Lesson 5: Solar inverter oversizing vs. undersizing

If you have a 3,000-watt solar panel array, it just makes sense that you’d pair it with a 3,000-watt inverter, or does it? In some cases, it may make sense to pair a smaller inverter, say 2,400 watts, with that 3,000-watt solar array. When you pair an inverter that is underrated for the amount of power the system is designed to generate, that’s called undersizing. There is also a situation where it may make sense to pair an inverter that’s rated higher than the solar array’s output. That’s known as oversizing. 

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Solar inverter undersizing causes clipping

When you undersize an inverter, you pair it with a system that can produce more power than the inverter is rated for. That can cause inverter clipping. Clipping happens when there is more DC power being fed into the inverter than it is rated for. When that happens, the inverter will produce its maximum output and no more. The excess amount of power is simply “clipped” off.

If you graph the daily power output of a solar system, the resulting graph will be a bell-shaped curve. It will begin curving up gradually in the morning, then it will reach its peak for about two hours in midday, then it will curve downward. When an inverter clips, some of that peak output during midday is lost, so the bell curve has a flat top.

When oversizing an inverter is a good choice

The only time that oversizing is a good idea is when the customer plans to add capacity in the future. By providing an oversized inverter, the customer would be saved the future expense of upgrading their inverter when they add panels to their system. There is a downside, however, because the undersized inverter never reaches its full power production, some potential power production could be lost.

Why undersizing an inverter can be a good choice

A solar system will only produce its peak power output under ideal conditions. Those conditions are a temperature of 25 degrees C, 1000W per square meter (m2) of sunlight, and an Air Mass Density of 1.5. These conditions may be present only a few times out of the year or perhaps not at all. Due to those limitations, a solar system is only rarely going to achieve its maximum output, if at all.

Undersizing may avoid a main panel upgrade

A home may be able to accommodate a solar system that can produce 10 kilowatts. At a voltage output of 220 volts, that would produce 45 Amps of current. But what if the home’s main panel can only accommodate 40 Amps? By substituting a 7.6-kilowatt inverter, the maximum power output can be kept below the home’s main panel’s rated capacity. That would then avoid a main panel upgrade and keep costs down for the homeowner.

Undersizing can result in higher daily power production

Undersizing will reduce the system’s power output under conditions that would result in the system reaching its peak output, but that would be true for only a couple of hours in the day. But an interesting thing happens with undersized inverters in the mornings and afternoons. Undersized inverters will ramp up quicker in the mornings, and ramp down slower in the afternoons. If you graph the power output, you’ll see a slightly lower peak production, but higher morning and evening production, resulting in a fatter power production curve. The result of this is that the undersized system would produce more power in total than a system that wasn’t undersized.

How much should you undersize an inverter?

According to the Clean Energy Council, you can have a solar array that can put out up to 30% more power than the inverter is rated for and remain within safe guidelines. The amount that you would want to undersize the inverter depends on the conditions that the system is installed in. Primarily, the DC-to-AC ratio, which is the ratio of DC current produced by the solar panels, versus the AC output of the inverter.

How the DC-to-AC ratio affects total system output

In an undersized system, the DC-to-AC ratio will be greater than one. If you don’t undersize enough, then the system will generate less power than it could in the mornings and evenings. But if you undersize it too high, you could lose power production in midday. The amount you want to undersize primarily depends on the location (city, state) that the system is located in, the angle that the system is mounted at, and whether the customer is going to be on a time-of-use rate plan. The analysis necessary to properly undersize the system is complicated, system designers will use often use simulation programs like PVsyst, PV*SOL, or SAM. 

The trend for homeowners who will be under time-of-use plans is to undersize as high as safely possible to maximize afternoon energy production, with DC-to-AC ratios as high as 1.5 to 1. The ideal DC-to-AC ratio would have the inverter working at between 85% to 95% of it’s rated capacity for as long as possible during the day.

Conclusion: Undersizing an inverter has become a best practice

A properly undersized solar system will produce the best power output for the system owner. It can also save the homeowner money if it enables them to avoid a main panel upgrade. As long as the undersizing stays within the 30% guideline recommended by the Clean Energy Council, the load on the inverter will not be severe enough to cause premature component failure.

The above general information is for illustrative purposes only and is not intended to be relied upon for any individual circumstance.

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Different Kinds of Solar Inverters

Microinverters

Microinverters are compact devices designed to convert the direct current (DC) output from solar panels into alternating current (AC) that can be utilized in homes. Similar in size to a WiFi router, these inverters are typically installed directly beneath each solar panel, often with one microinverter serving between one and four panels.

Benefits of Microinverters

1.Enhanced Efficiency: Unlike string inverters that are limited by the performance of the least efficient panel, microinverters operate on a parallel circuit. This setup ensures they are not constrained by the panel with the lowest output, thereby increasing overall efficiency.
2.Detailed Monitoring: Microinverters allow for detailed, panel-by-panel monitoring of energy production, offering more precise data than whole-system monitoring. This makes it easier to identify and address issues with individual panels.
3.Simplified Expansion: Adding capacity to a photovoltaic (PV) system is straightforward with microinverters. Installation involves simply adding an additional microinverter for every one to four new panels.
4.Safety Features: Microinverters can be quickly shut down, an essential feature required by recent electrical codes for safety in emergencies or during maintenance.
5.Durability: Typically, microinverters come with warranties lasting up to 25 years, significantly longer than the 8-12 years offered for standard inverters, highlighting their robustness and long-term reliability.

However, there are some disadvantages to consider:

1.Higher Initial Costs: Microinverters typically come with a steeper price tag, costing about $1,000 more than string inverters for a standard 5kW home solar installation. This increased upfront investment can be a significant consideration.
2.Service Challenges: If a microinverter fails, maintenance can be cumbersome. Technicians must access the roof, dismantle part of the mounting rack, and remove the solar panel to reach the faulty unit. This process is not only time-consuming but also potentially costly.

Microinverters are particularly advantageous in environments where solar panels experience variable shading, are oriented in different directions, or might require future expansion. Additionally, they are beneficial in regions where local regulations mandate rapid shutdown capabilities to enhance safety during emergencies. These features make microinverters a suitable choice for many, despite their higher initial costs and maintenance complexity.

Central (or string) inverters

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Central inverters, also known as string inverters, are devices designed to convert the direct current (DC) output from a series of solar panels into alternating current (AC) suitable for home or commercial use. These inverters are typically bulkier and installed in a centralized location, such as near the home's main electrical panel or on an exterior wall.

Benefits of Central Inverters

1.Cost Efficiency: On a per-watt basis, central inverters tend to be more economical than microinverters. This makes them particularly attractive for larger installations where cost-effectiveness is key.
2.Proven Reliability: With a longer history in the market compared to microinverters, central inverters are well-established and trusted by many installation professionals.
3.Simplified Hardware: Since only one inverter is required to manage multiple panels, there is less equipment to install and maintain, which can simplify the setup on rooftops.
4.Ease of Maintenance: Typically positioned in easily reachable locations, central inverters are generally simpler to service or replace compared to multiple microinverters installed on rooftops.
5.Optimal Performance Under Ideal Conditions: In environments without shading, where all solar panels are aligned and angled consistently, central inverters can deliver excellent performance, maximizing the energy output from the solar array.

However, there are some disadvantages to consider:

1.Shading Impact: Performance can be significantly reduced by shading or debris on even a single panel. In such cases, the output of the entire string of panels is compromised due to their interconnected nature.
2.Limited Monitoring Capabilities: Central inverters lack the ability to monitor individual panels. This can make it challenging to pinpoint which panel might be underperforming without detailed diagnostics.
3.Shorter Lifespan: Generally, central inverters need replacing every 10-15 years, which is less than the lifespan often offered by microinverters.
4.Expansion Challenges: Adding more panels can be complex with central inverters, particularly if the existing unit is nearing its capacity. This might necessitate the installation of a larger or additional inverters.
5.Single Point of Failure: A failure in the central inverter halts the entire system's electricity production. This contrasts with systems using microinverters or power optimizers, where only a portion of the system's output might be affected.

Power optimizers

Power optimizers are devices connected to each solar panel, functioning similarly to microinverters but with a distinct role. Instead of converting DC output to AC directly, power optimizers adjust the voltage and current to "condition" the DC electricity. This optimized DC is then sent to a centralized inverter for conversion to AC.

Benefits of Using Power Optimizers

1.Enhanced Efficiency: Power optimizers improve the efficiency at the panel level, compensating for losses due to shading, dirt, or differences between panels. This leads to a more consistent and higher overall system output.
2.Flexible Installation Options: They offer installation versatility, allowing solar panels to be set up in various orientations and tilts. This flexibility helps maintain high system efficiency regardless of individual panel positioning.
3.Detailed Performance Monitoring: Equipped with individual optimizers on each panel, these systems enable precise monitoring of each panel’s performance. This feature aids in early detection of issues, allowing for quicker resolution.
4.Increased Safety Measures: During installation, maintenance, or emergencies, power optimizers can reduce the DC voltage to safe levels, enhancing overall safety.
5.Hybrid Advantages: By combining features of both central inverters and microinverters, power optimizers provide the benefits of efficient performance with centralized AC conversion, capturing the best of both technologies.

However, there are some disadvantages to consider:

1.Increased Costs: Adding power optimizers to each panel elevates the initial investment, as each optimizer represents an additional component within the system.
2.Installation Complexity: The incorporation of power optimizers can complicate the installation process and the system’s overall wiring. This might extend installation times and introduce more potential points of failure.
3.Maintenance Challenges: Should a power optimizer malfunction, fixing it usually involves accessing difficult-to-reach areas on the roof or around the panel, which can be more cumbersome than servicing a centralized inverter.
4.Operational Efficiency: Although power optimizers enhance the efficiency of individual panels, they do consume some power themselves. This slight energy usage can marginally reduce the net efficiency of the entire system.
5.Central Inverter Dependency: Despite the benefits of individual optimizers, the system still depends on a central inverter for DC to AC conversion. Consequently, if the central inverter fails, it impacts the entire system's functionality, similar to setups without optimizers.

Deciding on the Right Size for Solar Inverters

1. Oversizing a solar array compared to the inverter’s capacity (with a DC-to-AC ratio exceeding one) can boost energy production throughout the day, particularly during early morning and late afternoon hours.
2. If a solar array generates more power than the inverter's capacity, the inverter will limit its output to its maximum rated power, a phenomenon known as inverter clipping.
3. To enhance energy production without encountering inverter clipping, one option is to add an additional inverter. Designers need to weigh the extra costs of purchasing and installing another inverter against the potential energy losses from inverter clipping when deciding to oversize the array.
4. Accurate energy production forecasts in solar project designs must consider inverter clipping to ensure that the estimated outputs realistically align with the system’s designed capacity.

Introducing Innotinum: Advanced Energy Solutions for Modern Homes

Innotinum's inverters are designed for optimal energy efficiency in modern homes, supporting PV, grid, and battery power with power priority management to minimize electricity bills and ensure critical loads are powered. These inverters feature a high-level integration with a DC-coupled system, reducing power loss and enhancing stability. Additionally, they are highly compatible with self-designed battery energy storage systems, ensuring deep communication and increased system reliability. This makes Innotinum inverters a smart choice for those seeking advanced, efficient energy solutions.

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