At STC and tested under load, voltage at max power (Vmp) is the highest operating voltage a module will produce. Vmp, adjusted for highest operating cell temperature, is used to calculate the minimum number of modules in series.
Open-circuit voltage (Voc) occurs when the module is not connected to a load. No current can flow in an open circuit and, as a result, Voc occurs at the point on the I-V curve where current is zero, and voltage is at its highest (Note: the module produces no power under open-circuit conditions.)
Voc is used to calculate the maximum number of modules in a series string. Because voltage rises as the temperature drops, calculations are performed for the coldest expected operating conditions. This ensures that NEC parameters and equipment voltage limitations are not exceeded.
At STC, and tested under load, the maximum power current (Imp)is the highest amperage a module can produce. Imp is used in voltage drop calculations when determining wire gauge for PV circuits. This is a design consideration rather than an NEC ampacity calculation, for minimizing voltage drop and maximizing array output.
Short-circuit current (Isc) is the maximum amperage that the module can produce. There is no voltage when a module is short-circuited, and thus no power. Isc is the measurement used to size conductors and overcurrent protection, with safety factors as required by the NEC.
Frequently, nominal operating cell temperature (NOCT) specifications are also listed on a manufacturer’s sheet. These are measurements calculated at different conditions than STC, using a lower sunlight intensity (800 W per m2); an ambient (not cell) temperature of 20ºC; and a wind speed of 1 meter per second; with the module tilted at 45°. The NOCT value itself is the cell temperature—given in degrees Celsius—reached under these conditions, Compared to the STC 25ºC cell temperature, the NOCT value will always be higher, usually by about 20ºC. NOCT values are used to mathematically calculate other test condition data points without resorting to further laboratory tests. NOCT conditions tend to more closely resemble the field conditions PV arrays generally operate in, and so give a perspective on “real-world” module operation.
Power tolerance is the range within which a module manufacturer is stating the module can deviate from its STC-rated Pmax, and thus what the manufacturer warranty covers. Common values are +/-5%, -0%/+5, and up to +/-10%. A 200-watt module with a +/-5% power tolerance could produce a measured output of 190 to 210 W. Finding modules with a -0% power tolerance can ensure the best value per dollar spent, and keep arrays operating at closer to predicted output.
Module Efficiency & Cell Efficiency
Efficiency is the measure of electrical power output divided by solar input. At STC, power in is equal to 1,000 W per m2 and power out is the rated Pmax point. Assuming a module sized at exactly 1 square meter, and rated at 150 W Pmax, module efficiency would be 150 W per m2 ÷ 1,000 W per m2, which equals 15%. The typical crystalline efficiency range spans 12% to 15%, but there are high-efficiency modules over 19%, and amorphous silicon modules on the low end with efficiencies around 6% or 7%.
Cell efficiencies will be slightly higher than module efficiencies because there is usually a small amount of empty space between cells. When deciding what module to purchase, if W per square meter (known as power density) is the driving factor, then a module with high efficiency should be chosen. But in many instances, there is plenty of room for an array and price per watt will be given higher priority than module efficiency.
Modules are directly affected by both irradiance and temperature, and because of environmental fluctuations, also experience power output fluctuations. When exposed to full sun, the cells will reach temperatures above the STC temperature of 25°C. And sometimes cell temperatures are lower than 25°C, such as on cold winter days.
Temperature coefficients are used to mathematically determine the power, current, or voltage a module will produce at various temperatures deviating from the STC values.
The temperature coefficient of open-circuit voltage is used to figure out the PV array’s maximum system voltage at a site’s lowest expected temperatures. The temperature coefficient of power can be used along with pyranometer-measured irradiance to calculate the power an array should be producing, which can be compared to actual output to verify proper performance.