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Note: This blog was originally published in 2018. It was updated in January, 2024 to reflect the most recent information. If you have any questions, please contact us.
As you likely know, solar cells produce direct current (DC) electricity, which is then converted to alternating current (AC) electricity by a solar power inverter. Converting energy from DC to AC allows you to deliver it to the grid or use it to power buildings, both of which operate with AC electricity. When designing a solar installation, and selecting the inverter, we must consider how much DC power will be produced by the solar array and how much AC power the inverter is able to output (its power rating).
This article will discuss some critical considerations for solar projects to ensure that the inverters in your designs are appropriately sized.
Specifically, we’ll examine the relationship between the amount of energy your solar array produces and the amount of power your inverter can output, and we’ll introduce the concept of inverter clipping.
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The DC-to-AC ratio — also known as Inverter Loading Ratio (ILR) — is defined as the ratio of installed DC capacity to the inverter’s AC power rating. It often makes sense to oversize a solar array, such that the DC-to-AC ratio is greater than 1. This allows for a greater energy harvest when production is below the inverter’s rating, which it typically is for most of the day.
The following illustration shows what happens when the power inverter’s DC/AC ratio is not large enough to process the higher power output of mid-day.
The power lost due to a limiting inverter AC output rating is called inverter clipping (also known as power limiting).
Figure 1: Inverter AC output over the course of a day for a system with a low DC-to-AC ratio (purple curve) and high DC-to-AC ratio (green curve). The chart represents an idealized case; in practice, power output varies considerably based on weather conditions.
While oversizing the solar array relative to the inverter’s rating can help your system capture more energy throughout the day, this approach is not without costs.
“Either spend money on an additional inverter or lose energy harvest to inverter clipping.”
What Figure 1 also shows is an effect called inverter clipping, sometimes referred to as power limiting. When the DC maximum power point (MPP) of the solar array — or the point at which the solar array is generating the most amount of energy — is greater than the inverter’s power rating, the “extra” power generated by the array is “clipped” by the inverter to ensure it’s operating within its capabilities.
The inverter effectively prevents the system from reaching its MPP, capping the power at the inverter’s nameplate power rating.
To prevent this, it’s crucial to model inverter clipping to design a system with a DC-to-AC ratio greater than 1, especially in regions that frequently see an irradiance larger than the standard test conditions (STC) irradiance of 1000 W/m2 (higher levels of irradiance lead to higher power output).
The US Energy and Information Administration (EIA) states, “for individual systems, inverter loading ratios are usually between 1.13 and 1.30.”
For example, consider a south-facing, 20°-tilt ground mount system in North Carolina (35.37° latitude) with a 100 kW central inverter. If we design the system with a DC-to-AC ratio of 1, it will never clip; however, we will also not fully utilize the AC capacity of the inverter. We have two options. Either spend money on an additional inverter or lose energy harvest to inverter clipping.
Knowing how much energy is clipped allows a designer to understand how effective the oversizing scheme is at increasing energy harvest, and ultimately determine what system configuration is the most cost-effective.
The chart below shows three DC-to-AC ratios and their estimated losses to clipping.
DC-to-AC Ratio Annual AC Energy Production Energy Lost to Clipping 1.0 163.06 MWh 0.0 MWh 1.3 193.86 MWh 1.8 MWh (0.9%) 1.5 217.24 MWh 11.0 MWh (4.8%)Table 1: Annual energy production out of a 100 kW inverter as a function of DC-to-AC ratio. As the DC-to-AC ratio increases, so does the AC output and clipped energy.
Aurora’s solar design and sales software automatically takes inverter clipping into account in its performance simulations. Our system loss diagram automatically calculates the amount of energy that is clipped throughout the year and the percentage of total energy that amount represents. Aurora’s NEC validation report ensures designs are code-compliant and appropriately sized so installers can be confident in their work.
Microinverters
A microinverter is a device that converts the DC output of solar modules into AC that can be used by the home. As the name suggests, they are smaller than the typical solar power inverter, coming in at about the size of a WiFi router. Microinverters are usually placed under each solar panel, in a ratio of one microinverter for every 1-4 panels.
Advantages of using microverters include:
: The output of string inverters is capped by the least-efficient panel in the string. In contrast, microinverters use a parallel circuit, so they aren’t limited to the least-producing panel.
: Since microinverters are paired to individual or grouped solar panels, users have granular access to production monitoring per panel instead of the whole system.
: Scaling up a PV system is as easy as adding one microinverter for every 1-4 new panels added to the system.
: Microinverters can be rapidly turned off, which is an important requirement in new electrical codes in case of accident or urgent servicing situations.
Microinverters can have up to 25-year warranties vs. 8-12 years for standard inverters.
On the other hand, cons include:
On average, microinverters can be over $1,000 more expensive than string inverters for a typical 5kW residential installation.
: Fixing or replacing a failed microinverter is more difficult, since you would need to go up to the roof, work the rack, and unbolt the panel to access the unit.
To sum it up, microinverters are best used in sites where the panels face multiple orientations, have shading issues (so that the least efficient panel doesn’t affect the whole system output), have a good chance of being scaled up in the future, and if the local electrical code requires a rapid shutdown capability.
To learn more about module-level power electronics, check out our article Module-Level Power Electronics (MLPE) for Solar Design: A Primer.
Central (or string) inverters
A central inverter, commonly referred to as a string inverter, is a device that converts the DC output of a string of solar panels into AC for home or commercial use. These inverters are typically larger and are installed at a central location, often near the home’s main electrical panel or on an external wall.
Advantages of using central inverters include:
: String inverters are generally less expensive on a per-watt basis compared to microinverters, making them more cost-effective for larger installations.
: Having been around longer than microinverters, central inverters have a proven track record and are trusted by many installers.
: With only one inverter needed for multiple panels, there’s less equipment to install and maintain on the roof.
: Since they are usually installed in accessible locations, central inverters can be easier to service or replace than multiple rooftop microinverters.
Performance in Ideal Conditions: In scenarios where there’s no shading and all panels have a consistent orientation and tilt, string inverters can perform exceptionally well.
On the other hand, cons include:
: If one panel in a string underperforms due to shading or debris, the output of the entire string can be affected.
: String inverters do not offer granular, panel-level monitoring. If there’s an issue, it can be harder to determine which specific panel is underperforming.
n: Central inverters often have a shorter lifespan than microinverters, typically needing replacement after 10-15 years.
: If you want to expand the system later, it may require a larger inverter or additional inverters, especially if the original inverter is operating near its capacity.
: If the central inverter fails, the entire solar system stops producing electricity, whereas systems with microinverters or power optimizers might only see reduced performance.
Power optimizers
Power optimizers are devices that are attached to each solar panel, similar to microinverters. However, instead of converting the DC output to AC, they “condition” the DC electricity by adjusting the voltage and current. This optimized DC power is then sent to a centralized inverter for conversion to AC.
Advantages of using power optimizers include:
: By optimizing the DC power at the panel level, power optimizers can counteract inefficiencies from shading, dirt, or panel mismatch.
: Like microinverters, power optimizers allow for panels to be placed in various orientations and tilts without compromising the entire system’s efficiency.
: With individual optimizers on each panel, users can monitor the performance of each panel separately, identifying potential issues early on.
: Power optimizers can reduce the DC voltage to a safe level during installation, maintenance, or emergencies, ensuring safety.
: Power optimizers combine the advantages of both central inverters and microinverters, providing efficient performance with a centralized conversion system.
On the other hand, cons include:
Introducing power optimizers increases the system’s overall cost, as you’re essentially adding another component to each panel.
: The addition of power optimizers adds complexity to the system’s installation and wiring, potentially leading to longer installation times or more potential points of failure.
: If a power optimizer fails, it may require a technician to access the roof or the panel’s location, which might be more challenging than servicing a centralized inverter.
: While power optimizers boost panel efficiency, they themselves consume some power to operate, which can slightly reduce the overall system efficiency.
: Even with power optimizers, the system still relies on a central inverter to convert DC to AC. If the inverter fails, the entire system is affected, similar to a system without optimizers.
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Solar inverter FAQs
What is a solar power inverter?
A solar power inverter is an essential element of a photovoltaic system that makes electricity produced by solar panels usable in the home. It is responsible for converting the direct current (DC) output produced by solar panels into alternating current (AC) that can be used by household appliances and can be fed back into the electrical grid.
What does a solar power inverter do?
A solar power inverter converts direct current (DC) output into alternating current (AC) for use in standard electronics, appliances, and more.
How does a solar power inverter work?
Solar panels produce electricity in direct current (DC). Direct current is basically electricity flowing in one direction. The problem is, homes and businesses run on alternating current (AC), which is electricity reversing directions many times per second. A solar power inverter runs direct current through two or more resistors that switch off and on many times per second to feed a two-sided transformer, creating alternating current usable in homes.
How long does a solar inverter last?
A solar power inverter typically lasts 10-15 years, so you’ll probably have to replace it some time during the life of a solar system.
What is a good DC-to-AC ratio?
A 1:0.8 ratio (or 1.25 ratio) is the sweet spot for minimizing potential losses and improving efficiency. DC/AC ratio refers to the output capacity of a PV system compared to the processing capacity of an inverter. It’s logical to assume a 9 kWh PV system should be paired with a 9 kWh inverter (a 1:1 ratio, or 1 ratio). But that’s not the case. Most PV systems don’t regularly produce at their nameplate capacity, so choosing an inverter that’s around 80 percent lower capacity than the PV system’s nameplate output is ideal.
As the world is following the process of sustainable development, the invention of Solar Panels is a revolutionary change. Bothering about solar energy is something that is a renewable, affordable, and inexhaustible source of energy. It is believed that – Solar energy is a cheaper source of energy that can be wasted, and we are doing the same thing. Therefore, it is analyzed that this is the high time to begin utilizing solar energy. Solar energy is available in abundance.
From the investment perspective, investing money in solar energy is a significant one-time investment. The continuous use of solar energy has resulted in the reduction of consumption of electricity, which directly affects the electricity bill. The availability of solar energy is in abundance, so it can be seized and used in several ways.
This renewable resource requires convertible equipment to put this energy into proper use. Therefore, the need for an inverter gave birth to solar inverters. Solar inverters are the essential elements of a solar panel system.
The energy absorbed from the sun is stored in the batteries in the form of direct current (DC); later, this stored energy is further converted into electrical energy, which is named alternating current (AC) with the help of solar inverters. In simple words, solar inverters are used to convert the variable direct current and alternating current so that they can be utilized as a power supply. All electrical appliances run on ACs, and therefore these inverters play a very vital role in the utilization of solar energy.
If a person or community wants to use solar energy as their main source of energy, then solar panels need to be installed along with solar inverters. Prior to the installation of solar panels at your home, it is mandatory to choose a size for solar inverters. The size of the inverter is an important matter of consideration.
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The variability of the solar inverter is determined by its size. The size of the inverter plays a significant role in overall electricity production, and as a result, it affects the supply of the solar inverter.
The DC rating of the solar panel determines the size of the inverter. It is important because, in the solar panel converter, the capacity of the inverter should be capable of handling DC electricity.
The installation services come with guidelines, and it is the role of the vendor to train their clients about the dependability of electricity supply on solar inverters.
In the market, solar inverters are available in differentiated sizes. The ratings of the solar panels are made in Watts. In the same way, a solar inverter is also rated in Watts and symbolically represented as (W). The solar energy system will be producing and transmitting DC electricity to your inverter, so there is always a requirement for an inverter that can handle the load and convert it to AC power. This tells the appropriate necessity of the solar inverter.
Geographical Factors
Geographical Factors play a vital role in sizing your solar inverter due to their impact on the production of solar energy. The locations that have high temperatures are expected to have a large amount of solar radiation. Thus such a type of area generates more electricity as compared to an area with low temperature.
These areas vary in temperature and solar radiation; they will produce different amounts of DC at a particular time.
Under typical weather conditions, the solar inverter is likely to produce maximum output with its listed DC ratings.
Site Conditions
Another major factor to consider is the location. Solar PV inverter sizing is influenced by the solar array’s design and area of installation. The tilt of solar panels directly impacts the amount of electricity produced.
Other weather conditions like dust or moisture are also liable to affect the electricity production in solar panels. These are the major factors that act as a problem in the pathway of the sunlight that reaches the array.
The installation of solar panels comes up with equipment efficiency to overcome these hindrances. But these factors affect the production of electricity.
Size of Solar array
The size of the solar inverter is a major consideration. The inverter is liable to handle the electricity generated by the DCs by the solar array.
The inverter must have a similar size as the DC rating specified on the solar panels. For instance, when installing a 6-kilowatt solar energy system, the inverter must be 6,000W, give or take a few watts.
The size requirements for inverters are listed on the product sheet of the solar panel. The capacity that can be handled by the inverter is also listed there. Always keep in mind that using the incorrect size will diminish the warranty scheme.
The efficiency of the inverter drives the efficiency of a solar panel system as the role of the inverters is to convert Direct Current (DC) into Alternating Current (AC). These are utilized by the electric grid. This leads many to wonder what effect over-sizing or under-sizing an inverter can have. This type of entry helps the users to make better decisions with regard to their current or future solar photovoltaic installation.
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The inverter used in the solar systems is optimally functioning within a predetermined operational ‘window’ (usually laid out in the inverter’s specifications) as the power generated by the system’s solar panels keeps on fluctuating. The ability of the inverter to convert its energy from DC electricity to AC electricity differs with factors. The inverter will operate optimally as long as the input from the panels falls under the range of the window.
Here we have mentioned a graph to clarify the concept. The graph depicts the red line representing average inverter efficiency, and the green arrow highlights the power output from your solar panels. The gray box shows the operational window of the inverter based on the input from the solar panels and the predetermined efficiency of the inverter. In such a scenario, efficiency is less than 83%, which would be considered ‘suboptimal’. Here the system must be sized to minimize the amount of time during the day that the inverter operates within this range.
In order to understand the concept of Under-sizing your inverter, let us consider the above-mentioned graph. Under-sizing your inverter states that the maximum power output of your system (in kilowatts – kW) will be determined by the size of your inverter. Regardless of the output of the solar panels, the power output will be clipped by the inverter so that it does not exceed the inverter’s rated capacity like 3kW, 5kW etc.
The installer suggests an undersized inverter if they determine that the amount of incident solar irradiation (sunlight) on your panels will be lower than expected. This is all because of your area of location & climate, the orientation of your panels, and other factors.
Inverter under-sizing is understood as ‘overclocking’. The concept has actually become a common and widely accepted practice in countries like Australia. One of the largest inverter manufacturing companies is SMA which is a respected name in the industry.
Under perfect conditions, the maximum power output of a solar system will be clipped back to the inverter’s output through overclocking through the middle of the day. They can also be profitable in the overall amount of energy (kilowatt-hours – kWh) generated. The production gain is aroused from additional energy being produced in the early morning and late afternoon as a smaller inverter will turn on sooner and off later and operate more efficiently with lower DC inputs.
As per the Clean Energy Council rules for accredited installers, the solar panel capacity can only exceed the inverter capacity by 33%.
Installing an inverter whose maximum capacity is greater than the nominal capacity of your solar panel array might be the correct option to go with if you are planning to expand your solar panel array in the future. The situation is not recommended over the globe. The overall energy yields from your solar system may be less than a perfectly-sized or under-sized inverter when they are oversized.
The inverters are designed to handle lower power inputs than their nominal capacity. One must raise a query to the installer about how your system will perform with an oversized inverter. The query may include- how would your overall energy yield be differentiated from the next 5-10 years with an oversized inverter v/s a ‘right-sized’ or under-sized inverter? Balance this against the cost of the various system configurations before making your final decision.
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