Module and Inverter Specifications

What does stringing modules mean in a solar powered system? Each solar module has two main attributes: voltage (V) and current (I).

Simple Example Module Current and Voltage attributes

You may need to brush up on how electrical circuits work, but if you recall you may connect electrical components in series or in parallel.

Simple Example of Modules connected in Series, Voltage Increases and current Remains the Same

Connecting a solar panel in parallel connects multiple strings together. Electrically, this means that the voltage of each string remains the same, but the current increases by the number of strings you have connected together. In the case below, the current from each string of 6.3 A is multiplied by 2 strings which would be 12.6 A total at the point of connection.

Simple Example of Two Strings connected in parallel, Voltage of both Strings Remain the Same and Current Increases

The combination of connecting solar modules in series and parallel creates an array that will then connect to the inverter.

Inverters typically have 4 main specifications that need to be met as to not void the warranty and for maximum performance:

• Minimum Operating Voltage
• Maximum Open Circuit Voltage
• Max Short Circuit Current or Max Current
• Max Power

The module specifications that are used to string size against the inverter specifications.

Most modern string inverters have a multiple power point tracking (MPPT) that operates within a specific voltage range.

Checking the minimum voltage does two things:

• Minimum Voltage is above the start-up voltage
• Minimum Voltage is within the lower MPPT range

Checking the maximum voltage does two things:

• Maximum voltage is below the inverter’s maximum (possibly damaging the inverter if not)
• Maximum voltage is within or close to the upper MPPT range

Checking the maximum Isc or maximum current does one thing:

• Maximum current is below the inverter’s maximum (possibly damaging the inverter if not)

Checking the maximum connected power is below the inverter’s max. This may be optional as some inverter manufactures have this specified and others do not.

Before string sizing we will need to check the inverter limits and peak performance ranges.

Example inverter data sheet

Max. PV Power: be sure that the total number of modules are below the inverter’s maximum.
Max. DC Voltage: be sure that the total modules in series are below, in this case, 600 V total.
MPPT operating voltage range / Start Voltage: be sure the minimum module voltage is above the operating MPPT lower range of the start voltage, whichever is higher. In this case it would be the 125 V.
Max. short circuit current current per MPPT: be sure that the total max current due to the number of strings connected in parallel per MPPT doesn’t exceed the Max short circuit current per MPPT. Now let’s take a look at the module data sheet for the important information for string sizing.

Example panel data sheet

String Sizing Calculation

Calculate Lowest Voltage

Ensure that the lowest voltage during the highest temperature is within the start-up voltage and MPPT range.

Find the high ambient temperature for the site and calculate the low voltage per module using the following equation:

(( T_high-25º ) * (%/C) * V_mp )+ V_mp = V_low

Once you find this voltage, find the minimum start-up or MPPT voltage for the inverter and calculate the minimum string length.

(Inverter Min Voltage) / ( V_low) = Minimum String Length

Calculate Highest Voltage

Ensure that the highest voltage during the lowest temperature is within the inverter’s max and near the upper MPPT range.

Find the low ambient temperature for the site and calculate the high voltage per module using the Voc and Tvoc with the following equation:

(( T_low-25º ) * (%/C) * V_oc )+ V_oc = V_high

Once you find this voltage, find the maximum voltage for the inverter and calculate the maximum string length.

(Inverter Max Voltage) / ( V_high) = Maximum String Length

Calculate Highest Number of Strings

Inverters have a maximum current limit, exceeding this limit may damage the inverter and void the warranty. Inverter limits can be displayed differently, sometimes it’s Isc and sometimes it is max current (typically 1.25 X Isc).

From the example above, it specifies a maximum short circuit current.

(Inverter I_sc Max) / (Module I_sc) = Maximum Number of Strings

For inverter data sheets that specify a maximum current limit:

(Inverter Max Current) / (Module I_sc X 1.25 ) = Maximum Number of Strings

Calculate Total Max Number of Modules / Max power

The inverter has a maximum PV power that can be connected to it. As we add modules, this increases the total power. We need to consider the maximum total number of modules that can be connected to the inverter. This may impact the number of strings or how how many modules per string you

(Inverter Max Power) / (Module Power) = Maximum number of modules

Iterate for the ideal array configuration:

After calculating the low and high limits, you now can finalize the array configuration within those limits along with other factors that may affect it. Some of these factors could be the module count that has already been purchased, physical space for modules, etc.

Lets run through an example.

Example

Location: Dixon, MO
Inverter: Sunny-Boy 3.0
Solar Module: EverVolt EVPV360

Calculate Lowest Voltage

Ensure that the lowest voltage during the highest temperature is within the start-up voltage and MPPT range.

The high ambient temperature for the site is found on energyresearch.ucf.edu and the 2% Avg. is 34º C.

High temperature

Calculate the low voltage per module:

(( 34º-25º ) * (-0.0026/C) *36.7V )+ 36.7V = 35.84 V_low

Once you find this voltage, find the minimum start-up or MPPT voltage for the inverter and calculate the minimum string length. We round this number up.

(125 V) / (35.84) = 3.48 = 4 Module Minimum String Length

Calculate Lowest Voltage

Ensure that the lowest voltage during the highest temperature is within the start-up voltage and MPPT range.

The low ambient temperature for the site is found at energyresearch.ucf.edu and the Min is -19º C.

Low temperature

Calculate the high voltage per module:

(( -19º-25º ) * (-0.0024/C) *43.9V )+ 43.9V = 48.54 V_high

Once you find this voltage, find the maximum voltage for the inverter and calculate the maximum string length. We round this down.

(600 V) / (48.54) = 12.36 = 12 Module Maximum String Length

Calculate Highest Number of Strings

Find the inverter’s maximum current limit and the module’s I_sc at STC. The inverter’s datasheet specifies max short circuit current per MPPT, so this is just I_sc.

(18 A) / (10.49 A) = 1.72 = 1 String Maximum per MPPT

If the Inverter data sheet specifies a max current of 18A, for example, then you would need to calculate max current per NEC. One method of max current is 1.25 X I_sc.

Calculate Total Max Number of Modules / Max power

Locate the inverter’s maximum PV power and the module’s power.

(4800 W) / (360 W) = 13 Maximum number of modules total

Iterate for the ideal array configuration:

After calculating the low and high limits, you now can finalize the array configuration within those limits. Here are the results we calculated:
Minimum String Length: 4
Maximum String Length: 2
Maximum Number of Strings: 1 Per MPPT
Maximum number of Modules 13 Modules

This inverter has 2 MPPT trackers, so a total of 2 strings can be connected to the inverter. We know that there can only be 13 modules maximum installed. We can have one MPPT with 6 modules in a string and the other at 7 modules in a string.

Results

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Results with UpTop Solar String Sizing Tool