In this document
In this document: We will explain the importance of DC/AC ratio, the different ways of calculating the DC/AC ratio and their uses.
The DC/AC ratio, also known as the Inverter Loading Ratio (ILR) or sizing ratio, is a fundamental parameter in the design and optimization of PV power plants. It describes the relationship between the installed capacity of the solar array (i.e. the DC output power) and the power conversion capacity of the inverters (i.e. the AC output power).
Importance of DC/AC ratio
In the PV industry, it is standard practice to oversize the DC solar array relative to the AC inverter output, resulting in a ratio greater than 1.0. Solar arrays rarely produce their full peak power due to real-world conditions such as suboptimal irradiance (due to sun position or clouds) or high temperatures. A higher DC/AC ratio ensures the inverter operates closer to its maximum capacity for more hours of the day. This maximizes the inverter utilization and improves the financial viability of a project. On the other hand, if the conditions are right and the solar array produces more energy than the inverters can process, the output of the power plant will be limited (phenomenon known as inverter clipping) and it will be losing potential revenue. Careful choice of the DC/AC ratio is therefore crucial for any PV project to find the balance between system costs and energy yield.
Methods of calculation
In the PV industry, the DC/AC ratio can be calculated in two primary ways depending on whether you are focused on general sizing or detailed technical performance assessment.
Nominal DC power to nominal AC power
This is the conventional method used by most simulation software and for general project descriptions. It uses the nameplate ratings of the components - power of the PV modules at the Standard Test Conditions (STC) measured according to the IEC 60904-1 standard, and rated (nominal) power of the inverters according to the IEC 60146-1-1 standard.
Formula
Advantage
As a conventional figure it is useful for describing PV projects and comparing their technical configuration on a high level.
Nominal DC power to maximum AC power
Inverters often have a “maximum AC power” higher than the “nominal power”, meaning they can actually output more AC power depending on their operating conditions such as ambient temperature and power factor management. The maximum AC power is also measured and specified according to the IEC 60146-1-1 standard.
This method for DC/AC ratio calculation is more useful for operational analysis. It compares the nominal DC capacity against the maximum AC output the inverter is capable of producing.
Formula
Advantage
More accurate description of the physical limits of the inverters which will cause clipping, setting more accurate expectations of the power delivered to the grid.
DC/AC ratio in Solargis Evaluate
In Solargis Evaluate we use the Nominal DC Power to Maximum AC Power (second method mentioned) to calculate the DC/AC ratio shown in the user interface. This is because the second method provides a more realistic view of when clipping occurs in real-world operation, leading to a more accurate simulation of the expected energy yield. With the DC installed capacity shown in the top left of the Energy System Designer interface, the DC/AC ratio allows you to quickly gauge the power output capability of your configuration, and inform your design optimization process.