How Does Sizing A Solar Inverter Work?

Solar inverters come in all different sizes, big and small. Similar to solar panels, the size of an inverter can be rated in watts (W). When it comes to solar inverter sizing, installers will consider three primary factors: the size of your solar array, geography, and site-specific conditions.

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Size of your solar array

The size of your solar array is the most important factor in determining the appropriate size for your solar inverter. Because your solar inverter converts DC electricity coming from the array, it needs to have the capacity to handle all the power the array produces.

As a general rule of thumb, the size of your inverter should be similar to the DC rating of your solar panel system; if you are installing a 6 kilowatt (kW) system, you can expect the proposed inverter to be around 6000 W, plus or minus a small percentage.

Inverter manufacturers typically list sizing guidelines for the array capacity their inverters can be paired with on their product spec sheets. If the size of the solar array paired with their inverter is outside the stated guidelines, manufacturers may void their warranty offering.

Geography

Geography also plays a vital role in sizing your solar inverter due to its impact on the production of your solar panel system. Properties in Arizona have higher solar irradiances (i.e., larger amounts of solar radiation) than properties in Vermont; as such, a rooftop 6 kilowatt (kW) system in Arizona should produce more power than a similarly sized system further north.

Because these two systems will produce different amounts of DC electricity at a given time, the inverters needed to handle that electricity load can also be different sizes. In areas with more sunshine and moderate temperatures, inverters will likely be sized closer to the solar array's overall wattage so it can handle close to the maximum power output of the array at any given point. Alternatively, if your solar array experiences lower amounts of solar radiation or high temperatures that decrease panel efficiency, it's less likely to produce that maximum power output defined by the DC rating under standard testing conditions (STC). A smaller, undersized inverter may get the job done in these scenarios.

Site-specific factors

The site and design specifics of your solar array will impact the size of your solar inverter. Like geography, the tilt and azimuth your solar array is installed at will affect how much electricity the system can produce. Environmental factors (such as shading, dust, etc.) will significantly affect how much sunshine reaches the array.

Solar installers will account for these considerations, equipment efficiencies, and more when estimating the overall production of your solar panel system. All will contribute to the overall derating factor of your system, which is used to help determine what your solar panel system will produce in a real-life scenario (as opposed to the STC specs determined in a lab.) Solar panel systems that experience more shade, sit at a sub-optimal tilt, or face east rather than due south have higher derating factors than systems on sunny, south-facing roofs.

Solar panel systems with higher derating factors will not hit their maximum energy output and can afford smaller inverter capacities relative to the size of the array.

The efficiency of the inverter drives the efficiency of a solar panel system. Inverters change the Direct Current (DC) from solar panels into Alternating Current (AC), which is what we use in our homes and businesses.

This article talks about how to pick the right size solar inverter. We also look at different solar inverter prices and brands, to help you choose the right one.

This leads many to wonder what effect over-sizing or under-sizing an inverter will have on overall system efficiency.

Understanding Solar Inverters

A solar system’s inverter works best within a specific ‘window’ of operation. This ‘window’ is the range that the inverter is designed to work in. You can usually find it in the inverter’s details or specifications.

As the power input from the system’s solar panels goes up and down, the inverter’s ability to efficiently convert it from DC electricity to AC electricity differs.

Solar Inverter Efficiency System Performance

As long as the input from the panels falls within the range of the window, the inverter can be considered to be operating optimally.

In the graph below, the red line represents an average inverter efficiency and the green arrow represents the power output from your solar panels.

The grey box shows the operational window of the inverter based on the input from the solar panels and the predetermined efficiency of the inverter.

Optimal Sizing for Maximum Efficiency

In this case, an efficiency of less than about 83% would be considered ‘sub-optimal’, and ideally the system should be sized to minimise the amount of time during the day that the inverter operates within this range.

Under-sizing Your Inverter

Using the graph above as an example, under-sizing your inverter will mean that the maximum power output of your system (in kilowatts – kW) will be dictated by the size of your inverter.

Solar inverter under-sizing (or solar panel array oversizing) has a become common practice in Australia and is generally preferential to inverter over-sizing.

If an inverter is under-sized, this should happen within certain parameters – which accredited solar installers will be familiar with.

The Role of Inverter Size in Solar Panel Output

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Regardless of the output of the solar panels, the power output will be cut off (‘clipped’) by the inverter so that it does not exceed the inverter’s rated capacity (e.g. 3kW, 5kW etc).

Your installer may suggest an undersized inverter if they determine that the amount of incident solar irradiation (sunlight) on your panels will be lower than expected – because of your location & climate, the orientation of your panels, or other factors.

Inverter under-sizing – sometimes referred to as ‘overclocking’ – has become a common and widely accepted practice in Australia – even endorsed by inverter manufacturer SMA, one of the largest and most respected names in the industry. (Read more about overclocking.)

Efficiency Gains at Different Times of the Day

Although 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, there can also be gains in the overall amount of energy (kilowatt-hours – kWh) generated.

The gains come 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.

The chart below offers an illustration of how the midday losses (red) associated with an ‘under-sized’ inverter can be offset by morning and afternoon gains (green).

Under the Clean Energy Council rules for accredited installers, the solar panel capacity can only exceed the inverter capacity by 33%. That means for a typical 5kW inverter you can go up to a maximum of 6.6kW of solar panel output within the rules.

Over-sizing Your Inverter

Installing an inverter whose maximum capacity is greater than the nominal capacity of your solar panel array may be an option if you’re looking to expand your solar panel array at some point in the future, but it is not generally recommended.

In this scenario, the overall energy yields from your solar system may be lower than of a perfectly sized or under-sized inverter – especially if it is significantly oversized.

Although inverters are generally designed to handle lower power inputs than their nominal capacity, there are limits to this.

It’s therefore important to ask your installer questions about how your system will perform in the event that an over-sized inverter is suggested – e.g. how would your overall energy yields differ over the next 5-10 years with an over-sized inverter vs a ‘right-sized’ or under-sized inverter?

Balance this against the cost of the various system configurations before making your final decision.

How to Calculate the Right Inverter Size

To calculate the ideal inverter size for your solar PV system, you should consider the total wattage of your solar panels and the specific conditions of your installation site. The general rule is to ensure the inverter’s maximum capacity closely matches or slightly exceeds the solar panel array’s peak power output. However, slight over-sizing of the solar panels compared to the inverter capacity (up to 133% under certain guidelines) can sometimes yield better overall efficiency due to the variable nature of solar irradiation throughout the day.

The Ratio for Inverter Sizing

The ratio for inverter sizing often depends on specific system requirements and local regulations. A commonly accepted ratio is that the total nominal power of the solar panels can exceed the inverter’s capacity by up to 133%, as per some guidelines by regulatory bodies such as the Clean Energy Council in Australia. This practice, known as “overclocking,” allows for more energy production in the early morning and late afternoon, although it may result in power clipping during peak sun hours.

Calculating the Capacity of an Inverter

The capacity of an inverter is determined by its maximum output in watts (W) or kilowatts (kW). To calculate the required capacity for your solar inverter, sum up the total wattage of your solar panels and adjust based on expected system efficiency, shading, and the specific energy needs of your household or business. Remember, solar inverters are rated based on their continuous and peak power output, which should align with your solar PV system’s output and your energy consumption pattern.

Should You Over-size or Under-size Your Inverter?

Deciding whether to oversize or undersize your solar inverter hinges on balancing efficiency with potential future system expansions. Over-sizing an inverter may be prudent if you anticipate increasing your solar array size, allowing for greater flexibility without the need for immediate inverter replacement. On the other hand, under-sizing, within regulatory limits, can lead to more efficient operation during low light conditions, although it risks energy clipping at peak times. It’s essential to evaluate your current and future energy needs, the solar irradiance patterns of your location, and regulatory constraints before making a decision.

Conclusion

Choosing the correct inverter size is crucial for the efficiency and effectiveness of your solar PV system. While oversizing can prepare you for future expansions, undersizing might optimize your system’s performance under certain conditions. The key is to strike a balance that aligns with your energy needs, potential growth, and the operational characteristics of solar panels and inverters. Consulting with a solar energy expert can provide tailored advice, ensuring that your system is well-equipped to meet your energy goals both now and in the future.

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