Demystifying Solar Module Temperature Coefficients

Many do-it-yourself system owners and even expert solar panel installers struggle with calculating temperature coefficients for a PV module. What they really want to know is “What do these specifications mean to my systems’ performance?” This article explains how ambient and cell temperatures affect solar module behavior and offers suggestions on how to improve performance.

Clarifying Module Output Power, Voltage & Current

A PV module consists of an array of individual cells, typically 36 or 60. A PV module’s temperature coefficients are mathematical expressions of how cell temperature affects module output power, voltage, and current. They describe how temperature affects module performance, and the negative coefficients for power and voltage help explain why your PV system may not meet performance expectations.

Power production may be lower than expected, particularly in high ambient temperatures, and depending on system architecture and configuration, a PV system may not be able to achieve target charging voltages.

PV module data sheets contain key performance specifications and other useful technical data. Rated electrical specifications are based on Standard Test Conditions (STC) – including the 25°C cell temperature reference – using a solar simulator in a laboratory environment.

STC specifications allow for useful comparison of PV modules, but they’re not necessarily an accurate indicator of real-world performance. Under optimal operating conditions (i.e., clear full Sun around noon and a well-aligned PV array), a module’s cell temperature will measure roughly 30°C (+45°F) above ambient. For the cell temperature to be about 25°C (77°F), the ambient temperature is typically about -5°C (23°F). That’s a cold operating condition!

Everything else being equal, a module’s power- and voltage outputs drop as cell temperature increases. The module’s temperature coefficients algebraically describe these reductions. The table below is an example of how ambient temperatures can affect optimal performance:

Ambient
Conditions
Ambient
Temp (°C)
Ambient
Temp (°F)
Cell Temp (°C) Performance
Loss
Example Power from
a 300 W Module
Very Cold -5 23 ~25 ~0% 300 W
Cool 10 50 ~40 ~6% 282 W
Moderate 25 77 ~55 ~12% 264 W
Hot 30 86 ~60 ~14% 258 W
Very Hot 35 95 ~65 ~16% 252 W

Optimizing PV Module Performance

Fortunately, there are several ways to optimize PV module performance in both hot and cold conditions. Two universal recommendations are to (1) increase PV array size by ~20% to compensate for hot temperature performance loss, and (2) not surface-mount PV modules. Instead, install them with a several-inch gap under the modules to allow for air circulation to help cool the modules and improve their performance.

Here are some architecture-specific suggestions for 12V and 24V systems to improve performance (verify compatibility of module and controller specification):

(A) For 12 V (nominal) systems using “12V modules” (36 cells) and 12V series (PWM) or basic MPPT controllers:

  1. Select PV modules with a relatively high STC MPP voltage specification (18V range). The high voltage will help achieve target charging voltages in hot temperature
  2. Connect individual modules in parallel to increase array current
  3. Use heavy-gauge wire to connect the module to charger and charger to the battery. This reduces voltage loss in the overall charging systems
  4. Consider using AGM batteries, which don’t use the high equalizing voltage required by flooded-cell batteries

(B) For 24 V systems using “12V modules” (36 cells) and 24V series or PWM controllers:

  1. Generally the same as (A) above, but connect two modules in series for each “24 V” string before connecting strings in parallel

(C) For 12 V systems using “12 V modules” (36 cells) and advanced MPPT controllers:

  1. Generally the same as (A) above, but connect two modules in series for each “24 V” string before connecting strings in parallel. The string voltage will be high enough to meet target charging voltages, and the MPPT controller can convert ‘excess’ voltage into additional charging current. This is especially helpful in cold ambient temperatures when module voltage can be relatively high.

(D) For 12 V systems using “grid-tie modules” (54 or 60 cells) and advanced MPPT controllers:

  1. Connect individual modules in parallel to increase array current
  2. Select controller nominal 12V output voltage (some controllers do this automatically)
  3. The module voltage will be high enough to achieve target charging voltages for common types of 12V battery banks (e.g., flooded or AGM), and the MPPT controller can convert ‘excess’ voltage into additional charging current. This is especially helpful in cold ambient temperatures when module voltage can be relatively high.

(E) For 24 V systems using “grid-tie modules” (54 or 60 cells) and advanced MPPT controllers:

  1. Connect two modules in series for each string before connecting strings in parallel
  2. Select controller nominal 24V output voltage (some controllers do this automatically)
  3. The string voltage will be high enough to achieve target charging voltages for common types of 24V battery banks, and the MPPT controller can convert ‘excess’ voltage into additional charging current. This is especially helpful in cold ambient temperatures when module voltage can be relatively high.

As operational temperature increases, PV module voltage and power decline. A module’s temperature coefficients mathematically express this behavior. However, careful system architecture and component selection can help mitigate or even eliminate the impact of these losses on system performance and your expectations.

If you’d like help optimizing your PV system’s performance, give us a call at 800-621-1271. We’ll be happy to review your requirements and design. Visit www.inverterservicecenter.com.


About the Author – Jim Goodnight – also known as “crewzer” — is a retired solar industry application engineer, product manager, and forum moderator and has previously written for Home Power and Solar Professional magazines.