Inverter sizing is a new feature that helps you verify the compatibility between your selected inverter and PV module library components.
When to use it?
- At the beginning of a new project, consider reviewing the PV modules and inverters planned for use. Verify product compatibility and select the optimal string size. This choice influences the layout, electrical topography, and yield analysis—such as comparing string versus central inverters.
- During the preliminary design phase, after selecting the primary option, consider evaluating a secondary PV module or inverter. This can help confirm how the product change impacts project details such as system size, yield, and LCOE.
- During the detailed design phase, you might make changes to the design intended for construction. You will assess the flexibility of these modifications to ensure the product remains compatible. For instance, if string sizes in a specific area need adjustment, you should also verify the connections to the MPPs.
How does it work?
The Inverter sizing interface
- From the dropdown menus, choose the manufacturer and model of the PV module and the Inverter.
- If the model you need isn’t available, you can easily add it from the database by clicking Manage Database. This opens a website in your browser that contains additional instructions on how to use it, which can be found here.
- It’s also important to know the design location. If you've selected a site, it'll be shown here. If not, click on Layout generation settings and update the Project location.
- The location determines the minimum and maximum temperatures based on data from the PVGIS database. You can also manually adjust these temperatures if you have a trusted alternative source. To revert to the PVGIS values at any time, click on the revert button (circular arrow) to the left of the values.
This temperature matters because it helps determine string size options. Low temperatures increase PV module and string voltage; high temperatures decrease it. Ensuring string voltage matches inverter requirements is crucial for optimal, safe operation. PVcase calculates this using the following formulas.
- The calculated sizes are displayed under “recommended range”. You can manually enter min. and max. values, but this might be outside the inverter's operating range. We recommend consulting with the manufacturer to confirm the sizing.
- Now that the string size is defined, it’s important to check how many strings can fit on the inverter.
This is done by checking the inverter's current limit against the strings' current. PVcase calculates each string's maximum power point current (Impp) from the PV module and compares it to the inverter's maximum current limit (Imax mpp). It checks each MPPT and indicates how many strings can connect to the inverter.
- Check the system's recommended maximum number of strings per inverter.
- If desired, manually enter the number of strings to customize your layout.
- While string size (voltage) is relatively fixed, the number of strings is flexible.
- Adjusting the number of strings lets you fine-tune and experiment with the inverter's overall DC/AC ratio.
- Once you've selected your settings, click Apply.
DC/AC ratio
This is also known as the DC/AC ratio, generally defined as R:
R = \frac{P_{DC}(kWp)}{P_{AC}(kVA)}
Where:
PDC = the power of all PV modules connected to the inverter expressed in kWp at STC
PAC = the maximum output of the Inverter expressed in kVA
Typically, the DC power generated by PV modules surpasses the inverter's AC power capacity, a situation called inverter undersizing. This results in a sizing ratio greater than 1. For instance, in northern Europe, string inverters are often sized at about 1.2, so for a 6kWp PV array, the inverter's AC output is limited to 5kW.
PVcase GM clearly shows the different string sizes and string-per-inverter combinations in the DC/AC ratio matrix. The recommended range is what we calculated earlier and is displayed in graphical format. The green dots represent your chosen values. They are green because they fall entirely within the recommended range.DC/AC ratio
This is also known as the DC/AC ratio, generally defined as R:
R = \frac{P_{DC}(kWp)}{P_{AC}(kVA)}
Where:
PDC = the power of all PV modules connected to the inverter expressed in kWp at STC
PAC = the maximum output of the Inverter expressed in kVA
Typically, the DC power generated by PV modules surpasses the inverter's AC power capacity, a situation called inverter undersizing. This results in a sizing ratio greater than 1. For instance, in northern Europe, string inverters are often sized at about 1.2, so for a 6kWp PV array, the inverter's AC output is limited to 5kW.
PVcase GM clearly shows the different string sizes and string-per-inverter combinations in the DC/AC ratio matrix. The recommended range is what we calculated earlier and is displayed in graphical format. The green dots represent your chosen values. They are green because they fall entirely within the recommended range.
You can also choose to ignore PVcase’s recommendations and overload the inverter. In such cases, the dots will turn orange, and an advisory message will appear in the Inverter sizing window. For example, this indicates that the number of strings exceeds the calculated limit.
To evaluate the impact of the DC/AC ratio, it’s essential to conduct a yield study using tools such as PVcase Yield or PVSYST. Only through this process can you accurately determine how inverter sizing will influence the PV system's performance.
MPP distribution
Each inverter includes an MPPT (maximum power point tracker) that monitors the changing DC voltage and current from the PV modules, converting it into a stable DC voltage to maximize efficiency when inverting to AC. The number of MPPTs varies by inverter model, ranging from 1 to over 10. Having multiple MPPTs allows a single inverter to effectively manage PV strings operating at different voltage levels.
When should strings be separated?
String voltage fluctuates based on environmental and design factors. If your strings differ in any of the following criteria, they must be connected to separate MPPTs to maximize efficiency:
- Inclination (Tilt angle)
- Orientation (Azimuth)
- Shading conditions
- String size (The number of PV modules per string)
Real-World Example: Shading on Multi-Row Fixed tilts
Consider a common fixed-tilt system with two rows of modules on a single frame. During winter, the bottom row experiences more shading than the top row.
- The correct setup: Connect the bottom-row strings to one MPPT and the top-row strings to a separate MPPT. Each string can then operate at its own maximum efficiency.
- The risk (Mismatch Losses): If both the shaded and unshaded strings share the same MPPT, the tracker is forced to drop down to the lower voltage of the shaded string. This drastically reduces the power output of the unshaded string, resulting in significant mismatch losses. Using separate MPPTs can significantly enhance the performance of fixed tilt PV systems.
PVcase GM enables you to manually set the maximum number of strings to connect to each MPPT. This setting is used during device generation, where the strings are allocated to the inverter's MPPTs based on optimal matching.
How do I generate with Inverter sizing?
After you apply your settings in Inverter Sizing, Smart stringing will be updated to match your chosen string size range. Now you can confidently string the site, knowing that the inverter will be capable of connecting to all the strings that you are generating.
In the Electrical Devices panel, inverter settings such as size, power, and the number of strings per inverter will be configured. To disable automatic inverter sizing and enter values manually, click Off.
Finally, the power values will be used for Cable sizing calculations. As usual, you can edit these values at any time.