String Inverter vs Central Inverter: Which is Right for You?

Central inverters and String inverters are two main types of inverters that dominate the solar market and these two variants are often preferred due to every individual’s varying requirements by considering factors like cost, lifespan, capacity, usage, and several others. Typically, an inverter is an essential component of the solar panel system that converts DC electricity into AC electricity.

In this review, you will find an interesting comparison of string and central inverters highlighted with their advantages and disadvantages, which helps to determine the best type of solar inverter for your solar projects.

What are String Inverters?

A solar string inverter is a type of PV system inverter specifically designed to connect either single or multiple groups of PV modules in series and the wiring connections are linked end-to-end to form a “string”.

The functioning of a string inverter is simple. The energy produced by the solar panels is fed into the inverter to convert the direct current (DC) into usable alternating current (AC) which can be used to power the appliances in homes or businesses.

These low-powered inverters are located in an electronic box that is installed on the exterior wall of your house or garage.

String inverters can easily handle multiple strings and solar panel connections. For instance, if four different groups (known as strings) of solar panels, have three solar panels on each string, then it makes a total of 12 solar panels wired to a single inverter.

You should carefully select the string size and capacity based on the calculated kilowatt (kW) output and the solar panel system’s total wattage. If you prefer the incorrect size strings, then the total system might waste the entire electricity.

Since the panels are connected in a series or string to the inverter, if one or more panels don’t produce much energy (because of shading, dirt, or several other issues), the remaining panels’ output on that string will be lowered. This remains a major disadvantage despite its simplicity in operation.

String inverters are commonly used in residential and commercial solar energy system installations in areas that have exposure to consistent sunlight and minimal shading effect. Despite this information, there is much more to learn about string inverters.

Advantages of String Inverters

These are the main benefits of using string inverters:

• Cost-effectiveness: Compared to other types of devices, string inverters are budget-friendly (both upfront and installation budget) and cost $0.25 per watt which is lower than the microinverter system ($0.50 to $1 per watt). Also, string inverters require minimal components and labor for installation.
• Higher efficiency rates: String inverters can achieve a maximum efficiency of around 98% due to the huge conversion volume of DC power at a time, which is higher than microinverters (96% to 97%).
• Easy maintenance: Mainly, the design and installation process of string inverters are straightforward and require minimal components, and labor for installation.
• Scalability: Larger string inverters are designed to be modular and scalable that can handle multiple string inputs which can prove to be beneficial for future expansion of the solar panel system. The inverters have integrated maximum power point trackers (MPPT) on the input side of the DC to boost the array of solar panels’ performance at the string level. Also, the latest string inverter models have a maximum of 12-15 MPPTs and handle dozens of PV string inputs.

Disadvantages of String Inverters

At the same time, there are certain drawbacks to using string inverters:

• Limited performance monitoring: String inverters have limited performance monitoring capabilities in comparison to microinverters. This is because the individual panel-level tracking isn’t provided due to the limited development in technology which makes it challenging to assess the individual panels’ performance.
• Huge installation space requirements: At times it can be challenging for commercial buildings with limited area to install string inverters which requires huge space.
• Decreased reliability upon failure of a single panel: String inverters have maximum risks of failure compared to microinverters as the failure of one panel can affect the entire string’s performance, thereby diminishing the overall energy production. Here are more details about common solar inverter failures you should know.
• Minimized efficiency due to shading: String inverters are sensitive to shading and the effect of shading on a single panel can reduce the overall efficiency of all panels connected to the string.

What are Central Inverters?

Central inverters are large-sized devices operated in solar power plants to perform the DC-to-AC conversion process (Direct current to Alternating current) which can be fed into the electrical grid for future use. These high-wattage inverters are installed outdoors and are specifically designed to resist excessive power as well as function at a fixed voltage and frequency.

The operation of the central inverter is easy to comprehend. Certain electrical components such as MOSFETs, IGBTs, and diodes help in transforming DC to AC power. Multiple solar panels are connected in parallel to the combination box, which feeds integrated DC power into the central inverter, which gets converted into AC power. The output voltage and frequency of the central inverter are aligned with the grid voltage and frequency.

A central inverter’s input comes from the combined direct current (DC) circuits of all groups (called strings) of the solar panel system. A minimum number of input devices called Maximum Power Point Trackers (MPPTs) handle these inputs to manage the energy production optimally.

Typically, central inverters are large with each one ranging between 1-5 MW per unit, but a majority of solar power projects use one or more central inverters to generate more than 10 MW of electricity.

Advantages of Central Inverters

These are the beneficial features of using central inverters:

• Higher efficiency: The conversion efficiency range from 95% to 98% is higher for central inverters which means that they can convert a maximum percentage of direct current produced by the solar panels to the alternating current fed into the electricity grid.
• Cost-effectiveness at scale: Central inverters are cost-effective for large solar power plants and their cost per watt is less ($0.18 / watt) than microinverters ($0.50 to $1.00 / watt) or string inverters ($0.25 / watt) to lower the capital expenditure (CAPEX).
• Longer Lifespan to reduce replacement frequency: Central inverters have a longer lifespan (15–20 years) than string inverters and microinverters (10–15 years), thereby reducing the frequency of replacement and leading to cost savings over a certain period.

Disadvantages of Central Inverters

You need to look at these drawbacks before purchasing central inverters:

• Single point of failure risk for the entire system: If a single point of failure occurs, then the entire functioning of the solar system goes down. This occurs even if one solar panel is shaded or performs poorly due to other reasons.
• Lack of power management flexibility: Central inverters have lower flexibility in the context of power management. This means that they cannot control power on a per-panel basis during outages, blackouts, or failures in one panel that affects the overall performance of the solar system.
• Size and Weight challenges: Central inverters are large and bulky which makes the installation and maintenance process much more complex. As a result, a large amount of separate space is required for installation.

String Inverter vs Central Inverter: Detailed Comparison

Typically, the comparison process of string and central inverters can be performed by considering the four important factors such as their capacity, efficiency, usage, and price.

Capacity

Here is the capacity of central and string inverters listed below:

• Central Inverters: 100 kW to 1,200 kW for solar power projects.
• String Inverters: 3 kW to 20 kW for residential purposes. Larger installations can range upto 350kW per unit.

Efficiency

Central inverters are more efficient than string inverters due to the higher conversion efficiency rate of DC electricity to AC electricity.

• Central Inverters: 95% to 98% peak efficiency. There are chances for efficiency loss to occur due to module mismatch if the central inverters have 2,000 to 3,000 solar panels approximately operating from a single MPPT.
• String Inverters: 98% peak efficiency. Using MPPTs per inverter, the efficiency rate can be higher than 4% to 8% greater than central inverters.

Usage

Both central and string inverters have different usage as mentioned below:

• Central Inverters: Used in large-scale commercial installations like solar power plants to withstand higher power capacity.
• String Inverters: Used in residential building installations or smaller commercial spaces which include solar parking spaces, charging station projects, and several others.

Price

Here is an in-depth analysis of the cost implications and maintenance demands linked with central and string inverters:

Central Inverters:

• Cost Implications: The cable cost is 33% higher than string inverters with power losses more than 1%. Less upfront costs than string inverters. Central inverters cost $0.18 per watt.
• Maintenance Demands: Central inverters need to be housed and require a combiner box to integrate a maximum number of 20 PV strings. As a result, you might require ten combiner boxes to connect with the inverter.

String Inverters:

• Cost Implications: Despite the lower cable costs, the upfront costs are twice that of central inverters. String inverters cost $0.25 per watt.
• Maintenance Demands: Housing isn’t mandatory for string inverters. However, solar power plants with string inverters need additional AC cables. This is because the AC cable losses might be more than 0.5% than central inverters. Unlike the central inverters, AC combiners aren’t mandatory for the latest string inverters, since they are included in the transformer stations.

Conclusion

Selecting the right type of solar inverter is essential to align perfectly with your solar project scale, efficiency requirements, and budget to ensure long-term reliability.

Central inverters with lower upfront costs and larger physical size have a larger power conversion per unit by using longer-sized wire.

On the other side, string inverters with a modular design have a small power conversion per unit and higher upfront costs than central inverters. For each row of panels, 10-20 input strings connecting from the PV modules to inverters can be shorter.

However, central inverters are more affordable in price than string inverters due to fewer DC components, higher power quality, and density, which makes them affordable for large-scale utility installations. But for the smaller solar projects, string inverters could prove to be the ideal choice with the easier service available.

Finally, it is important to consult with solar energy experts before purchasing the solar inverter to tailor the choice according to your specific needs. The invaluable suggestions provided by the solar consultants help to promote renewable energy rather than just designing and installing solar systems. It includes tricks like budget saving and energy efficiency optimization. Furthermore, these consultants guide you on the proper maintenance and monitoring of solar panels to ensure peak performance.

Overall, choosing the best type of inverter and seeking professional guidance is essential to install a perfect solar system and keep yourself updated with the new trends, strategies, and efficient practices in the solar energy industry.

How Should I Select the Best Inverter Type for My Solar Project?

While choosing the best type of solar inverter for your solar project, you need to consider these important factors:

1. Wattage Rating
2. Input Voltage
3. AC Output
4. Surge Power
5. Efficiency Rating
6. Warranty and Support
7. Compatibility
8. Brand and Reputation
9. Cost