Stringing, Micro-Inverters, and Power Optimizers

Summary:

This article outlines key design considerations in OpenSolar, focusing on string inverters, micro-inverters, and DC optimizers. It explains how these components interact with OpenSolar’s solar modeling engines—PVWatts and the System Advisor Model (SAM)—which are used to estimate system performance. This guidance pertains to the Project Zone > Design tab within the OpenSolar application.

Table of Contents

String inverters

Micro-inverters

Effect of Optimisers/Micro-inverters using PVWatts

Effect of Optimisers/Micros using System Advisor Model (SAM)

String inverters

OpenSolar currently supports two stringing methods: Manual and Automatic. Proper string configuration is essential for accurate energy modeling, especially when using SAM.

Manual Stringing

Manual stringing gives users full control over how modules are connected to the inverter’s MPPT inputs. This method is recommended for advanced users who require precise design customization.

To configure manual stringing:

1. Navigate to the Design tab or the Inverter section.

2. Select an inverter from the dropdown list.

3. Click the + String button to add MPPTs and strings.

4. Click and drag across panels to assign them to a string.

Note: SAM accounts for mismatch losses if panels with different orientations are assigned to the same MPPT. To improve modeling accuracy, OpenSolar recommends assigning all panels in a system appropriately using stringing tools.

Auto Stringing

Automatic stringing is a simplified method that uses predefined rules:

1. Select an inverter from the dropdown menu.

2. Click the Auto String button.

3. The system will automatically assign strings based on layout and inverter specs.

Micro-Inverters

When a micro-inverter is selected for the system:

• OpenSolar automatically calculates the number of micro-inverters based on the module count.

• Each panel operates independently at its own maximum power point.

• The modeling engine groups panels by orientation and assigns each group to a separate MPPT input in SAM.

This setup minimizes mismatch losses and improves shading tolerance.

DC Optimizers

Optimizers are configured via:

Control > Design & Hardware > Other Components

To configure DC optimizers:

1. Ensure the component is defined with type set as DC Optimiser.

2. Add the optimizer to your component library.

3. In the panel settings, activate Use DC Optimiser.

OpenSolar calculates the optimizer-to-panel ratio (e.g., 1:1 or 2:1) based on voltage, current, and power constraints derived from component specifications.

Impact of Micro-Inverters and Optimizers in PVWatts

Using micro-inverters or DC optimizers changes modeling assumptions in PVWatts:

• Mismatch Loss: Reduced to 0%.

• Shade Mitigation Factor (SMF): Set to 33%, reducing the effect of shading by one-third.

Although PVWatts is simplified and does not fully capture orientation-based or panel-level mismatch, OpenSolar accounts for some of these effects by grouping subarrays and modeling them independently.

Impact of Micro-Inverters and Optimizers in SAM

SAM offers a more granular representation of performance. Default loss values vary depending on system configuration. In the following table, soiling refers to losses from dirt, leaves, or dust covering panels. DC wiring refers to resistive losses in DC wiring, and mismatch refers to losses from panels not operating at MPP, due to manufacturing, shading or soiling differences.

These default values reflect nominal conditions. OpenSolar's 3D model will accurately account for the impact of shading on available irradiance, but mismatch losses reflected in the global loss factors do not scale with shading. SAM accounts for additional shading losses impacting string inverter projects but not module-level optimization projects as follows:

Shading Effects:

• Micro/optimizer systems: Shading is applied linearly (e.g., 20% shading = 20% power loss), with panel-level resolution.

• String inverter (no optimizer): Beyond the standard linear shading, SAM uses a validated lookup table to produce additional non-linear shade impacts to account for shade-related mismatch.

Configuring Losses and Simulation Settings

OpenSolar enables tailored loss configuration via simulation presets. Users can apply different global loss factors for each system type.

Accessing Simulation Settings:

1. Go to Control > Design & Hardware > Setback & Design Settings

2. Edit an existing settings preset or create a new one.

3. Expand the Simulation Settings section.

Here, users can configure:

• Soiling loss

• DC wiring loss

• Module mismatch loss

Note: OpenSolar integrates SAM with proprietary 3D shade modeling. SAM assumes all modules in a string operate at the same maximum power point, and shading is treated as a uniform loss unless optimizers or micro-inverters are used.

What is Shade Mitigation Factor (SMF) for microinverters:

Microinverters and DC Optimisers are ideal for conditions where the modules are partially shaded during a time of peak sun exposure. They have the potential to recover the shading losses by a certain factor that is known as shade mitigation factor (SMF).  

SMF represents the annual percentage of shading losses that can be recovered through the use of module level power electronics (microinverters or DC optimisers) in partially shaded PV systems. The SMF can be used to calculate an updated shade loss term by the following equation [1]:

Shading Loss (new) = Shading Loss (%)  x (1 − SMF)

OpenSolar assumes a default SMF value of 0.33 based on the recommendation provided here.

[1] More information about this can be found on the NREL Website: Modeling Microinverters and DC Power Optimizers in PVWatts.