any device.

Inverter for storage systems

Storage PV system allows to store energy when energy consumption is lower than the generated energy and to give it back during the opposite situation. That’s why it is important to consider a hybrid inverter. This will offer you the possibility to convey the generated but not used energy into special batteries, whether or not you will proceed with the installation of the storage system.

How much does an inverter cost?

Before listing the prices, we need to distinguish the power of the inverter:

• For domestic use until 6kW, it may cost up to 700€ or 1000-1200€ for a hybrid inverter
• For commercial use from 10 to 20 kW, it can cost between 800€ and 2000€