Optimizing Array-to-Inverter Power Ratio

Developers and PV system designers are challenged to stay competitive in the constantly evolving solar marketplace. With the steep decline of solar module prices, designers are exploring the economic benefits of increasing the array-to-inverter power ratio. Here I discuss dc loading in general, then focus on array-to-inverter sizing design approaches. I describe some scenarios supporting the trend for higher dc load ratios and present some relevant production modeling results. I also discuss typical inverter operational limits related to dc loading.

DC Loading

The array-to-inverter power ratio is defined as the relationship between array capacity in dc watts and inverter capacity in ac watts. Array capacity is determined by the array nameplate power rating under standard test conditions (STC), meaning at 1,000 W/m2, 25°C cell temperature and a reference solar spectral irradiance of air mass 1.5. The total inverter maximum output power rating determines the inverter capacity. For example, if you connect a solar array with an STC rating of 575 kWdc to one or several inverters with a total maximum rated output power of 500 kWac, then the resulting array-to-inverter power ratio is 1.15, or 115% (575 kWdc ÷ 500 kWac). Other terms for array-to-inverter power ratio include dc load ratio, dc-to-ac ratio, oversizing ratio and overloading ratio.

Evolving Design Practices

System design approaches to dc loading have evolved over the past 5 years and are closely tied to module price trends. The traditional design approach to dc loading is conservative, as it was a direct response to high module prices. Newer design approaches use higher loading ratios, in large part because module prices are an increasingly smaller percentage of total project costs.

Traditional approach. When module prices were high, the system designer’s main goal was to define a dc load ratio that ensured none or very little of the power produced by the expensive PV array was wasted. Designers specifically wanted to avoid inverter power limiting, which occurs whenever the array is capable of producing more power than the inverter can process. They would typically determine a project’s optimal dc-to-ac sizing ratio by analyzing the annual energy production per kilowatt of PV energy at different loading ratios. This kWh/kW metric is known as specific yield and is a measure of production efficiency.

The traditional design approach generally results in dc load ratios within a 1.1–1.2 range, depending on the project location and design details. These conservative overloading ratios allow the designer to offset a variety of environmental and system-level loss factors—such as cell temperature, irradiance, tilt angle, soiling, module mismatch, array degradation, conductor resistance and so forth—without exceeding the inverter capacity under typical real-world conditions.

Higher dc loading approach. As module prices have fallen, PV system design philosophy has shifted. Rather than focusing on production efficiency and maximizing the output of each individual module, designers have begun designing for maximum financial efficiency at the system level. In many cases, the incremental cost to increase PV array capacity is small compared to the value of the associated energy production gains. This allows system designers to capitalize on higher dc load ratios—up to 1.5 and in some cases even higher—despite the potential for PV power generation to exceed inverter capacity during peak hours.

Rationale for High-DC Loading

Project developers and system designers might opt to increase dc loading beyond 1.2 for several reasons. Higher dc load ratios allow designers to get more value from fixed development costs. They also allow designers to capitalize on high-value energy rates, or time-of-delivery or time-of-day (TOD) rate structures that incentivize summer production. Increasing the dc load ratio is a compelling design approach when there is a limit on ac system size but no corresponding limit on dc system size. It also allows designers to increase production in response to suboptimal conditions such as cloudy climates or long-term array degradation.

Fixed development costs. When system designers increase the dc-to-ac load ratio beyond 1.2, the total system cost does not increase in direct proportion to the increase in array capacity since many of the project costs remain the same. These fixed costs include permitting, interconnection fees, legal fees, and inverter and ac interconnection hardware costs. Therefore, the cost to add array capacity is limited to the material and labor costs associated with adding more modules and dc balance of system components. By increasing the dc loading ratio, designers may be able to take full advantage of these fixed development and structural costs.

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