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Design Irradiance:

The Design Irradiance represents the amount of energy per second that the sun provides at a given location, usually represented by Latitude and Longitude coordinates. In areas such as Denver, CO, or Bozeman, MT, where the elevation is significantly higher than sea level, the amount of energy per second tends to be greater than 1000 W/m2. In areas like Seattle, WA, or Portland, OR, where cloud cover is present for much of the year, the amount of energy per second tends to be smaller than 1000 W/m2. The value 1000 W/m2 is the average value for the world, near sea level. If you want to find more detailed information you can look at the National Renewable Energy Laboratory website.

Hottest Ambient Temperature:

The hottest ambient temperature selection is less critical and the average high temperature can be used for this calculation. The high temperature is used in the calculation of the minimum max power voltage, the input current, and the input power. The inverter is more tolerant of exceeding the maximum input current by a few milliamps than it is of exceeding the maximum input voltage. The Coefficients of Voltage and Power are usually negative and will tend to decrease the voltage and power at higher temperature. As a result, the danger of exceeding the inverter's maximum inputs is reduced.

Inverter Minimum DC Input Voltage (VMIN):

This is the lowest DC input voltage at which the inverter can continue to operate. This depends in part on the AC line voltage, but will not vary more than a few volts in either direction. In the sizing calculation, the minimum input voltage is calculated using the maximum power voltage of the string of panels, and corrected for temperature using the high ambient temperature.

Inverter Minimum Power Point Tracking DC Input Voltage (VMPTmin):

This is the lowest voltage at which the inverter can maintain power point tracking. The inverter will continue to operate at voltages below this value, down to Minimum DC Input Voltage, but will not maintain the maximum power point of the array.

Inverter Maximum Power Point Tracking DC Input Voltage(VMPTmax):

This is the maximum DC input voltage at which the inverter can maintain power point tracking. The inverter can continue to operate at input voltages above this level, but will not maintain the maximum power point of the array.

Inverter Maximum DC Input Voltage (VDCmin):

This is the absolute maximum DC voltage that can be applied to the inverter without damaging any of the components. If this limit is exceeded, the warranty on the inverter will be voided, and any resulting damage is the responsibility of the person, or persons, installing the inverter. In the String Sizing Program calculation, the maximum input voltage is determined using the open circuit voltage of the string of panels, and corrected for temperature using the lowest ambient temperature.

Inverter Maximum DC Input Current (IDCmax):

This is the maximum input DC current that should be connected to the inverter at any time. The output current for a panel is usually described in its specification sheet. This number is modified by the ambient temperature, as well as heating from the sun, in a way that will actually increase the current from a panel when it gets hot. 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 Power (PDCmax):

This is the recommended maximum DC input power that should be attached to the inverter. This number is in STC watts, which is usually the nominal output rating of the PV panels. This can be larger than the maximum rating of the inverter because STC watts are usually not the amount of power that the inverter is actually receiving. For a brief introduction to this concept please see this discussion of STC vs. PTC ratings for PV panels.

Inverter Maximum AC Output Power (PAC):

This is the maximum output power rating of the inverter. All Sunny Boy Inverters will produced up to this maximum rated output power provided there is sufficient DC input power from the PV array.

Module Temperature Coefficient of Power (PKpmp):

In order to determine how much the panel's overall output power is affected by changes in temperature, we need to know the Temperature Coefficient of Power. Like the Coefficients of Voltage and Current, the magnitude of the Coefficient of Power represents how much, or how little, changes in temperature will affect the performance of the panels. As you will notice, the Coefficient of Voltage is usually much larger, 10 to 100 times larger typically, than the Coefficient of Current. This means that the Coefficient of Power is influenced more by the Coefficient of Voltage than the Coefficient of Current. The maximum input power is actually a fuzzy limit. This means that if you were to exceed this value with a given number of panels, you might not actually damage the inverter, as you would with too much voltage or current, but you may see decreased performance. For more information about why the limit is fuzzy, please see this discussion about the different ways to rate panels. For more information about the possibility of degraded performance, please see this discussion about the derating mode.

Module Temperature Coefficient of Current (IKimp):

In order to determine how much the panel's output current is effected by changes in temperature, we need to know the Temperature Coefficient of Current. Like the Coefficient of Voltage, the magnitude of the Coefficient of Current represents how much, or how little, changes in temperature will affect the performance of the panels. You will notice that the Coefficient of Current is typically much smaller than the Coefficient of Voltage, usually between 10 and 100 times smaller. The Temperature Coefficient of Current is used with the highest temperature selected to determine the maximum number of strings that can be paralleled into the inverter. Each string must have the same number of panels. As long as that number fits within the voltage window specified by the minimum and maximum input voltages, this should result in a system that will function well.

Module Temperature Coefficient of Voltage (VKvoc):

In order to determine how much the panel's output voltage is affected by changes in temperature, we need to know the Temperature Coefficient of Voltage. The magnitude of the coefficient is used in the String Sizing Calculations to determine how much, or how little, changes in temperature will affect the panels. For the maximum input voltage calculation, the Temperature Coefficient of Voltage is used with the low temperature to determine the maximum number of panels that can be used in a string. For the minimum input voltage calculation, the coefficient of voltage is used with the high temperature to determine the minimum number of panels that should be used. Any configuration between these two extremes can be used as long as they do not exceed the maximum current or maximum power input thresholds.

Module Maximum Power Current (IMP):

This is the current that the panel produces when exposed to sunlight and a complete circuit with a load impedance is attached in such a way that power produced by the panel is maximum. This current is determined using the Standard Test Conditions, and like the Short Circuit Current is subject to change based on the temperature. The Maximum Power Current is affected by temperature in the same way as the Short Circuit Current.

Module Short Circuit Current (ISC):

This is the current that is produced by a PV panel when the load circuit impedance is reduced to zero Ohms, creating a short circuit. This rating is again determined using the Standard Test Conditions and is subject to change based on the temperature of the panel. Unlike voltages and power, the current is usually positively effected by temperature, such that the current will increase with temperature. This increase is very small compared to the reduction of voltage with temperature.

Module Maximum Power Voltage (VMP):

This is the voltage that the panel produces when exposed to sunlight and a complete circuit with a load impedance is attached in such a way that power produced by the panel is at its maximum. This voltage is again determined using the Standard Test Conditions, and like the Open Circuit Voltage, is highly dependent on the temperature of the panel. Like the Open Circuit Voltage the Maximum Power Voltage is reduced with increases in temperature.

Module Open Circuit Voltage (VOC):

This is the open circuit voltage of the panel when it is exposed to sunlight. A PV panel will produce electricity from the sun because of the photoelectric effect which converts the sun's energy into electricity. The open circuit condition is applied to the inverter when the panels are exposed to any sunlight, even indirect, and the inverter is not producing power. This value is highly dependent on the temperature of the panel, and like nominal power decreases with increased temperature.

Module Nominal Output Power (Module PNOM):

This is the nameplate rating or nominal output power of the panel under Standard Test Conditions. When modules are mounted in a PV system they are usually operating at temperatures that far exceed the temperature used for the Standard Test Condition. For more information about the differences between Standard Test Conditions and real operating conditions please read STC, PTC, and Power Production from our FAQ's section. The power that is produced from a PV panel is inversely proportional on the temperature of the PV cells. At higher temperatures, the PV panel will produce less power than its nominal STC rating.

Mounting Style:

The mounting style choices that are available represent the most common styles for mounting panels in a system. The first choice, "Mounted flat against the roof", means that the panels are going to be mounted in the same geometric plane as the roof pitch, and that there will be less than 6 inches of clearance between the backs of the panels and the top of the roof. The second choice, "Tilt-angle mount or ground mount", means that the panels will be mounted in such a way that bottom edge of the panels will be significantly closer to the plane of the roof pitch than the top edge, or that the panels will be mounted on the ground on a non-mobile rack. The third choice, "Pole or Tracker mount", means that the panels will be mounted on a single pole, or on a Tracker device for following the sun across the sky. These three choices effect the calculation by determining the rise in temperature above ambient that the panels will experience in each installation style. The difference in temperature rise is directly attributable to the amount of air circulation behind the panels which can be used for cooling. The temperature rises are 35°C, 30°C, and 25°C, respectively for the three mounting options.

Open Circuit Voltage:

This is a term in electronics that means 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.

Strings:

A string of panels is meant to represent a number of PV panels connected in a series circuit, and attached to a Sunny Boy Inverter. The maximum number of panels in a string is determined by the Open Circuit Voltage at the coldest temperature selected. The minimum number of panels in a string is determined by the Maximum Power Voltage at the hottest temperature selected.

Short Circuit Current:

This is a term in electronics that means the current between two points that are connected by load circuit with an impedance of zero Ohms, or no resistance. This is typically the greatest amount of current that can be delivered by a circuit between these two points.

Photoelectric Effect:

The Photoelectric Effect was first described by Albert Einstein and was the work that actually won him the Nobel Prize in Physics, not his more famous work in relativity. Essentially, the photoelectric effect says that by shining light of certain wavelengths onto a metallic surface, and for the purposes of this discussion the silicon in PV panels is a metal, you can produce electrons which can be used to create an electric current. This current can be used like any other DC current, so in effect, because the light comes from the Sun, this is "free" energy that we can use. For more detailed information about the photoelectric effect please see these websites, the content of these websites are the property of their respective owners:

• University of Colorado
• Physlink.com
• JAVA applet provided by Michigan State University<//link>
• Georgia State University

Units:

This selection will switch the temperature selection boxes from °F to °C, whichever is more comfortable for you.

Configurations:

The configurations displayed here are recommendations that will work for the given inverter. These recommendations are not intended to be used as predictions of system performance, and are only to be used as informational regarding the number of panels that could be used with a given inverter. These configurations are intended to be used in such a way as to ensure that the inverter will not be damaged and are therefore inherently conservative in nature.

Coldest Ambient Temperature:

The coldest ambient temperature selection should be conservative and as close to the 30-year record low, if not lower, for the area in which the array will be installed. This temperature is used in the calculation of the maximum input Open Circuit Voltage to determine the maximum number of panels that can be used in each string. This temperature is critical because if the maximum open circuit voltage exceeds the inverter's limits, then the inverter may be damaged in an over-voltage situation. The highest input voltage is recorded in the inverter's non-volatile memory and damage produced in this way will void the warranty.