Considerations When Pairing Solar Panels & Microinverters

There is a wide and ever-growing variety of microinverters, optimizers, and solar modules on the market. A common question we see: Which panel and microinverter (or optimizer) should I pair?

To sort out this matter we look at several of the product datasheets from our current lineup as of July 2021. In this article we’ll dive into current, voltage, and power considerations, as well as site specifics when making this design decision. You’ll want to have both the microinverter and solar panel datasheets in front of you when making a selection.

For this example we’ll use the following datasheets

Enphase IQ 7 and IQ 7+ Micro Inverters Datasheet

APSystems QS1 4-Module Micro Inverter - Datasheet

Trina Solar 395W Panel Datasheet

LONGi Solar 540W Panel Datasheet

Also, consider the temperature extremes of your location:

Solar Reference Map

We’ll use weather data from San Francisco International Airport for this example.

You’ll need to note the extreme low, as well as high temperature for the location of your installation.

In this example we’ll use the following:

High Temperature at 3.5” above roof: 42 °C

Extreme Minimum: 2 °C

Now we’ll take a look at current for compatibility:

We want to make sure that the panels will never supply more current than the microinverters are rated for. Referring to the Trina 395 datasheet, with the current-voltage curve in the top right corner of page 2, we can observe that the current is highest when the voltage is lowest, this is the short circuit current (Isc) at standard test conditions (STC). Because site conditions will be different than that of STC, and current is proportional to temperature in this application, we must consider site conditions to calculate the site specific Isc. The steps for this are as follows:

Identify each panel’s short circuit current (Isc) at STC.

Calculate site specific Isc . To do this take your site high-temperature difference from STC (25°C) and apply the temperature coefficient of Isc to get the high temperature Isc estimation for your site. 

Formula:((Tsite-Tstc)*Cisc+1)+Isc 

Example for Trina 395 with 42°C site high temperature.((42-25)*0.04+100)/100*12.21A=12.29A

Identify each microinverter’s maximum DC short circuit current

Now compare the microinverter Max DC short circuit current to the calculated panel Isc at your site. Select only panel and microinverter pairs for which the Max DC short circuit current is not exceeded by the panels. With high power panels coming onto the market this may become a growing concern.

Ne we’ll look at voltage for compatibility:

For pairs which have passed the current requirements, we will then check that we do not exceed the microinverter’s voltage rating. 

First we find on the panel datasheets: Open Circuit Voltage (Voc) and Temperature coefficient of Voc (Cvoc).

Next we calculate the maximum voltage of the panel at your site. Voltage is inversely proportional to temperature in this application, so we’ll use the minimum temperature at the site to calculate the maximum voltage supplied by the panel.

Formula:((Tsite-Tstc)*Cvoc+1)+Voc  

Example for Trina 395 with 2°C site extreme minimum.((2-25)*-0.25+100)/100*41.0V=43.4V

Microinverter - Maximum input DC voltage

Now compare the microinverter Maximum Input DC voltage to the calculated panel Voc at your site. Select only panel and microinverter pairs for which the Maximum Input DC voltage is not exceeded by the panels. Again, with high power panels coming onto the market this may become a growing concern. This is exemplified above comparing the LONGi 540 with the IQ7, which is incompatible in the case for this site, and at STC as well.

Finally, we’ll dive into power, to evaluate the quality of the pairings.

Commonly the microinverter will deliver less maximum continuous power than the panels maximum power output (Pmax). Depending on site conditions, the actual power output from the panels may be fairly close to the maximum continuous power output of the panels, or quite disparate in some poorly matched instances. We’ll need to again check the datasheets to begin making this consideration.

First look at the microinverters Maximum Continuous Power:

Next, note the Pmaxfor each solar panel. This is typically the nameplate capacity (395W for the Trina 395, and 540W for the LONGi 540)

Now consider the difference between the panel and microinverter power ratings, as well as site specifics. In a site with ideal conditions, a panel such as the LONGi 540 may be way oversized for the IQ7+ or QS1, while they technically could work. This oversizing means that the microinverters cannot output as much power as they are receiving from the panels, which is known as clipping loss. The LONGi 450 may be a better alternative in that case, while clipping losses are likely to still occur. The Trina 395 is a closer match, and depending on the site and conditions, may or may not exhibit clipping losses. For instance, shading and sky conditions, as well as array azimuth and tilt can all factor into the extent to which clipping losses may occur.  

**Note that some microinverter manufacturers may specify power ratings specifically for compatibility, please review all documentation provided by the manufacturer before making a decision.**

For further considerations on solar inverter clipping, see our related article.

A resource for checking solar module compatibility is available through Enphase.

Also check out SolarEdge’s Site Design Calculator for power optimizer recommendations based on array size and PV module selection.