Understanding Solar Inverters

hoosing the most suitable photovoltaic modules for your needs is essential, no doubt about that. However, the modules aren’t the only important thing when planning your own PV system: it comprises a whole set of equipment, with each piece playing a vital role. So you should really put much thought into deciding on the components of your photovoltaic system to ensure maximum yield.

One of the critical PV system elements is definitely an inverter, sometimes also called a solar converter – the device that makes power generated by the modules suitable for home use. But why do the modules produce power that cannot be used without an intermediary device? We must first look at some of the basics to figure this out.

What are inverters for home solar systems?

An electric current is created within a photovoltaic cell – the smallest building block of a module. It is made from two thin silicon layers, with one being positively charged and the other negatively charged, and these layers create an electric circuit when placed in contact. The moment sun rays hit the surface of the cell, they force the electrons within it to move through the circuit, generating an electric current. This type of current, where electrons flow in one direction around a circuit, is called direct current (DC).

Direct current is ideal for storing electricity within a battery, this is why portable electronics working on batteries (e.g.laptops and smartphones) use only DC power. Most video and audio processing devices, such as TVs and radios, also use direct current for most operations, and battery electric vehicles commonly run on DC motors. Additionally, high-voltage DC electric transmission systems allow more efficient power transmission over long distances.

However, our home appliances mostly use alternating current (AC). While in DC, the electrons move only in one direction, in AC, they jump back and forth at regular intervals. AC is produced by a wire spinning in a magnetic field. As it rotates, it enters a different magnetic polarity at regular intervals, so the current periodically alternates in direction.

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AC electricity is generally used more widely for power generation because it is cheaper and simpler to produce. AC is also the primary power transportation method over short and medium distances because AC moves through the transmission lines without significant power losses and is easier to adjust to different voltage levels. Since the power we pull from the grid is alternating current, and household electrical outlets are designed for AC, most home appliances such as refrigerators, dishwashers, and washing machines operate on AC.

Thus, to use the electricity generated by our photovoltaic power system, we need it to be transformed into an alternating current - and this is where a home solar inverter comes into the picture. Apart feom making the power usable, modern models maximize power production thanks to a built-in maximum power point tracker and ensure seamless communication with the utility grid.

Solar powered inverter working principle

So how does a solar power converter transforms DC to AC? Direct current can be represented as a consistent straight line, whereas AC current is a continuously changing sine wave, also known as a pure sine wave. First, a solar powered inverter generates vibrating pulses through an integrated circuit at a specific frequency, amplifying the produced voltage to bring the current to the required level.

There are two types of inverter technology: pure sine wave inverters and modified sine wave inverters. In a pure sine wave type, the produced voltage comes in the form of a smooth wave indistinguishable from the usual AC sine waves, while modified sine wave inverters abruptly switch the polarity from positive to negative, creating a square or stair-step pattern.

The inverters of modified sine wave type are cheaper but limited in application. The jagged unstable wave they generate can negatively affect some sensitive equipment; however, they can be used to run simple appliances like old TVs and water pumps. Pure sine wave inverters, by contrast, can power much more sensitive electronics, such as newer audio and video devices. But the quality comes at a higher price: a pure sine wave solar converter can cost about twice as much as a modified sine wave invertor.

Inverter types according to the connection with the panels

Now that we’ve discussed the internal structure of solar panels and inverters let’s move on to find out how they can be connected. Three solar inverter types are distinguished by how they receive and process the power from the modules: string inverters, microinverters, and string inverters paired with optimizers.

String solar inverters

String solar panels converter is the oldest and the most popular type. In a string system, the modules are wired together into a “string,” which, in turn, is linked to the standalone central unit. It receives the current generated by all the modules of the system and transforms it into usable power. Unlock sustainable energy with an integrated solar roof, harnessing sunlight for eco-friendly power generation and efficient energy use.

Most string inverters for solar systems can manage up to 30 photovoltaic modules, depending on the voltage of both the modules and the inverter and the number of strings the latter can handle. But in most cases, residential systems only need one or two string inverters for solar power, most commonly installed on a wall close to the main switchboard.

String options are the simplest and the cheapest, typically costing from $1,000 to $2,000. They are ideal for installations without shading problems, easier to monitor, and generally considered the most reliable of all, as there are fewer parts in a system that can potentially fail.

However, this type of inveters is less efficient in the case of partial shade. As the modules are linked in series, if one of them is shaded, the entire series will be affected, and the power system performance will be limited to the worst-performing module. Additionally, a system with a string solar powered inverter will be more challenging to expand, as adding new modules will require them to be linked to a new separate inverter.

One more disadvantage is the warranty length: string models typically come with a warranty period of only 8-12 years. And finally, system monitoring can be more complicated as it will be impossible to track the performance of each individual module.

Microinverters

Unlike string type models, microinverters are mounted on the back of every module in a system, and each microinverter operates independently from the others. Therefore, if one module is shaded or underperforming for any other reason, it won’t affect the rest. Microinverters allow for more straightforward and quick monitoring and performance tracking, which helps to diagnose and fix problems faster. Scaling up the photovoltaic system is also much simpler: all you have to do is install a microinverter for every module you add. Plus, they usually last longer and come with a 25-year warranty.

The most obvious drawback is the price: microinverters for a standard-sized residential system will cost around $3,000. Also, microinverters are installed on the roof and hence more difficult to access, so maintaining and repairing them is more complex and costly.

Solar power optimizers

There is a third type beyond string and microinverters: DC power optimizers, sometimes referred to as "electronic shepherds" for solar panels inverter systems. To be more precise, they are complementary devices to traditional string inverters that give them properties of microinverters. Optimizers, just like microinverters, are mounted on the roof and located under each module, but unlike microinverters, they do not create alternating current. As the name implies, their main task is to “optimize” the power from the modules ensuring they perform at their best in the given conditions. Afterward, the optimizers send the power down to the central inverter for solar panel.

But why install optimizers if you can have microinverters? The most distinct advantage is the price, as optimizer systems are generally cheaper than microinverters – their price tag averages about $2,400 for a typical-sized photovoltaic system. And besides, some find optimizers more reliable and durable because keeping one centralized unit in the garage is generally less risky than having on your roof multiple inverters exposed to external shocks.

Nevertheless, optimizers are not without flaws, with the biggest being a limited product selection. Plus, installing optimizers won't shield your photovoltaic system from any potential issues with a central solar powered inverter.

Inverter types according to the presence of grid connection

Some residential systems are connected to the public electric grid and don’t need a battery, while others operate independently, relying on battery storage. Depending on what photovoltaic system you wish to install, you may need different solar panels and inverters. Let us look at the types of solar inverter suitable for different PV systems and how they work.

On grid

On grid systems are the most popular type for domestic energy generation. These systems work without batteries, and any surplus of electricity they produce is exported to the local utility grid, for which homeowners usually receive credits partly offsetting their power bills. On grid systems use special grid connected inverters that must not only transform direct current into the alternating current but also condition it for the power transmission lines. To ensure power compatibility with the electric utility infrastructure, an on grid solar inverter accurately synchronizes the voltage, phase, and frequency of the sine wave with those of the wave in the grid.

Off grid

Some photovoltaic systems work independently from the utility and require a battery to store the electricity the modules generate. Unlike a grid on inverter, a solar battery inverter doesn’t have to match the phase and frequency with the electromagnetic wave from the transmission lines. Nevertheless, they have a more complex structure with a greater number of elements, and therefore their cost is averagely two times higher than that of on grid options.

Hybrid

Hybrid solar panels inverter systems have both a grid connection and a battery for storing unused electricity. Hybrid type inverters can function in different modes: feed the power from the modules to the grid in AC form, send it to the battery in DC form, or even use the grid power to recharge the battery during the hours when public electricity is cheap. Thus, they combine the properties of both on-grid and off-grid solar battery inverters.

Best manufacturers comparison: Solaredge vs. Enphase

Some of the largest manufacturers with cutting edge products are Fronius, Huawei, SMA, and SunPower, but just two brands currently dominate around 90% of the U.S. market. Here's a quick comparison of the benefits SolarEdge and Enphase have to offer.    

Efficiency: Solaredge is slightly ahead

SolarEdge sells a single-phase HD-Wave inverter solar power system consisting of two elements: the central inverter with 99% efficiency and the power optimizer with 99.5% efficiency. Thus, the overall efficiency stands at 99.25%, which means only 0.75% of power is lost during transforming direct current into alternating current.

Enphase manufactures microinverters. Its newest product is the IQ 8 Series, with an efficiency of only 97%. However, the efficiency comparison is not entirely fair due to the different technology types used.

Warranty: Enphase has a better offering

Both companies offer excellent warranties covering replacement labor and shipping costs. But Enphase provides a warranty of 25 years, while the SolarEdge inverter is covered only with a 12-year warranty. However, SolarEdge offers an extended warranty for an extra fee.

Reliability: Both have strong and weak points

Enphase systems are generally considered more reliable as they do not have a central point of failure: if one microinverter fails, the rest will keep on functioning. This cannot be said about the SolarEdge inverter, as its collapse will mean the whole photovoltaic system will stop working.

Meanwhile, the SolarEdge inverter system employs better monitoring technology as compared to Enphase: it tracks power output, consumption, and technical performance. This visibility allows for registering even minor changes and noticing malfunctions as early as possible.

Scalability: Enphase solution allows for more flexibility

With the SolarEdge solution, the photovoltaic system scalability is limited to the size of the central inverter, which can only handle a certain number of modules. The Enphase option, by contrast, enables you to scale your system by simply adding new modules and microinverters.

Cost: Solaredge system is cheaper

Neither of the two manufacturers offers cheap products, but the pricing is reasonable given the quality. Enphase will likely cost more as microinverter technology is more sophisticated. But if you plan to expand your solor system in the future, it will be a worthwhile investment.

Still have questions? Watch this video about inverters and how they works