Determining Module Compatibility

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One of Enphase Energy's biggest technical questions is about module compatibility and how to calculate. The document below explains this in detail. Please post comments below.

Technical Brief
Determining Module Compatibility

Generally a 60-72 cell module will be electrically compatible with the M190-72; and a higher voltage 72-84 cell modules will be electrically compatible with the M210-84. System designers need to determine that the specifications of a PV module will not exceed the microinverter model’s nameplate ratings under all expected environmental conditions for a given application. The module data sheet contains all of the necessary module specifications to make this determination. The ASHRAE Handbook is the recommended source for weather data in a particular location. This document details how this determination can be made and assumes the specifications of a 60 cell module.

Expected Voc:
There are two considerations for Voc; highest expected temperature and lowest expected temperature. First, the system designer should consider Voc at the highest expected cell temperatures to verify that the Voc will meet the inverter’s minimum start up voltage.

This can be calculated based on the module’s rated specifications:

(Voc) + (Voc)*(Highest Expected Cell Temp.-STC)*(Temp. Coefficient %/100) > 28V

Example (220W Standard Test Condition rated module):

36.6 + (36.6)*(75C-25C)*(-.34/100) > 28V
36.6 + (36.6)*(-.17) > 28V
30.38V > 28V

Second, the system designer should consider Voc at lowest expected cell temperatures to verify that the Voc will not exceed the maximum input voltage of the inverter (56V).

This can be calculated based on the module’s rated specifications as follows:

(Voc) + (Voc)*(Lowest Expected Cell Temp.-STC)*(Temp. Coefficient %/100) < 56V

Example (220W Standard Test Condition rated module):

36.6 + (36.6)*(-40C-25C)*(-.34/100) < 56V
36.6 + (36.6)*(.221) < 56V
44.69V < 56V

Expected Vmp:
There are two considerations for Vmp. First, the system designer should consider Vmp at the highest expected cell temperature to ensure that the inverter will be able to perform Maximum Power Point Tracking.

When calculating module Vmp it is acceptable to use the temperature coefficients provided for Voc. Most module manufacturers do not list temperature coefficients for Vmp but when they are listed, they should be applied to this calculation.

High temperature Vmp can be calculated based on the module’s rated specifications as follows:

(Vmp) + (Vmp)*(Avg. High Expected Cell Temp.-STC)*(Temp. Coefficient %/100) > 22V

Example (220W Standard Test Condition rated module):

(28.7) + (28.7)*(75C-25C)*(-.34/100) > 22V
(28.7) + (28.7)*(-.17) > 22V
23.82V > 22V

Second, the system designer should consider Vmp at the lowest expected cell temperature to verify that Vmp less than or equal to the inverter’s highest MPPT limit. Again this will ensure that under most conditions the inverter is operating within the MPPT range.

This can be calculated based on the module’s rated specifications as follows:

(Vmp) + {(Vmp)*(Avg. Low Expected Cell Temp.-STC)*((Temp. Coefficient %/100))} < 40V

Example (220W Standard Test Condition rated module):

(28.7) + (28.7)*(-25C-25C)*(-.34/100) < 40V
(28.7) + (28.7)*(.17) < 40V
33.58V < 40V

Expected Isc:

One of two methods may be used here. If the maximum fuse rating of the module is less than or equal to the inverter’s rated maximum short circuit current, it is acceptable and no further consideration must be given. If the fuse rating is greater than the inverter’s max DC short circuit current rating, the following calculation must be made for Isc at high temperature:

Isc + (Isc)*(Highest Expected Cell Temp. – STC)*(Isc Temp. Coefficient %/100) < 12A

Example (220W Standard Test Condition rated module):

8.24 + (8.24)*(75C-25C)*(.053/100) < 12A
8.24 + (.21836) < 12A
8.46 < 12A

System Design:
It is most important to verify that the module Voc and Isc will not exceed the microinverter’s nameplate ratings under all expected weather conditions. The system designer is responsible for determining the appropriate module to use for a specific installation in order to achieve the maximum energy harvest. For more information about appropriate module power rating refer the white paper “Module Rightsizing”.

Please note: When verifying compatibility between microinverters and solar modules, it is always recommended that an Electrician or Solar Professional verifies calculations.

Enphase Energy FAQ

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Posted 3 years ago

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Bob Greene, Champion

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Good information. Thanks - I live and work in an area that has historic low temperatures below zero. The other thing to remember is that the high temperature is the ambient temperature of the module on the roof. 130-140 degrees F are not uncommon roof temperatures in the summer.