Everything You Need to Know About Inverter Sizing

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    At Power Northwest, we understand that every solar system can be unique to every home or business. For this reason, one of the most important things we look at when installing solar systems is inverter sizing. This article will go into the basics of what an inverter is, the types of inverters, inverter power outputs, and how the DC-to-AC size ratio is vital in making a solar system run as efficiently as possible.

    What is an Inverter?

    A solar inverter is an essential component of a solar energy system. It is responsible for converting the direct current (DC) electricity produced by solar panels into alternating current (AC) electricity, which is compatible with standard electrical appliances and can be used in homes, businesses, and the grid. The primary function of a solar inverter is to ensure that the energy generated by the solar panels is efficiently transformed and utilized.

    It also provides safety features such as isolation from the grid during power outages to protect utility workers. In addition to the conversion process, modern solar inverters often include advanced features like maximum power point tracking (MPPT), monitoring capabilities, and compatibility with energy storage systems, enabling efficient power generation, system monitoring, and integration with battery backup solutions.

    Types of Solar Inverters?

    There are two main types of solar inverters commonly used in solar systems: string inverters and microinverters. String inverters, also known as central inverters, are centrally located and connected to multiple solar panels arranged in series, forming a string configuration. String inverters are cost-effective and suitable for large-scale installations where simplicity and overall system efficiency are prioritized. However, monitoring capabilities typically apply at the string level rather than at the individual panel level.  They also tend to perform worse in shaded applications unless additional hardware such as DC to DC optimizers is included.

    Microinverters, on the other hand, are installed on each individual solar panel. This allows for advanced monitoring capabilities at the panel level, enabling real-time performance tracking, issue identification, and system optimization. Microinverters, or micros, perform well regardless of shading, varying orientations, tilts, or panel count.  Microinverters provide greater system flexibility and safety features such as rapid shutdown, ensuring the safety of installers and utility workers. 

    Microinverters are commonly chosen for residential and smaller commercial installations where panel-level monitoring and optimization are desired.  Whereas string inverters are the preferred choice in large commercial or utility-scale applications where there is minimal shading.  If your home is unshaded, a string inverter, being more economical, could produce the same amount of power for less money, making it a better investment. 

    Tesla (String) Vs Enphase (Micro)

    To help you get an idea of some of the key differences between (micro) and (string) inverters, here is an example between Tesla’s string inverters and Enphase’s microinverters:

    Panel-level monitoring capabilitiesEmphase (micro)
    Converts energy at one central locationConverts energy individually at each panel
    Warranty: 12.5 yearsWarranty: 25 years 
    Shading may affect the overall energy production of a systemShading will only affect the individual solar panel’s energy production
    CheaperMore expensive 
    Unable to monitor individual solar panel performance Panel level monitoring capabilities
    Table: Tesla VS Enphase Inverters

    Inverter Power Outputs?

    Since inverters convert DC power to AC power the output of the inverter is measured in either power (kW AC) or current (amps) and voltage (typically 240v AC).  For example, the Tesla string inverter has a power output of 7.6 kW AC or 31.6 amps at 240v AC.  An Enphase IQ-8+ micro inverter has a power output of 290 W AC (.29 kW AC) or 1.21 amps at 240v AC.  If you multiply that by the number of microinverters on your solar system, you’d arrive at the system size in kW AC or amps AC.

    An important thing to note is that the DC size of a solar system is determined by the panel wattage times the number of panels, whereas the AC size of your system is determined by the power output of your inverter(s).

    What is inverter clipping?

    Inverter clipping occurs when an inverter output is exceeded by the power input.  For example, if you pair an IQ-8+ inverter with a 370W DC panel, the maximum power output that you will ever see is 290W AC, limited by the inverter.  Now, a 370W panel will almost never actually produce 370W in the field.  That rating was given to the panel under ideal laboratory conditions.  In the real world, a 370W panel may produce 50 to 350W throughout the day depending on the angle of sunlight, but will almost never reach the full laboratory-rated value.  It will, however, spend the majority of the time producing below the 290W rating of the microinverter.

    In fact, we’ll intentionally undersize the inverter, knowing this fact.  It’s a common practice that helps make solar systems more financially viable, providing a better return on investment.

    Choosing DC/AC Sizes

    Ideally, a solar system setup should have minimal inverter clipping. Sizing your solar system appropriately, specifically the DC-to-AC size ratio, can help mitigate clipping. It is best when the total capacity of your solar panels (DC size) is slightly bigger than the peak capacity of your inverters (AC size). To set up an efficient solar system, we recommend a DC-to-AC ratio of 1.25:1, or as close as possible to that ratio.  This typically provides the best return on investment for residential applications in the Pacific Northwest where we often see sub-optimal conditions with partially shaded applications or frequent cloudy days.

    Why a DC-to-AC ratio of 1.25:1? 

    The energy your solar panels produce is always lower than the max ratings stated by manufacturers. Solar manufacturers frequently test their products under perfect laboratory conditions, but in the real world, many factors reduce the effectiveness of solar panels. These factors may include things like tree shading, roof incline, weather conditions, and other environmental factors. That is why it is always best to size your system appropriately to avoid any unnecessary expenses. A DC-to-AC ratio of 1.25:1 ensures that your inverters are being used to their full potential.

    Calculating the Size Ratio of Your Solar System

    To calculate the DC-to-AC ratio you must first calculate their individual capacities. For example, to determine your DC size, you multiply the number of solar panels by their maximum output in watts. Let’s say you have (20) panels on your roof, each with a 370W capacity, (20) x 370W = 7,400W DC, or 7.4 kW DC. Your AC size is calculated similarly. If you have (20) microinverters, each with a peak output of 290W, (20) x 290W = 5,800W AC, or 5.8kW AC. 

    Solar Panels: (20) x 370W = 7,400W = 7.4kW DC size

    Microinverters: (20) x 290W = 5,800W = 5.8kW AC size

    DC/AC Size: 7.4kW/5.8kW = 1.28:1

    The higher the ratio, the more likely your solar panels are producing energy that your inverters can’t handle. On the contrary, the lower the ratio, the more likely you’re overspending on inverter capacity that you’ll never use.

    enphase iq8 inverter on a solar panel

    Guide to Enphase Inverters

    The following chart is a guide to the different microinverters Enphase offers. Keep in mind that there are many configurations that may provide a more efficient solar system unique to you:

    Inverter Peak Power OutputSuggested Solar Panel RangesAn Example PairingDC-to-AC Size Ratio
    IQ-8245W235-350W(20) x 310W = 6,200W(20) x 245W = 4,900W1.26:1
    IQ-8+290W235-440W(20) x 370W = 7,400W(20) x 290W = 5,800W1.27:1
    IQ-8M330W260-460W(20) x 410W = 8,200W(20) x 330W = 6,600W1.24:1
    IQ-8A366W295-500W(20) x 450W = 9,000W(20) x 366W = 7,320W1.23:1

    Contact Us Today

    Whether you want to know more about the different types of inverters or about inverter clipping, Power Northwest is here to answer any of your questions. We strive to optimize your solar system for your unique home or business as best as possible. Get in contact with Power Northwest today to receive a quote for an installation that is right for you.

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