Glossary

Definitions, Questions and Answers

STC Rating
The Standard Test Condition (STC) is an instantaneous solar panel rating under controlled conditions. The Standard Test Conditions for a module are: solar irradiance of 1000 W/m2 , with the cell temperature maintained at 25°C and zero wind speed for cooling effect. These conditions are rarely, if ever, encountered in the real-world; therefore we are blessed with multiple other confusing terminologies.
PTC Rating
PTC (PVUSA or Performance Test Conditions) are defined as 1000 W/m2 plane-of-array irradiance, 20°C ambient temperature, and 1 m/s wind speed. PTC differs from standard test conditions (STC) in that its test conditions of ambient temperature and wind speed will result in a cell temperature of about 50°C, instead of the 25°C for STC. This rating was developed in an attempt to simulate real-world conditions. With elevated operating temperatures, the module/array produces less power, therefore the PTC rating of a module is typically 89% of the STC rating.
Inverter Efficiency
Efficiency for an inverter can be defined as power-out divided by power-in. The inverter efficiency varies with ambient temperature, DC input voltage, and inverter’s operating power level. As a result, defining inverter efficiency is an interesting game. The California Energy Commission has created a "weighted" inverter test procedure to create a level playing field. This weighted inverter efficiency is known as the CEC inverter efficiency.
CEC Rating
The California Energy Commission (CEC) has created the CEC system rating for determining rebate levels. This CEC rating for a solar array is simply the PTC rating of a solar array times the inverter's CEC efficiency.
Inverter Power Rating:
This is the maximum output power rating of the inverter.
Inverter Maximum DC Input Current:
This is the maximum input DC current that should be connected to the inverter at any time. It is important not to exceed the maximum input current of the inverter as this may damage the components and will void the warranty.
Inverter Maximum DC Input Voltage:
This is the maximum DC voltage that can be applied to the inverter. Exceeding this value can damage the inverter. In the Selection Guide output results, the maximum input voltage is determined using the Open Circuit Voltage corrected for the user-selected minimum temperature.
Inverter Minimum Power Point Tracking DC Input Voltage:
This is the lowest voltage at which the inverter can continue its maximum power point tracking routine, which optimizes the power production. The inverter will continue to operate at voltages below this value, down to Minimum DC Input Voltage, but will not maintain the maximum power production.
Inverter Minimum DC Input Voltage:
This is the lowest DC input voltage for which the inverter can operate. In the Selection Guide output results, the minimum input voltage is calculated using the Max Power Voltage corrected for the user-selected maximum temperature.
Module Nominal Power Rating:
This is the nameplate rating or nominal output power of the panel under Standard Test Conditions. At higher temperatures, the PV panel will produce less power than its nominal STC rating.
Cell Temperature:
We use a standard 30°C as the cell temperature above ambient temperature for all calculations.
Lowest Ambient Temperature:
The lowest ambient temperature selection should be conservative and as close to the 30-year record for the system's geographic location. This temperature selection is used to adjust the open circuit voltage and ensure the inverter's maximum voltage is not exceeded.
Highest Ambient Temperature:
The highest ambient temperature selection should be conservative and as close to the 30-year record for the system's geographic location. This temperature selection is used to adjust the maximum power (operating) voltage and ensure the inverter does not shutdown due to low voltage during the hottest part of the day.
Strings:
A number of PV panels connected in a series circuit represent a "string". The maximum number of panels in a string is determined by the Open Circuit Voltage at the lowest temperature selected. The minimum number of panels in a string is determined by the Maximum Power Voltage at the hottest temperature selected.
Module Open Circuit Voltage:
This is an electronics term for the voltage between two points when they are not connected by a load circuit. This is typically the greatest amount of voltage that can be provided by a circuit to these two points.
Module Maximum Power Voltage:
This is the operating voltage point where a solar module delivers maximum power. The Maximum Power Voltage changes primarily with sun intensity and cell temperature. A grid supplementing inverter has a maximum peak power tracking routine which tracks the changing Maximum Power Voltage throughout the day.
Module Short Circuit Current:
This is an electronics term for the current between two points that are connected by load circuit with zero resistance. This is the greatest amount of current that can be delivered by a circuit.

Calculation Method for Matching Array and Inverter Voltage Windows:

The solar array minimum and maximum voltages are calculated as follows:

Vmax=XSeries*(Voc+(Ltemp-25°C)Vtemp)

Vmin=XSeries*(Vmp+(Htemp+∆V-25°C)Vtemp)

Where:

Vmax = Maximum temperature adjusted open circuit voltage encountered
Vmin = Maximum temperature adjusted open circuit voltage encountered
XSeries = The number of modules in series
Voc = Open circuit voltage of the module, experienced by the array is subjected to sunlight or just before the inverter is starting, also known as the no-load voltage.
Vmp = Maximum power voltage of the module. This voltage exists when the array / inverter is operating and producing power in ideal sunlight conditions.
Ltemp = The design minimum temperature experienced in daylight conditions
Htemp = The design maximum temperature experienced in daylight conditions
∆V = The change in temperature between ambient temperature and sunlight soaked solar modules
Vtemp = The solar module manufacturer’s voltage drop due to temperature (expressed in V/ºC)

What is the magical 25 in the equation? This is the temperature at which the module is rated by the manufacturer. These equations adjust the temperature above or below the nominal rating at 25 ºC. The temperature adjustment is then multiplied by the appropriate voltage condition to judge whether or not the array and inverter are compatible.

Please contact us with any questions at Technical Support.