Inverters: What are they and which ones are suitable for off-grid, hybrid, and grid-connected systems?

If you're interested in solar panels or energy storage systems, you'll inevitably need to purchase an inverter (the term "invert" translated from English refers to turning, reversing, or converting)—the device that converts the direct current (DC) produced by solar panels and/or stored in batteries into alternating current (AC). All household appliances and other electrical devices operate on AC, and the entire electrical grid functions on alternating current. Therefore, energy stored in solar panels or energy storage systems cannot be utilized without an inverter.

Inverters are available in a wide range of power levels, functions, and price categories, so it’s essential to choose the right inverter for your needs. This is the goal of this post—to help navigate the diverse world of inverters a bit better. To avoid getting too technical right away, let’s first look at some general possible use cases that can help narrow down the selection of suitable inverters, and we will categorize inverters into three groups: off-grid inverters (with and without grid input), hybrid inverters, and string or grid-tied inverters.

Off-grid systems: What to consider when choosing an inverter?

A completely off-grid system means there is no connection to the grid, and the household generates the necessary electricity for its own use, often using solar panels or a generator. A key component of such a system is usually energy storage, as solar panels only produce electricity during the day, while consumption can occur at night as well. The general structure of the system is illustrated in the following diagram. The inverter directs the energy taken from the solar panels or storage into consumption as needed. Such a system is particularly suitable for vacation homes and households where connecting to the grid would be very costly.

In such a system, the suitable type of inverter is off-grid inverters, which allow for the connection of both energy storage and solar panels and/or generators as inputs, but do not necessarily have a connection to the power grid. Off-grid inverters are typically single-phase, meaning that the output of the inverter is a single phase of alternating current electricity. It is assumed that the number of consumers in such a household is limited, and one phase is sufficient.

Off-grid inverters are usually in a lower price range because (a) the flow of energy is one-way (from solar panels or batteries to consumption), (b) there is no need to manage simultaneously drawn electricity from the grid and electricity drawn from one’s own production/storage source, and (c) there is no need to balance three phases. It should be noted that more complex inverters, such as hybrid or grid-tied inverters, can be used as off-grid inverters, but these are not cost-effective solutions, and some functionality remains unused.

You can find the off-grid system-compatible inverters available in Thunor's online store here. We will discuss more specific parameters below.

On-grid or grid-connected consumption systems: What to consider when choosing an inverter?

In an on-grid system, the household can combine the necessary electricity from its own production with electricity drawn from the grid. In such a system, energy storage is added as a backup or to store excess electricity produced for later use. In this context, there is no possibility of returning electricity to the grid in such a system—this will be discussed in more detail below in the context of hybrid systems. The general structure of the system is illustrated in the following diagram. The inverter assesses the current consumption to determine whether to (a) draw electricity from the grid for storage or consumption, (b) store excess energy, or (c) use electricity from the battery when there is no grid connection, directing energy taken from solar panels or storage into consumption as needed. Such a system is particularly suitable for households and businesses when there is an elevated risk of energy supply interruptions due to storms or other circumstances.

In such a system, the suitable types of inverters are off-grid and on-grid inverters, which have an input for connecting to the power grid. In the case of three-phase grid connections, the inverter must also be three-phase to avoid overloading one phase in the house excessively. For several inverters, there is also the option to create a so-called backup phase or critical phase from one output, to which critical consumers can be connected. For example, in a situation where there is a risk of power loss due to weather conditions, there may be a desire to ensure electricity for critical consumers such as basic lighting, refrigerator, water pump, and other essential devices, while excluding electrical consumers like TVs, underfloor heating, and other convenience devices.

Inverters suitable for such systems are similarly in a lower price range as off-grid inverters, since the flow of energy is still one-way (from solar panels, batteries, or the grid to consumption). However, compared to completely off-grid inverters, more intelligent control and configuration are now required, as well as the existence of operating modes that allow you to set when to draw electricity from the battery and when from the grid. All hybrid inverters can be used in this system, but the resale of electricity back to the grid is limited.

You can find the on-grid inverters available in Thunor's online store here. We will discuss more specific parameters below.

Hybrid systems: What to consider when choosing an inverter?

Hybrid systems allow for simultaneous consumption of self-produced (directly from the panels) or battery-stored electricity and electricity drawn from the grid, similar to the previously described on-grid system. The only difference is the capability of a hybrid system to sell electricity back to the grid if desired. In such a system, energy storage is added for (a) backup, (b) storing excess electricity produced for later consumption, or (c) timing sales during periods when prices are higher compared to the current moment. Since the capacity of solar parks in electricity production has significantly increased, electricity prices are typically low during very sunny periods, as everyone produces and sells solar energy to the grid at the same time. By storing the produced energy in storage and selling it when the sun is not shining (and widespread production is absent), it is often possible to earn higher profits from energy sales. Therefore, hybrid inverters together with energy storage enhance the profitability and return on investment of existing or newly created solar parks. The general structure of the system is illustrated in the following diagram and is similar to the on-grid solution. An important addition is the capability to sell electricity back to the grid. Such a system is particularly suitable for households and businesses whose energy production exceeds their consumption needs and who wish to participate in stabilizing the energy market while earning additional income from it.

Such a system can only be implemented with hybrid inverters, and not all inverters can be connected to the electrical grid. A list of suitable hybrid inverters (and string inverters, discussed below) is defined by Elektrilevi and can be found here. To connect a device to the grid, it is necessary to obtain consent from Elektrilevi. Relevant guidelines can be found on Elektrilevi’s website. The reason for this is that devices connected to the grid must comply with the general parameters of the grid (for example, fluctuations in the output current) and allow Elektrilevi to remotely disconnect the device from the grid.

You can explore the hybrid inverters available in Thunor's online store here. We only offer high-quality Deye hybrid inverters approved by Elektrilevi, which allow for all previously described use cases. More specific parameters will be discussed below.

Grid-selling production stations: What to consider when choosing an inverter?

For solar parks whose main goal is to produce and sell electricity to the grid and have no significant consumption, it is most cost-effective to use so-called string inverters for selling to the grid, which convert the electricity generated from the connected solar panels into suitable alternating current for the grid. In such a system, the inverters are connected in parallel, meaning that the electricity generated by each string of solar panels is summed and sold to the grid. In this type of system, the use of energy storage allows for better timing of electricity sales, thereby increasing the return on investment, similar to the logic of hybrid inverters. It is important to note that string inverters only operate when connected to the electrical grid; thus, in the event of a grid outage, electricity cannot be used for personal consumption with a string inverter. A simplified system structure is presented in the diagram below.

Similar to hybrid inverters, only inverters that are on Elektrilevi's list of approved inverters can be connected to the electrical grid. Since the capacity of such parks usually exceeds the 15 kW production limit, the park does not qualify as a small producer and is classified as an electricity producer according to Elektrilevi's definition. The requirements set by Elektrilevi and the process for establishing a park are explained on their website.

In Thunor's online store, we offer hybrid inverters that allow larger energy stations to add storage capacity. In such a system, hybrid inverters are connected in parallel based on the same principle as shown in the diagram above. An energy storage system is added to the system. Thunor batteries can be combined in one storage system up to a total of 16 units, ensuring a total storage capacity of 227.2 kWh (see more on the Thunor website). You can check out the hybrid inverters available in Thunor's online store here. We only offer high-quality Deye string inverters approved by Elektrilevi. More specific parameters will be discussed below.

More Detailed Inverter Parameters

While the general types of inverters were discussed in detail earlier, each inverter type has specific parameters that help find a suitable inverter for your system. The best overview of a specific inverter's parameters can be found on its data sheet. Each inverter available in Thunor's store has a corresponding data sheet linked to the product.

Let’s examine the inverter parameters based on one of the hybrid inverters we offer, the Deye SUN-5/6/8/10/12K-SG04LP3-EU models (data sheet). An excerpt from the data sheet is presented below.

The most important parameters to consider when selecting an inverter in relation to the battery are as follows:

• Battery type – the type of energy storage. This model is compatible with both lead-acid batteries and lithium-ion energy storage systems, including those produced by Thunor, which you can read more about on Thunor's website.

• Battery voltage range – the acceptable voltage range for the energy storage. This model is suitable for low-voltage (48V) batteries (including Thunor batteries), but not for high-voltage batteries.

• Max charging and discharging current – the current rating at which the battery can be charged and discharged. For these models, the possible current ratings start from 150 amps and go up to 240 amps. The charging current determines how quickly the battery can be fully charged or discharged. For example, a Thunor battery can be charged at 140 amps, allowing it to be fully charged and discharged in about 2 hours (280 Ah capacity ÷ 140A).

The most important parameters to consider when selecting an inverter in relation to an existing or planned solar park are as follows:

• Max DC input power – the actual input power of the solar park, which must be equal to or less than the capacity allowed by the selected inverter. For example, for the Deye 12K model, the solar park capacity must remain below 15.6 kW.

• Rated PV input voltage – the permissible cumulative input voltage for the solar park. For Deye models, the allowed input voltage is set at 550V (160-800), meaning that tests have been conducted at 550V input voltage, and the parameters specified in the data sheet were determined; however, the inverter allows input voltages in the range of 160-800V.

• MPPT Voltage Range – the voltage range for the solar park in which productivity is most efficient. For Deye models, the corresponding range is 200-650V.

• PV input current – the permissible summed current rating for the solar park. The Deye data sheet indicates 13+13, which means that both solar park current inputs of the inverter can handle currents of up to 13A.

• No of MPP trackers – the number of inputs for solar panels that can be connected in series. For the given Deye models, the inverter can accommodate two groups of series-connected solar panels.

The most important parameters to consider when selecting an inverter in relation to the output are as follows:

• Rated AC Output Active Power – the power output provided by the inverter under test conditions. Max AC Output Active Power refers to the maximum possible power output.

• AC Output Rated Current – the current output provided by the inverter under test conditions. Max AC Output Current refers to the maximum possible current output.

The remaining parameters in the AC Output Data section are technical parameters related to the inverter's compatibility with the grid.

It is very important to note that for most inverters, including Deye inverters, protection against excess electrical energy is ensured locally to safeguard the inverter against minor current fluctuations. However, the inverter does not guarantee the overall safety of the entire system, for example, against lightning strikes, and additional equipment must be installed in the system for that purpose. Nonetheless, for instance, the Deye model has built-in protection against current fluctuations for both DC and AC currents.

In conclusion, when selecting an inverter, general non-technical parameters are also important, such as the acceptable operating temperature range, the presence of a cooling function to prevent overheating, noise level, dimensions and weight, waterproof rating (Protection Degree, IP-class), warranty conditions, and many other factors.

Summary of Inverter Selection

In summary, if you are interested in solar panels or energy storage systems, you will likely need to acquire an inverter. The inverter converts the direct current (DC) stored in solar panels or batteries into alternating current (AC), which is used by household appliances and electrical devices. Inverters come in various types depending on the needs of the system:

• Off-grid systems: Suitable for areas without an electrical grid. Off-grid inverters connect energy storage and solar panels but are not grid-connected.

• On-grid systems: Suitable for households that utilize the grid and wish to use the energy produced by solar panels and add energy storage for backup.

• Hybrid systems: Suitable for households that want to use their produced energy and grid-connected electricity simultaneously and sell electricity back to the grid for additional income.

• Solar parks and grid-selling production stations: Use string inverters to convert the electricity produced by solar panels into AC suitable for the grid.

When choosing an inverter, you must consider the parameters of the system, such as the characteristics related to energy storage, solar parks, and output. Important parameters include battery type, voltage and current ranges, and input and output power.

In addition to technical parameters, other factors such as operating temperature range, cooling function, noise, dimensions, weight, waterproofing, warranty, and other characteristics are also essential. Choose an inverter according to your specific needs and system characteristics.

If you have any questions, feel free to get in touch with our specialists via Thunor's website or e-mail info@thunor.eu.