If you're interested in solar panels or energy storage systems, you inevitably need to acquire an inverter. An inverter is a device that converts the direct current (DC) produced by solar panels and/or stored in batteries into alternating current (AC). All household appliances and electrical devices operate on AC, and the entire electrical grid runs on AC as well. Therefore, without an inverter, it's not possible to use the energy stored in solar panels or energy storage systems.

Inverters are available in various power ratings, functions, and price ranges, making it essential to choose the right one for your needs. The aim of this post is to help you navigate the diverse world of inverters. To avoid getting too technical right away, let's first explore the general use cases, which will help narrow down the selection of suitable inverters. We'll categorize inverters into three groups: grid-connected and grid-disconnected off-grid inverters, hybrid inverters, and string or on-grid inverters.

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

A completely off-grid system, where there's no connection to the electrical grid, allows a household to generate its own power using sources like solar panels or generators. Energy storage systems, often essential components of such systems, come into play because solar panels produce electricity only during the day, while consumption can occur at night. The basic setup of this system is illustrated in the following diagram. An inverter directs the energy from solar panels or storage to consumption as needed. Such a system is particularly suitable for cottages and households where grid connection is prohibitively expensive.

In such a system, the suitable type of inverter is off-grid inverters, which allow the connection of both energy storage and solar panels and/or generators as input but do not necessarily have a grid input. Off-grid inverters are typically single-phase, meaning the inverter's output is a single phase of alternating current (AC). Presumably, the number of consumers in such a household is limited, and one phase is sufficient.

Off-grid inverters are generally in a lower price range because (a) the flow of energy is unidirectional (from solar panels or batteries to consumption), (b) there is no need to simultaneously manage electricity from the grid and electricity taken from your production/storage input, and (c) there is no need to balance three phases. It should be noted that more complex inverters with different operating principles, such as hybrid or on-grid inverters, can be used as off-grid inverters, but they are not cost-effective solutions, and some functionality remains unused.

You can explore off-grid inverters suitable for such systems in Thunor's online store here. We'll discuss more specific parameters below.

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

In an on-grid system, a household can combine its required electricity with the electricity it generates from the grid. In such a system, energy storage is added either as a backup or to store excess generated electricity for later consumption. In this context, there is no option to feed excess electricity back into the grid, as explained in more detail below in the section on hybrid systems. The general structure of the system is illustrated in the diagram below. Depending on current consumption needs, the inverter assesses whether to (a) obtain electricity from the grid for storage or consumption, (b) store excess energy, or (c) use electricity from the battery in the absence of a grid connection. Such a system is primarily suitable for households and businesses facing a higher risk of energy supply interruptions due to factors like storms or other circumstances.

In such systems, both off-grid and on-grid inverters with a grid input are suitable. In the case of three-phase grid connections, the inverter must also be three-phase to avoid overloading a single phase in the house. For some inverters, there is also the option to create a so-called backup phase or critical phase, to which critical consumers can be connected. For example, in situations where there is a risk of power loss due to weather conditions, you may want to ensure power to critical consumers such as basic lighting, refrigerators, water pumps, and other essential appliances while excluding non-essential consumers like TVs, floor heating, and other convenience appliances.

Inverters suitable for such systems are similarly priced in the lower range, as the flow of energy is still unidirectional (from solar panels, batteries, or the grid to consumption). However, compared to entirely off-grid inverters, these systems require smarter control and configuration, as well as the presence 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 sale of excess electricity back to the grid is limited.

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

Hybrid systems: What to consider when choosing an inverter?

Hybrid systems, similar to the on-grid systems described earlier, allow simultaneous consumption of self-generated (directly from panels) or battery-sourced electricity and grid-sourced electricity. The key difference is the capability of hybrid systems to sell excess electricity back to the grid if desired. In such a system, energy storage is added either as (a) backup, (b) storage for excess generated electricity for later consumption, or (c) for timed selling when prices are higher compared to the current moment. As the capacity of solar parks in electricity generation has significantly increased, electricity prices are usually low on very sunny days since everyone is generating and selling solar energy to the grid simultaneously. By storing the generated energy in a battery and selling it when there is no sun (and limited production), it is often possible to earn a higher profit from energy sales. Therefore, hybrid inverters, in combination with energy storage, enhance the profitability and return on investment of existing or planned solar parks. The general system layout is illustrated in the diagram below and is similar to the on-grid solution. An important addition is the ability to sell electrical energy back to the grid. Such a system is primarily suitable for households and businesses whose energy production exceeds their consumption needs and who wish to participate in ensuring energy market stability 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. The list of suitable hybrid inverters (and string inverters, discussed below) is defined by Elektrilevi in Estonia and can be found here. The allowed inverters list might vary by market. Obtaining permission from Elektrilevi (in Estonia) or the relevant authority in a different market is required to connect the device to the grid. Relevant instructions can be found on Elektrilevi's website (in Estonia). The reason for this is that devices connected to the grid must meet the general parameters of the grid (such as current fluctuation) and allow Elektrilevi to remotely disconnect the device.

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

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

For solar parks whose primary purpose is to produce and sell electricity to the grid, and there is no significant consumption, it is most cost-effective to use string inverters, which convert the electricity generated by the connected solar panels into suitable alternating current for the grid. In this system, inverters are installed in parallel, meaning that the electricity produced by each string of solar panels is summed up and sold to the grid. Using energy storage in such a system 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 work when the electrical grid is operational, so in the event of a grid outage, you cannot use electricity for your own consumption with a string inverter. The simplified system layout is presented in the diagram below.

Similarly to hybrid inverters, only inverters listed by Elektrilevi (in Estonia) are allowed to be connected to the electrical grid. Since the capacity of such parks typically exceeds the 15 kW production limit, they do not qualify as small-scale producers, and they are considered electricity producers according to Elektrilevi's definition. The requirements set by Elektrilevi and the process for establishing such a park are explained on their website.

In Thunor's online store, we offer hybrid inverters that also allow for the addition of storage capacity for larger energy plants. In this type of system, hybrid inverters are connected in parallel, following the same principle as shown in the diagram above. An energy storage system is added to the setup. Up to 16 units of Thunor batteries can be added to a single storage system, providing a total storage capacity of 227.2 kWh (see more details on Thunor's website). You can explore hybrid inverters offered in Thunor's online store here. We offer only high-quality Deye string inverters approved by Elektrilevi. More specific parameters will be discussed below.

Detailed Inverter Parameters

While we have covered general types of inverters in detail, each type of inverter has specific parameters that allow you to find the right inverter for your system. For the most comprehensive information on a specific inverter's parameters, you can refer to the inverter's data sheet. Each inverter available in Thunor's store is linked to its respective data sheet.

Let's examine the inverter parameters based on one of our offered hybrid inverters, 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 choosing an inverter in relation to a battery are as follows:

• Battery type - The type of energy storage system. This model is compatible with both lead-acid batteries and lithium-ion energy storage systems, including those produced by Thunor. More details can be found on Thunor's website.

• Battery voltage range - The suitable voltage range for the energy storage system. This model is compatible with low-voltage (48V) batteries (including Thunor batteries) but not with high-voltage batteries.

• Max charging and discharging current - The maximum current at which the battery can be charged and discharged. For these models, the available current ranges from 150 amps to 240 amps. The charging current determines how quickly the battery can be fully charged and discharged. For example, Thunor batteries can be charged at 140 amps, allowing the battery to be fully charged and discharged in about 2 hours (280Ah capacity ÷ 140A).

The key parameters to consider when choosing 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 selected inverter's allowed input power. For example, with the Deye 12K model, the solar park's power must remain below 15.6 kW.

• Rated PV input voltage - The cumulative allowable input voltage of the solar park. For Deye models, the specified input voltage is 550V (160-800), meaning that testing measurements have been conducted at 550V input voltage, and the data sheet parameters are defined based on this voltage. However, the inverter allows input voltage in the range of 160-800V.

• MPPT Voltage Range - The voltage range of the solar park where efficiency is most effective. For Deye models, this range is 200-650V.

• PV input current - The cumulative allowable input current of the solar park. On Deye's data sheet, "13+13" indicates that both input current channels of the inverter can handle current up to 13A.

• Number of MPP trackers - The number of strings of solar panels that can be connected in series. For Deye models, two strings of solar panels can be connected to the inverter.

Regarding output-related parameters, the following are essential to consider:

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

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

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

It's crucial to note that for most inverters, including Deye inverters, protection against excess electrical energy is provided locally to safeguard the inverter against minor current fluctuations. However, inverters do not protect the entire system from events like lightning strikes, so additional system components need to be installed for such protection. Deye models, for instance, have built-in protection against DC and AC current fluctuations.

Finally, when choosing an inverter, non-technical parameters such as the operating temperature range, cooling function for overheating protection, noise generation, cabinet size, weight, waterproofing, warranty terms, and other factors are also essential to consider.

To summarise

In summary, when you are interested in solar panels or energy storage systems, you will likely need to acquire an inverter. An inverter converts the direct current (DC) generated by solar panels or stored in batteries into alternating current (AC) used by household appliances and electrical devices. There are different types of inverters available to suit the needs of your system:

• Off-grid systems: Suitable for areas without access to the electrical grid. Off-grid inverters connect to an energy storage system and solar panels but do not have a grid connection.

• On-grid systems: Suitable for households connected to the grid that want to use energy generated by solar panels and add energy storage for backup.

• Hybrid systems: Suitable for households that want to use their self-generated energy simultaneously with grid-supplied electricity and potentially sell excess energy back to the grid for additional income.

• Solar parks and grid-selling power plants: Use string inverters to convert electricity generated by solar panels into grid-compatible AC power.

When choosing an inverter, consider the parameters of your specific system, such as the type of energy storage system, characteristics of the solar park, and output requirements. Important parameters include battery type, voltage and current range, input power for solar panels, input voltage range, MPPT voltage range, input current for the solar park, and the number of MPP trackers.

Besides technical parameters, other factors like operating temperature range, cooling function, noise, dimensions, weight, waterproofing, warranty, and additional features are essential to consider. Select an inverter that aligns with your specific needs and system requirements.

For any questions, you can always reach out to our specialists through Thunor's website or via email at info@thunor.eu.